In the use of op amps, the idea of ​​the most elementary hardware designers is that there is only one parameter, the gain multiple. Of course, this is the basic capability of an op amp, but obviously only knowing the magnification factor cannot be said to be able to use an op amp. Maybe you know about op amps, you know that differential amplifiers amplify differential signals, and you even know about common-mode rejection ratios. So, let’s talk about the significance of common mode level for op amp and signal system today.

Here is a typical signaling system:

We can see that the important point in this system is the reference potential of the subsystem, and the common-mode level of the op amp is also a problem of the reference voltage. The common-mode voltage of an op amp is defined as:

The calculation formula Vcm=(Va+Vb)/2 can be understood as the middle potential point of the differential two-terminal signal of the differential op amp.

So what is the meaning of the common mode level of the op amp? This can be traced back to the amplifier circuit of the triode:

The amplification of the transistor needs to configure the static operating point, that is, it is necessary to set the bias voltage to make the amplifier in the amplified state instead of the saturated or cut-off state. Therefore, we need to provide a bias voltage for AC signal amplification, and for a dual-supply operational amplifier, the common-mode voltage 0V is also its common-mode voltage. Therefore, we can see that we do not need to amplify the common-mode voltage, and the function of the common-mode level is to provide a “platform”.

Common Mode Voltage and Noise:

In fact, the first picture reflects the interference problem of the common mode voltage. The difference of the reference potential will lead to the difference of the common mode level. For the signal detection system, the source of the interference is the capacitive coupling from the nearby electric field and the inductance from the nearby magnetic field. Coupling, and electromagnetic coupling of space radiation signals. The following figure shows several interference characteristics of signal cables. We can use twisted pair and shielding to deal with some interference.

The following are ways to suppress common-mode interference on transmission lines: use suppression devices and isolation.

For example, when we deal with AC signal amplification, we often isolate the DC level through capacitors to avoid affecting the common-mode input of the subsequent stage.

The common-mode interference caused by the reference potential can also be isolated by means of power supply to form two relatively independent power supply systems.

Finally, I will introduce the detection system of high common-mode voltage, which is widely used in BMS. We need to monitor the voltage of battery packs connected in series and parallel, and there will be high common-mode voltages reaching hundreds of V. The common-mode voltage input of a general op amp is smaller than that of the power supply.

Here is a special differential detection op amp, taking AD629 as an example:

The internal structure of the op amp is relatively simple:

It seems that we can actually do it directly with ordinary op amps, but in fact we can see that differential detection is performed on common-mode signals of hundreds of V. If the feedback resistors do not match, it will cause common-mode to differential-mode output, which is extremely large. Affecting the output voltage is actually a problem of common-mode rejection. Especially when the detection current is very small, the output voltage is in mv, and the resistance inside the integrated chip can maintain a good matching degree because of the semiconductor manufacturing process.

The following figure is the common mode rejection performance:

Summary: It is simple to say, the common mode voltage of the op amp is just a form of the input of the op amp, the important thing is that we need to understand its influence and function.

Reviewing editor: Tang Zihong
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