Author: Hooman Hashemi, product application engineer, ADI company

brief introduction

When connected to the sensor, the instrument amplifier (IA) is powerful and versatile, but there are some limitations that will hinder the design of variable gain IA or programmable gain instrument amplifier (pgia). In some literatures, the latter is also called software programmable gain amplifier (SPGA). We need this kind of pgia because it is often required to adjust the circuit according to various sensors or environmental conditions. With fixed gain, the system designer may have to deal with poor SNR, which will reduce the accuracy. My colleague published “analog dialogue” article “programmable gain instrument amplifier: finding the right amplifier”, which discusses a variety of technologies that help to create a precise and stable pgia. The article points out the possible defects of this design and shows a comprehensive survey of available solutions and technologies. In this article, I will introduce another tool and method to promote this work. I will introduce each design step one by one, so that you can quickly master the external component values required to create a precision pgia using the newly released instrument amplifier.

A new architecture of instrument amplifier

The common instrument amplifier architecture is shown in Figure 1.

Figure 1. Classic instrument amplifier

The gain is set by the value of the external resistor RG. To create pgia using such devices, simply switch the value of RG. This switching is usually done using analog switches or multiplexers. However, some non ideal behaviors of analog switches complicate this task – such as the on resistance of the switch, the channel capacitance, and the change of the channel resistance with the applied voltage.

Figure 2 shows a modified version based on the standard instrument amplifier structure. Note how the RG pin is decomposed into ± RG, s and ± RG, F, which are led out separately and configured from the outside of the device package.

Figure 2. Lt6372-1 architecture allows some ia internal nodes to be configured

The architecture shown in Figure 2 has an important practical feature: the instrument amplifier can be configured to switch between several different gain values, while minimizing the gain error caused by the switching resistance. This feature can be used to create pgia.

As mentioned above, any resistance programmable instrument amplifier can change its gain by switching the value of the gain resistance. However, this approach has obvious disadvantages, such as:

  • The nominal value of switch on resistance (RON) and its variation will cause large gain error.
  • Since the required switch Ron value is low, a high gain value may not be achieved.
  • Switching nonlinearity will cause signal distortion. This is because the signal current flows directly through Ron, so any change in its value with voltage will cause distortion.

As shown in Figure 3, when the lt6372-1 is configured as pgia, these problems can be alleviated because the RG, F and RG, s pins are led out separately. In this schematic diagram, the signal generated by Wheatstone bridge (composed of R5 to R8) is amplified to provide four possible gain values, which can be selected by the user according to the selected SW1 switch position. Using the lt6372 series pin arrangement, we can create a pgia to obtain the required gain value by changing the RF / RG ratio.

In addition, U1 and U2 analog switches Ron as gain error sources are minimized because it can be connected in series with the inverting port of the input stage and its feedback resistance. After this configuration, Ron only accounts for a small part of the total internal 12.1 K Ω feedback resistance, so it has little effect on gain error and drift. Similarly, the Ron value accounts for only a small part of the total feedback resistance, and its value has little effect with the change of voltage, so the distortion caused by switching nonlinearity can be minimized. In addition, the input stage of this device is composed of current feedback amplifier (CFA) architecture. Compared with the traditional voltage feedback amplifier, it allows less bandwidth or speed change when the gain changes. 1 the combination of all these factors allows us to use low-cost external analog switches to create a precision pgia with precision gain stepping.

1 the closed-loop bandwidth of CFA is inversely proportional to the value of RF, while the bandwidth of traditional voltage feedback architecture is inversely proportional to the gain (RF / RG).

Figure 3. Lt6372-1 pgia bridge interface provides four gain settings

Figure 4 shows a simplified diagram of pgia, showing how the different taps of ladder resistance (realized by a total of 8 analog switches, shorting 2 at a time to set the gain) configure the circuit. In this figure, two switchpacks are described by one of four possible gain values; – RG, s and + RG, s pins are shorted to Rf3 / rf4 junction.

Figure 4. Block diagram of lt6372-1 and simplified external connection of pgia (gain switch not shown)

Design steps for calculating the gain of external resistance

Figure 3 shows the complete pgia configuration, including the required switches, which can adapt to any size of gain range. There are four possible gain values, but you can increase this value by adding more switches to the design. As mentioned earlier, the feature of allowing the configuration of RG, F and RG, s pins allows us to increase the gain by increasing RF and reduce RG to reduce the gain, so as to create a multifunctional pgia. In order to calculate the gain, we can count the feedback resistance as the internal 12.1 K Ω adjustment resistance plus other resistors connected in series with RG and F on the RG, f to RG and s ports. In contrast, the gain setting resistance is the total resistance between + RG, s and – RG, S. To sum up:

RF = 12.1 K Ω + resistance between RG, F and RG, s above each of the two input amplifiers

RG = + resistance between RG, s and – RG, s

In this configuration, the possible range of gain is 1 V / V to 1000 V / v. When the switches on U1 and U2 switches are set to short circuit pins S3 and D3, the corresponding RF and RG values and the generated gain are as follows:

RF = 12.1 kΩ + 11 kΩ + 1.1 kΩ = 24.1 kΩ

RG = 73.2 Ω + 97.6 Ω + 73.2 Ω = 244 Ω

G = 1+ 2RF/RG = 1 + 2 × 24.1 kΩ/244 Ω = 199 V/V

It is easy to see that determining which value to use for external resistance is an iterative and interrelated process, and the possible gain values interact to affect the selection of resistance to use. For ease of reference, table 1 lists some common gain value composition values, but there may be many other gain combinations (g).

Table 1. Composition values of some pgia gain combinations

To determine the value of pgia

We can use the formula in equation 1 to sequentially calculate the values of individual resistors in the gain network. The way in which the resistance is determined by this equation is shown in Figure 3, and case 2 in Table 1 (gains of 2, 20, 200 and 500 V / V) is used as an example. Feedback resistance and gain setting resistance are interactive; Therefore, the formula must be a series in which the current item depends on the previous item. The calculation formula is as follows:

Here are some definitions:

RF1 = 12.1 K Ω (built-in resistance of lt6372-1)

M: Number of gains (4 for this circuit)

GI: gain example (in this example, G1 – G4 are 2, 20, 200 or 500 V / V respectively)

i: Varies from 1 to (m-1) to calculate RFI + 1

Equation 1 can be used to calculate the feedback resistance required for any gain combination. A dummy variable (J) acts as a counter to maintain the continuous total number of previous feedback resistors.

U before calculation, it is recommended to draw a resistance network similar to the network shown in Fig. 3. There are (2) in this network × M) – 1 resistor, where M = number of gains. In this example, M = 4, so the resistance string will contain 7 resistors. You need to find the value of equation 1 for I = 1 → (m – 1).

G1 = 2,G2 = 20,G3 = 200,G4 = 500 V/V

According to equation 2:

Find the value of equation 1 in an iterative manner according to I = 1 → (m-1)

The center resistance Rg can then be calculated using the following equation:

After the last step of calculation, all four resistance values in Table 1 have been calculated, and the design calculation process is completed.

Measured performance diagram

The following figures show the performance that can be achieved with this pgia configuration:

Figure 5. Pgia large signal frequency response

Figure 6. Relationship between pgia CMRR and frequency

The switching capacitor of adg444 causes some obvious peaks in the small signal frequency response at the lowest gain setting (G1 = 2 V / V) (see Figure 7). This phenomenon only occurs when a lower gain setting is adopted, because the bandwidth of the lt6372-1 is extended enough to be affected by the PF capacitance of the switch. Methods to solve this side effect include selecting switches with lower capacitance (e.g. adg611 / adg612 / adg613 with 5 pf capacitance), or limiting the minimum gain setting of pgia.

Figure 7. Pgia small signal low gain peaking

conclusion

This paper introduces how to use the pin arrangement of the newly released lt6372 series devices to add gain selection function to the instrument amplifier. The characteristics of this pgia are analyzed, and its design steps and performance measurements are described in detail. The lt6372-1 has high linearity and provides precise DC specifications and performance, so it is very suitable for such solutions.

Introduction to the author

Hooman Hashemi joined ADI in March 2018 and engaged in the application development of new product index testing and displaying product characteristics and uses. Hooman previously worked at Texas Instruments for 22 years as an application engineer, focusing on the high-speed product series. He graduated from Santa Clara University in August 1989 with a master’s degree in electrical engineering; He graduated from San Jose State University with a bachelor’s degree in electrical engineering in December 1983.

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