Noise is one of the signals that must be processed in electronic design. We all know that there are two types of amplifier noise: one is the external noise, which comes from the outside of the amplifier; the other is the internal noise, which comes from the device itself. It is very important to deal with the noise of the amplifier to improve the performance of electronic products. Here, we elaborate the principle of amplifier noise in the form of question and answer, and give some suggestions This paper expounds some practical skills such as how to deal with amplifier noise.

How to accurately measure the internal noise of Q1 amplifier? What factors does it have to do with? What problems should we pay attention to when testing?

[answer] for the measurement of the noise of the amplifier, generally speaking, the input of the amplifier is connected to 0, the output is passed through a low-pass filter, and then the high-precision ADC is used to sample and do FFT, or the oscilloscope is used to see the output.

Q: which noise parameter should we refer to when judging the performance of the amplifier?

Q3 [q] can you give a detailed introduction to the principle of lower amplifier noise? thank you!

[answer] Figure 1 shows the amplifier noise model. Amplifier noise can be divided into two types: voltage noise (VX) and current noise (IX). In practical circuits, amplifiers consist of many transistors, all of which have noise. Fortunately, the noise of all transistors can be reduced to the input of the amplifier.

Figure 1

The voltage noise specification is usually expressed in two ways in the data book, which are

When looking at the noise characteristics in the data book, you must know whether it is converted to the input or output. The noise characteristics of most amplifiers are converted to the input, which is almost the default algorithm for op amp datasheet. However, for other types of fixed gain amplifiers (such as differential amplifiers), the noise may be reduced to the output. Note that this input noise is amplified by the amplifier.

For example, for an amplifier with in-phase gain of 10, the output noise will be 10 times of the noise given in the index. The noise gain of some circuit configurations may be greater than the signal gain, and the inverse configuration is a good example. When the signal gain is – 1, the noise gain is actually 2. In order to determine the actual noise gain, all external voltage sources should be short circuited. At the same time, the RTI noise of the noise amplifier can be regarded as the noise appearing at the positive input of the amplifier. If the circuit is analyzed with this assumption, the gain accepted by the noise should be determined.

The noise characteristics of the instrument amplifier are slightly different from that of the operational amplifier. For the operational amplifier, all the internal transistor noise can be converted to the input. In other words, all the noise sources will be scaled according to the gain ratio. Otherwise, some noises in the circuit will be scaled according to the gain ratio, while other noises are independent of gain. Here, the amount of noise related to gain noise is shown as Eni, and the amount of noise independent of gain is shown as eno. There is a formula for the relationship between the two in the data manual.

Besides voltage noise, the amplifier also has current noise. If there is a resistor at the input, the current noise will interact with it and produce voltage noise. For example, most source voltages have a certain resistance. After all, converting a high impedance signal source to a low impedance signal source is one of the reasons for using operational amplifiers. The current noise flows through the resistor connected to the amplifier, resulting in voltage noise. Generally speaking, the higher the input bias current of the amplifier, the higher the current noise.

Figure 2 shows the configuration of voltage follower with a certain source resistance. The current noise of the op amp will interact with the signal source resistance and generate some additional noise at the output. Figure 3 shows how the resistor in the feedback path interacts with the current noise. The current noise flows through the parallel combination of the feedback resistors, generating an additional noise source at the input, and then the noise source is amplified by the amplifier to the output.

Shows how the resistance in the feedback path interacts with the current noise

Q4 [q] how to estimate the input and output impedances when designing amplifiers with operational amplifiers? Hope to use some practical indicators to express. What is the relationship between low noise and magnification?

[answer] generally, for op amps, we think that the input impedance is infinite and the output impedance is 0. (you can refer to the data manual of specific model for specific value). So the input and output impedance of the circuit can be calculated based on this condition.

In terms of noise, the RTI index given in the general manual is the noise seen from the input, so the output noise will be multiplied by the magnification.

Q5 [question] I use opa2350 amplifier. The input is clean and the output waveform is accompanied by 200 mV clutter. Where is it possible that it is introduced from? If it is caused by the side resistance or the amplifier itself, how to avoid it.

[answer] there may be many reasons, but generally there will not be such a large clutter. It is recommended that you carefully check the circuit settings, especially the gain configuration, and try to configure it to gain greater than 1, so as to improve the stability of the circuit. If the resistance thermal noise is large, you can consider appropriately reducing the resistance. The characteristics of the op amp itself cannot be changed.

Q6 how to reduce the internal noise and external noise of the device?

[answer] the internal noise of the device can’t be changed. You can limit the external noise by selecting the external bandwidth

[Q7 question] power matching and noise matching between capacitive sensor and input amplifier. Low capacitive sensor works at low frequency and presents high output impedance, which requires input amplifier to have high impedance input characteristics. However, high input impedance conflicts with low noise design. How can we reasonably solve this problem.

[answer] Hello, the capacitive sensor itself can improve the signal noise (kt / C) according to its capacitance value. There is no need to use a large impedance at the input end of the op amp itself. It can also improve the noise by adding input filtering according to the bandwidth of the op amp.

Q8 [question] I have a second-order or fourth-order low-pass filter composed of AD620 and OP07. Ad7732 is used for analog-to-digital conversion and ad7732 is used for reference voltage. The sampling rate is within 250Hz and the microprocessor controller is ATmega128, the output waveform will produce one second (1 Hz) up pulse noise, the inspection is sure to be generated by the internal circuit noise, but I do not know what caused it, what is the reason for this noise, how to eliminate it?

[answer] for such a problem, we should first look at the hardware circuit or ADC sampling. You can first use the oscilloscope to see the ad7732 input signal to see if it is normal. If there is no problem with the input, then check the ADC, the problem may be in the software.

Q9 〔 question: 〕 based on your experience, what are the characteristics of LC circuit filtering and operational amplifier circuit filtering, and where are they used?

[answer] LC filtering is simple, but the filtering effect is not as ideal as active one. Active one can amplify the signal at the same time, but passive one can’t.

Q10 [question] ask the experts, knowing the source of the noise, how to effectively reduce the overall noise of the amplifier? thank you!

[answer] now that you know the source of the noise, you need to analyze where the main part of the system noise is. For this part of the noise, we can find a way to reduce the noise by filtering or selecting a higher precision op amp.

Q11 [question] which model is the current integrated operational amplifier with the minimum noise and the maximum gain bandwidth product? thank you!

[answer] generally, it is difficult for gain bandwidth product and noise to achieve the highest performance at the same time. It depends on your system accuracy and bandwidth needs. You can see if ad8221 and ad8228 meet your needs.

Q12 in noise analysis and error analysis, what kind of noise and error can be calculated with root mean square, and what kind of noise needs to be added directly to the total noise?

[answer] the noise of any part of the system should be added to the total noise in the form of vector.

Q13 [q] what is the significance of knowing these noise parameters in the actual circuit design?

[answer] it is helpful for you to deal with the noise of the system. The noise parameters are very important for high-precision design.

Q14 how to consider the quantization noise of ADC?

[answer] quantization noise exists in theory and cannot be removed, which is also the source of theoretical SNR of 6.02n + 1.76.

Q15 amplifier can also be used capacitance and inductance devices, the noise of these two components such as consideration and description?

[answer] this problem is not sure. Inductance and capacitance are definitely noisy, but they are not considered.

Q16 [q] the internal resistance of the signal source is 500m Ω, and the signal size is about 50mV. How to design the amplifier circuit and PCB

In this case, to choose an op amp with high input impedance and low input bias current, try AD549

Q17 [Q:] just started, I would like to ask that RF amplifiers are generally power amplifiers (I understand that amplifying current under low supply voltage, right?) However, some components require the input RF signal voltage to be 5V. How to achieve this?

[answer: the amplified voltage or current has nothing to do with the supply voltage. If some power devices require a certain input power of the signal, we can consider using prediriver, such as adl5323.

Q18 is the effect of temperature on noise negligible? When designing, which effects on noise need special attention?

[answer] in most cases, the influence of temperature can be ignored. When designing, you should consider the noise performance of the resistor. We should also understand the bandwidth characteristics of the circuit, so as to reduce the voltage noise.

Q19 [question] please introduce the relationship between the noise source of amplifier and temperature.

[answer] generally, the voltage noise density and current noise density at room temperature will be given in the chip data of amplifier, but the relationship between noise and temperature will not be given.

Q20 [q] how to ensure the automatic equalization of the two differential signals when the differential signal enters the ADC chip?

The driver chip of ADC can ensure the equalization of differential signal.

Q21 [question] in the circuit design, the problem of signal crosstalk often appears in the actual debugging. I don’t know what the way of signal interference is, how can we reduce the occurrence of this situation? thank you!

[answer] we can consider using shield or adding ground shield to improve crosstalk.

Q22 [q] how can noise be reduced with high bandwidth?

[answer] try to narrow the bandwidth, otherwise the noise will not be reduced

Q: is the noise of a q23 differential amplifier lower than that of a single ended amplifier? Why?

[answer] we can’t guarantee that the noise is related to the internal structure, gain and bandwidth of the op amp. There is no fixed rule for the noise level of the op amp. The application is still based on the equivalent input noise on the datasheet.

Hello, q24! A DC level is added to an amplifier to adjust the offset of the amplifier, and the DC level is generated by a DAC. DAC has a certain distance from the amplifier, so the distance from DAC output to amplifier input is about 2000 mil. In this process, the long line will introduce some noise. What are some good suggestions to reduce the noise! thank you!

[answer] in this case, generally, the wire will not introduce too much noise. I don’t know what accuracy of your DAC is. In my opinion, the noise of DAC should be much greater than that of wire, so we don’t need to think too much about the wire.

Q25 〔 question: 〕 when the bandwidth performance requirements conflict with the noise performance requirements, how should we choose?

[answer] according to the requirements of your system, find a balance between the two

Q: is there a unified standard for noise index? Is it possible to choose directly without considering so many complicated calculation formulas?

[answer] the standard of noise is the same. If you don’t want to consider too many formulas, you can choose the one with small number. For low-frequency signal, choose the one with small voffset, and for high-speed signal, choose the one with small noise spectral density.

Q27 how to eliminate the high frequency interference of power line? Thank you, experts

[answer] for the power supply, it is better to use two capacitor filters at the position where the chip is connected close to the pin. The recommended circuit will be found in the general data manual.

Q28 [q] how to eliminate noise by single point grounding or multi-point grounding? What’s the difference between them? thank you!

[answer] single point grounding: refers to only connecting one piece of ground at the power supply pin of the chip. This is to prevent the backflow of the digital power supply from affecting the ground of the analog circuit. It is also used when the analog digital chip is on one board. Because the two pieces of ground must be connected together, it is generally selected at the junction of the analog and digital ground. Multipoint grounding refers to: the chip’s grounding anchor should be grounded nearby, do not lead a long line to the ground again.

Q29 [question] now I have a headache about noise. Can you calculate her noise level for a specific circuit? For example, how much MV is the output noise of a co directional amplifier in the circuit? If I connect a comparator later, what is the false alarm probability?

[answer] the calculation of op amp noise is quite different from that in circuit application. It is recommended to estimate the reference noise in datasheet, and evaluate the actual noise level of op amp according to the evaluation board and standard test method.

Q30 [q] can you tell us something about the noise from the sensor? thank you!

[answer] different sensors have different noise power, including internal inherent noise and external interference noise, including power frequency interference and radio frequency interference. You can see the specific noise for specific sensors

Q31 〔 question: 〕 for the 8-bit or 10 bit AD module with 5V power supply and reference, the 20kHz signal amplification is about 100 times. What is the difference between the rail to rail and low noise operational amplifier of single power supply and that of dual power supply? Please list some single power supply and dual power supply operational amplifiers that meet the requirements?

[answer] single power supply or dual power supply has no influence on the characteristics of op amp. For example, a nominal 5V op amp can also be powered by + / – 2.5V. For your requirements, to select the op amp whose gain bandwidth product is greater than or equal to 20kHz * 100, it is recommended to use the parameter search function provided on ADI website to select the appropriate op amp. The op amps that meet the bandwidth requirements include ad8672, ad8605, ad8519, etc.

Q32 [question] how to measure noise accurately without introducing measurement noise?

[answer] if you want to get the most accurate noise, use the root mean square measurement method. This method will take all the noise into account, but the disadvantage is that the measurement time is long and the amount of data is large.

Q33 〔 question: 〕 the power and noise matching problem of preamplifier and capacitive sensor, from the power matching needs high input impedance, from the noise matching needs and source impedance matching, how to compromise the two?

[answer: Nicolle] what kind of signal is the output of the sensor? Can you give an example to illustrate this problem.

Q34 [q] can you recommend a new RF low noise amplifier which is simple and adjustable (for example, adjust the magnification with potentiometer)?

[answer] I’m not sure what your frequency is. High frequency amplifiers (such as adl5521) usually have fixed gain. Of course, if the frequency is not too high (below a few hundred MHz), you can also consider ad8331. Potentiometer is not recommended. The effective bandwidth of potentiometer is generally not more than 1MHz

Q35 operational amplifier input stage has JFET, MOSFET and bipolar transistor, from the noise point of view, should choose which input operational amplifier?

[answer] generally, bipolar transistors have the lowest noise.

Q36 [question] I am looking for a design scheme of the flattest group delay filter. Can ADI provide such a scheme? Is there a low voltage (1.2V) high output current (20mA) op amp?

[answer] “flattest group delay filter” should refer to linear phase filter. You can use Bessel filter. ADI website has an online filter design tool, you can choose the response of the filter, including Bessel filter. ADI has no amp of 1.2V power supply, and the minimum power supply is 1.6V, which is op290.

Q37 〔 question: 〕 in the case of large impedance noise, is there any other way to adjust the design circuit and replace the impedance?

[answer] do you mean RF, RG in amplifier circuit? Because the greater the resistance, the greater the thermal noise, which cannot be changed. Therefore, we should choose the appropriate RF, RG resistance according to the specific situation.

Q38 〔 question: 〕 what kind of resistance can reduce resistance noise better?

[answer] it’s related to the materials of resistance, such as film resistance and carbon resistance. In addition, the larger the resistance, the greater the noise.

Q39 [q] how can the analog ground and digital ground of a / D converter be divided to better reduce noise?

[answer] there is no definite conclusion about whether analog land and digital land need to be separated. Some of them are divided into one ground plane, others are divided into two areas and connected with short lines under ad, and the methods are quite diverse. The main thing is to separate the analog and digital parts as far as possible and keep a certain distance. The analog signal and digital signal should not be crossed. The filter capacitor of the power supply should be close to the chip as far as possible.

Q40 [q] how to arrange the boards to minimize the noise?

[answer] this is not a simple sentence, such as complete ground, decoupling, division of analog and digital ground, etc. You can check the previous articles of elec666.

Q41 [Q:] what I want to ask is not about the noise of the amplifier. What I want to ask is how to eliminate the low frequency noise of hundreds of kHz from the power supply and the ground of the amplifier!? Or is there a good way to filter out the low frequency noise?

[answer] good earth, shielding and good power decoupling

Q42 [question] what should be paid attention to when amplifying tiny signals? What should be considered in device selection, layout and wiring? Which products of your company are suitable for this kind of application? thank you

[answer] if it’s a low-speed signal, you’d better analyze the noise source of the system and filter or pre process it according to the analysis in the seminar. The requirements for layout and wiring are relatively low.

Q43 [q] what kind of noise performance should we pay attention to in ultra-low or high temperature environment?

[answer] in this case, you need to pay attention to the performance of temperature drift.

Q44 〔 question: 〕 what kind of noise has the greatest influence in the amplifier?

[answer] there are external noise and internal noise. Internal noise comes from resistor, amplifier and ADC. The influence of these parameters on the amplifier noise is very important, which needs to be measured in specific applications.

Q45 [q] what is the principle of resistance noise in internal noise?

[answer] internal noise refers to the noise generated by the device itself in the signal link. So resistance noise is actually produced by RF, RG and other resistors in the amplifier circuit. The principle is the thermal noise of resistance.

Q46 [question] how can I reduce the noise when I use the OP4177 to play large and small signals? thank you

[answer] generally, the noise of the op amp itself is relatively small. We should pay attention to reduce the source noise at the input end of the op amp, such as resistance thermal noise and the output noise of the preamplifier (if there is a buffer). Before inputting the op amp, we should add a low-pass filter according to the bandwidth of the op amp.

Q47 [q] in the second or fourth-order low-pass filter composed of AD620 and OP07, ad7732 is used for analog-to-digital conversion, chip recommended by ad7732 data manual is used for reference voltage, sampling rate is within 250Hz, and ATmega128 is used for microprocessor controller. When the filtering frequency of low-pass filter is lower, the sampling rate is also lower, such as 10Hz sampling 4Hz In the case of filtering, the signal drift of the equipment sampling is very serious. What is the reason and how to eliminate it? The capacitance and resistance of the fourth-order or second-order filter are calculated with the help of microchip’s filtercad.

[answer] under normal circumstances, the lower the sampling rate, the smaller the root mean square noise and the higher the peak to peak resolution, that is, the code hopping situation will be improved.

Q48 [Q:] Hello, I am engaged in the development of testing instruments. I have been using the amplifier and AD converter of ADI company for a long time. My question is whether the input impedance of the instrument has an impact on the noise of the collected signal, how to affect it, and how to suppress it?

[answer] the input impedance will certainly not affect the noise of the signal, but the input impedance itself will bring additional noise and affect the accuracy.

Q49 [question] may I ask: through what scheme can the current noise of the amplifier be reduced? Is it suppression on the front end or filtering on the back end?

[answer] the current noise can be reduced by reducing the resistance in the feedback path.

Q50 〔 question: 〕 two of a four op amp are used to form a charge amplifier, one is used to form a differential amplifier, and the other is used to form a multi-channel feedback filter. When there is no signal input, there is a lot of noise. Single power supply. How to solve it?

[answer] can you send the model and circuit to us [email protected]

Q51 [q] what are the advantages of dual power supply and single power supply in front-end low-noise design?

It depends on the range of the input signal and the design of the point source of the system. If the input signal is bipolar and has a level below 0V, you need to use dual power supply or provide DC bias at ref pin.

Q52 how to eliminate transistor noise more effectively?

[answer: Neil] for a certain type of transistor, due to its design and process limitations, the noise characteristics have been determined, and it is impossible to change its noise performance until the user end.

Q53 [Q: uestcc3200732] Why does ad8221 have low frequency jitter of about 60uv in use?

[answer] what is the frequency of this low frequency? Generally, this kind of noise is likely to come from the surrounding environment, such as 50 Hz power line noise, or 60 Hz display refresh frequency noise. You can change the environment, change the power supply, and test to see if there is still this noise.

Q54 how to overcome the noise interference of amplifier in PCB design? thank you

[answer] when arranging the board, keep the amplifier away from high-power and high-frequency devices, divide analog ground and digital ground, decouple power ground, etc.

Q55 〔 question: 〕 what are the advantages of instrument amplifier in anti-interference ability compared with single ended amplifier?

[answer] compared with general operational amplifiers, instrument amplifiers have stronger common mode rejection ratio and are more suitable for amplifying tiny differential signals.

Q56 is there a preamplifier for audio? Please recommend one. Thank you!

Q57 〔 question: 〕 how to choose the right amplifier in the actual circuit design, for example, using a 12 bit AD converter to sample several hundred millivolts of signal. How to evaluate the influence of amplifier noise on measurement results?

[answer] first of all, we need to check the relevant operational amplifier data manual to find the voltage and current noise index (generally, the noise density is given from the input). According to the introduction in the slide, combined with the bandwidth and magnification in the specific application, the output noise is calculated. Of course, the thermal noise of the resistor used in the amplifier circuit should also be taken into account.

Q58 [q] which medical amplifier has a better noise figure?

Q59 [question] may I ask: what is the principle of noise produced by op amp? How can it be reduced?

[answer] as long as it is an active device, there will be irregular motion of electrons, which will produce noise. For a specific op amp, the noise generated by it is fixed, so it can only be filtered out of the bandwidth of interest by adding a filter.

Q60 〔 question: 〕 when single ended input, which is better, the reference point ground or the ground bias DC voltage?

[answer] it mainly depends on your specific application. If you want to get a reference point close to the ground, then ground it. If you need a biased output, you can connect the reference point to the bias voltage.

Q61 [question] how to use software to remove some internal noise and reduce the impact on ADC results?

[answer] in software, the method of averaging is often used, that is, sampling more points and averaging.

Q62 [q] what are the main causes of external noise? What factors and the surrounding environment affect it?

[answer] external resistance, temperature, power noise, high frequency signal coupling to signal input and output, etc.

Q63 〔 question: 〕 I would like to ask that the input of the instrument operational amplifier should be filtered to prevent RFI. If the resistance is 100k, the noise will reach 40nv. Is it unnecessary for the instrument operational amplifier to select 8nv noise?

[answer] in the system, all noises are added in the form of equal amplification. So the low noise of instrument amplifier is helpful to the noise performance of the whole system.

Q64 〔 question: 〕 the instrument operational amplifier has a large output of 50 Hz when there is no input signal. How to suppress it?

[answer] you can choose a better instrument amplifier, such as ad8221, or you can use hardware or software as notch filter, but the decoupling of instrument amplifier and the wiring of analog part are also very important.

Q65 [question] I’d like to ask a question about the current noise of operational amplifier circuit. Generally, due to the power consumption, we will choose large resistance in the design, but this will increase the current noise affected by current noise. How to solve this contradiction?

[answer] this is a problem often encountered in system design. There is no unified answer. You can only balance power consumption and noise according to your application requirements.

Q66 [q] what kind of amplifier is better for receiving laser signal?

[answer] the laser signal needs to be converted into the corresponding current or voltage signal. Please select the appropriate op amp model according to the current and voltage range.

Q67 [question] when is the right time to choose the instrument amplifier?

[answer] when the signal is very small, but the common mode interference is very large, such as ECG and EEG.

Q68 [q] just now, experts answered me that one is infinite and the other is infinitesimal. But when I was not designing, if I wanted to do impedance matching, how should I design it? Was it designed according to infinity and infinitesimal?

[answer] in general applications, it is not necessary to match the impedance of the op amp. For RF amplifiers, the chip is usually internally matched with 50 ohm, such as adl5521.

Q: when Q69 is used as a charge amplifier, is the input impedance an important factor?

[answer] the input impedance will affect the accuracy and time of integration. Simulation and tradeoff should be carried out according to the index of the system.

Q69 〔 question: 〕 for the normal operation of the amplifier circuit, how to give consideration to the index of offset voltage and bias current when selecting the op amp?

[answer] This mainly depends on the application. For example, to detect a relatively large current signal, use the sampling resistor to convert it into a voltage, and then amplify it. At this time, the bias current parameter is not very important, because it has a relatively small impact on the sampling resistor. If we look at the offset voltage, it is a fixed voltage, so the impact is not big. We can use it in the subsequent circuit In fact, the key to cancel is offset drift, which is its temperature drift characteristic. But generally, a larger offset will produce a larger temperature drift, so we usually choose an op amp with a smaller offset.

Q70 〔 question: 〕 how does the noise of operational amplifier affect the final measurement result in high precision measurement?

[answer] the premise to consider is that the noise in the system should be less than 1 LSB of the measurement resolution. From the perspective of operational amplifier, first of all, we need to check the relevant operational amplifier data manual to find the voltage and current noise indicators (generally, the noise density from the input side is given). Combined with the bandwidth and magnification in the specific application, the output noise is calculated. In addition, the thermal noise of the resistor used in the amplifier circuit should also be taken into account.

The feedback resistance of the q71 amplifier should not be too large. What factors should be taken into consideration for specific selection? Does the same problem exist in the selection of gain resistance of instrument amplifier?

[answer] it is mainly determined by the noise of the resistor and the product of the resistor and the current noise of the amplifier. The gain resistor of the amplifier will have the same problem, but it is not a feedback resistor, so the influence is not the biggest relative to the whole system.

Q 72 why is the amplifier with high input impedance and low output impedance?

[answer] the higher the input impedance of the amplifier, the smaller the current obtained from the signal source, the smaller the voltage drop on the internal resistance of the signal source, and the signal voltage is added to the input end of the amplifier with the minimum loss. Similarly, at the output end, the lower the output impedance is, the smaller the voltage loss added to the output internal resistance is, and the load will get as high output voltage as possible.

Q73 how is the feedback resistance calculated?

[answer] for operational amplifiers, the gain of the circuit is determined by the feedback resistance and the gain resistance. Relative to the amplitude of the signal, you can choose a resistance of the same size. If the resistance is too small, more noise will be introduced. If it is too small, the power consumption of the system will increase.

Q74 [question] does ADI have the relevant op amp calculation software?

[answer] welcome to pay attention to E-enthusiasts.

Q75 is circuit board design related to circuit noise? How to reduce the noise of the circuit?

[answer] to reduce the noise, the first thing to do is to choose low noise devices. From the perspective of circuit board wiring design, the main consideration is to avoid the interference of high noise circuits (such as digital circuits) on the board to low noise parts (such as analog signals).

Q76 [question] what are the ways to introduce external noise into the target system and how to suppress it? thank you!

[answer] it depends on the type of noise. For example, RFI filter and power filter capacitor can be added to RFI interference. If the environmental interference is serious, you can try to use shielding box and so on.

Q77 how to reduce the input noise of high speed ADC?

Q78 [question] I’m looking for a 50 Hz / 60 Hz notch filter. Can ADI provide a single chip solution that can switch between two devices without too many external resistors and capacitors. To cope with the over sampling between the instrument amplifier and ADC?

[answer] you can refer to the notch circuit designed by our ECG reference.

Q79 〔 question: 〕 in PCB wiring, the input ground wire is considered to surround the wiring to reduce the interference, but when the system ground wire is connected to the field ground, the output signal interference is more serious. I don’t know why, please ask the experts for guidance.

[answer] it may be that the ground noise on site is relatively high. Can you measure whether the ground noise on site is very high and whether the connecting wires are too long? You’d better reduce the length of the connecting line as far as possible, and connect a magnetic bead at the grounding point to suppress the high-frequency noise. If the noise on site is too large, you can consider isolating the system ground and the site ground during the isolation period. For example, adum series products.

Q80 [q] the gain of the first stage is the total gain. Sometimes it may be too large. Is there a maximum limit?

[answer] for the voltage feedback type, the configurable gain is limited by the gain bandwidth product of the op amp. For example, if the gain bandwidth product of a voltage feedback operational amplifier is 100MHz and the input signal frequency is 10MHz, the maximum configurable gain is 10. If you want to achieve greater gain, you need to consider using multi-stage amplification.

Q81 〔 question: 〕 in the project, switching power supply is used to supply power, and the output of amplification circuit contains high-frequency component, which affects the signal quality. How to deal with it? Consult experts

[answer] keep the switching power supply away from the analog circuit, and increase the filtering power for the ground output of the switching power supply. You can try different combinations of resistance capacitance and inductance capacitance. At the same time, the analog circuit should do a good job in decoupling. Generally, 0.1uF and 0.01uF capacitors should be connected in parallel in the power supply. You can try to change 0.1uF to 10uF or 100uF.

Q82 〔 question: 〕 for the thermal noise caused by the amplifier itself and the surrounding high-power radiator, what kind of suppression or noise reduction design has been adopted in the new design concept of ADI amplifier?

[answer] for the amplifier itself, the progress of technology and technology has made the noise of the amplifier lower and lower. As for the peripheral high-power devices, we can only reduce radiation and temperature drift.

Q83 [question] what’s the difference between comparator and op amp?

[answer] Operational amplifier is an electronic device designed to work under the condition of negative feedback. The key point of the design is to ensure the stability of this configuration. Other parameters such as slew rate and maximum bandwidth are the tradeoff between power consumption and architecture. On the contrary, the comparator is designed to work in an open-loop structure without negative feedback. These devices usually do not work through internal compensation, so they are fast Degree, i.e. propagation delay and slew rate (rise and fall time) are maximized on the comparator, and the overall gain is usually small.

Q84 [question] Hello, please give an example to illustrate that “focusing the total gain on the first stage is conducive to reducing noise”. thank you.

[answer] Taking two-stage amplification as an example, the first stage is G1, and the noise figure is F1, while the second stage is F2, so the total noise figure is as follows:

FTOTAL= F1 + （F2 -1）/G1

It can be seen that the larger the G1 is, the smaller the noise figure is.

Q85 [question:] Hello, teacher. I’m particularly confused about the common ground of logarithmic mode, such as the common ground of 0 ohm resistor, the common ground of magnetic beads, etc. What kind of common ground is better, and what specific attention do you have? thank you!

[answer] the equivalent circuit of the magnetic bead is equivalent to a band stop wave limiter. It can only significantly suppress the noise at a certain frequency point. When using it, it is necessary to estimate the noise frequency in advance so as to select the appropriate model. 0 ohm resistance is equivalent to a very narrow current path, which can effectively limit the loop current and suppress the noise. The resistance has attenuation effect in all frequency bands.

Q86 〔 question: 〕 what are the main indexes of noise parameters and what do they represent?

[answer] current noise and voltage noise should be considered in operational amplifier. In the data book, it is usually given in the form of noise density.

Q87 〔 question: 〕 in our application, we use multi turn potentiometer to measure the angle position, and use 5V switching power supply to supply power. The measured voltage is transmitted to MCU for AD sampling. The number of sampling bits is 10bit, but when the measuring object is not moving, the sampling value always jumps by 2-4 units. When the voltage is measured with a multimeter, there is almost no change. How can we solve the problem of jumps in the circuit?

[answer] this state is normal. If no other noise is introduced into the input (which can meet the accuracy requirements of ADC), the ADC itself will also have the problem of significant digits. The instability of the last bit does not mean that the ADC performance does not meet the index. You can use this jump output signal to calculate the RMS of ADC Noise, you can also input a full range of positive selected wave to test its dynamic performance.

Q88 〔 question: 〕 in bioelectrical signals, as long as the signal passband of 10K and the minimum signal gain of 1uv are 100, is there a better amplifier?

You can try ad8221, which has a noise density of 8nv / sqrthz.

Q89 is resistance noise not considered in general circuits?

Q90 [question] can you explain specifically the meaning of “concentrating the gain on the first stage amplifier is better than distributing the gain to the second stage amplifier to reduce the noise”?

[answer] the gain of the amplifier will increase the input noise. If the gain is concentrated in the first stage, the noise introduced is only that of the previous stage. If the gain is distributed in two amplifiers, the noise will come from two stages and be amplified at the same time.

Q91 〔 question: 〕 using ad8551, the output resistance of the sensor is about 80K, the output signal is UV, and the amplification is 100 times, is that ok?

[answer] because the impedance of your sensor is very large, the noise introduced by it is very large, which is larger than your input signal. Even if the ad8551 can achieve relatively low noise, it can’t.

Q92 〔 question: 〕 the internal resistance of the sensor is 100k and the output is UV. If you want to amplify it 100 times and the bandwidth is 10Hz, how to design the amplifying circuit and how to avoid noise?

[answer] due to the large internal resistance of the sensor, you’d better not use the single ended method to connect the circuit. For low noise measurement applications where the sensor is located outside the board, you’d better use differential input amplifiers such as instrument amplifiers.

Q93 what is the source of noise? How can it be eliminated?

[answer: ADI expert] noise is divided into internal noise and external noise. External noise can be reduced by filtering, while internal noise, that is, the noise of devices, cannot be eliminated.

Q94 is there a general noise unit?

[answer] in general, you can use the formula of 1 K Ω → 4 NV / √ (Hz) to evaluate the noise.

Q95 [question] now many MCU products come with AD peripherals, but there are many ad conversion chips in the market, such as your company’s many high-precision conversion chip products. I have compared them. Special ad conversion chips can achieve very high precision and sampling speed, such as 16bit or more, which often has a sampling rate of 100 megabits per second, while MCU’s ad rarely has 14bit Above all, the sampling speed is also slow, but these two points are not absolute. The performance indicators of the newly launched MCU products in these two aspects are also constantly improving, and compared with the use of special ad conversion chip, the use of its own ad peripherals saves the trouble of interface with MCU. Excuse me, special ad conversion chip In addition to sampling accuracy and sampling accuracy, what other advantages does the conversion chip have?

[answer] precision and sampling speed are the main advantages of single chip ADC. Comparing our aduc7 and aduc8 series single chip microcomputers with corresponding precision ADC, their indexes are almost the same, but they are more flexible for sampling rate, range of input signal, number of channels and other indexes, so the specific selection of ADC depends on your application requirements.

Q96 〔 question: 〕 how to deal with the noise of the circuit in the design of MP4 on board? Is the circuit safer?

[answer] for the MP4 circuit, the main thing to deal with is the noise caused by electromagnetic interference. You need to pay attention to the distribution of each functional module on the PCB. You can take some methods to remove the interference, such as power decoupling, shielding and filtering.

Q97 how to lower the offset in amplifier?

[answer] there are very few op amps that can adjust the offset externally. Now the offset of op amps is usually very small. If you have to adjust it, you can remove the offset in the digital field.

Q98 how to reduce the noise in input stage?

[answer] the smaller the resistance of the input electrode and the noise of the input electrode, the smaller the noise itself will be. At the same time, the current noise and voltage noise of the op amp should be as low as possible.

Self excitation and noise suppression of Q99 amplifier

[answer] the problem of self excitation should be solved at the beginning of the circuit design, so that the phase margin of the circuit is at least 45 degrees. Firstly, low noise op amp should be selected. Secondly, a filter can be added to the output to remove the noise.

Q100 [Q:] selecting high-quality resistors can effectively suppress current noise, but the cost is relatively high. How to balance the selection of resistors between cost and quality?

[answer] it depends on the designer’s consideration, that is, the choice between product performance and cost.

Q101 [question] please focus on how to deal with the noise caused by the components in the design of micro signal conditioning, and how to deal with the power supply noise better in a circuit.

[answer] select low noise components to reduce noise from the source. The power supply can be realized by LDO. If DC-DC is used, the output of DC-DC needs to be filtered by multi pole LC.

Q102 [question] in the case of low frequency measurement, should analog ground and digital ground be separated? Or a common land? thank you

[answer] the analog and digital circuits should be separated, and finally connected together at the ADC. For example, you can see the chip information of sigma delta ADC such as AD7705 or PCB diagram of evaluation board

Q103 [question] may I ask what I said just now: 1 kiloohm corresponds to 4nv / root hz9 kiloohm corresponds to 12 times noise?

[answer] the noise corresponding to 9K Ω is the product of 4nv / Hz and 3.

Q104 how to reduce the internal noise and external noise of the device?

[answer] first of all, what are the external and internal noise sources? Then according to the specific noise source to take noise reduction treatment. For example, if your current noise is too large, you can reduce the resistance value.

Q105 〔 question: 〕 explain in detail some matters needing attention when single power supply?

[answer] whether the range of input signal is within the range of single power supply.

Q106 how to provide the bandwidth of amplifier?

[answer] Sometimes the requirement of the best bandwidth performance may conflict with the requirement of the best noise performance. For bandwidth, we want each gain stage to have an approximate gain, while for noise, we want the first stage to have all gains. The front stage applies as much gain as possible

Q107 [Q:] Hello, why are many noises expressed in the form of “MV / (root Hz)”. thank you!

[answer] it’s just one of the methods to describe the noise characteristics: spectrum noise density.

Q108 [question] in the application of in-phase amplifier, the bias resistance from + to ground becomes the input impedance of the amplifier. In the application requiring high input impedance, the thermal noise of the resistance and the noise generated by the noise current of the amplifier can not be ignored. How to balance? But in practical application, the resistance can not be omitted because the bias voltage needs to be provided?

[answer] the trade-off depends on whether high input impedance or noise is more important to your system.

Q109 [question] what should I know about the noise of operational amplifiers?

A: first of all, we must pay attention to the difference between the noise generated by the operational amplifier and its components and the noise generated by the external interference or useless signal and the voltage or current noise generated at one end of the amplifier or the noise generated by its related circuits.

Interference can be expressed as spike, step, sine wave or random noise, and interference sources exist everywhere: mechanical, close to the power line, RF transmitter and receiver, computer and internal circuits of the same equipment (for example, digital circuit or switching power supply). It’s a big problem to recognize interference, prevent interference from appearing near your circuit, know how it comes in and how to eliminate it or find a way to deal with it.

If all the interferences are eliminated, there will be random noise related to the op amp and its resistive circuit. It constitutes the ultimate limitation of operational amplifier’s control resolution.

A: the output noise of operational amplifier is measured by voltage noise. But both voltage noise source and current noise source can produce noise. All internal noise sources of operational amplifiers are usually converted to the input, that is, they are regarded as random noise generators which are not related or independent to the two inputs of ideal noiseless amplifiers in series or in parallel. We think that there are three basic sources of op amp noise

(1) A noise voltage generator (similar to offset voltage, usually in series with in-phase inputs).

(2) Two noise current generators (similar to bias current, discharging current through two differential inputs).

(3) Resistor noise generators (if there are any resistors in the op amp circuit, they also produce noise. This kind of noise can be regarded as coming from current source or voltage source, which is very common in a given circuit.

The voltage noise of operational amplifier can be as low as 3 NV / Hz. Voltage noise is usually emphasized as a technical index, but in the case of high impedance, current noise is often the limiting factor of system noise performance. This situation is similar to offset. Offset voltage is often responsible for output offset, but bias current is really responsible. The voltage noise of bipolar operational amplifier is lower than that of traditional FET operational amplifier. Although it has this advantage, the current noise is still relatively large. The current FET operational amplifier can keep low current noise and achieve the voltage noise level of bipolar operational amplifier.

Q111 Q: what are the design skills of low noise system?

A: the first trick of low noise system design is to get as much gain as possible in the front-end application. Figure 4 shows two examples of an amplifier front-end with a gain of 10. It can be seen that it is much better to apply all the gains to the first stage than to distribute the gains to the two stages.

Note that sometimes the requirements for optimal bandwidth performance may conflict with the requirements for optimal noise performance. For bandwidth, we want each gain stage to have an approximate gain, while for noise, we want the first stage to have all the gains.

Figure 4

The second trick is to pay attention to source impedance. There are two reasons for this: first, the larger the source impedance, the greater the system noise; second, the amplifier must match the source impedance well. If the source impedance is higher, the current noise characteristic may be more important than the voltage noise characteristic.

The third trick is to pay attention to the feedback resistance. If you choose an ultra-low noise operational amplifier and use a large feedback resistance, it is impossible to realize a low-noise circuit. In the in-phase (Fig. 5) or inverse configuration, pay attention to that the feedback resistance is equivalent to the noise source equivalent to the output. The other resistors are equivalent to the voltage source at the input, or more accurately, the voltage source at the input in reverse configuration. As mentioned earlier, when designing low-noise systems, the first stage application has high gain, in which case RG noise is dominant.

Editor in charge: PJ