In the use of op-amps, the most elementary hardware designer’s idea is to have only one parameter, the gain multiplier. Of course this is the basic capability of an op-amp, but obviously knowing only the amplification is not enough to say that you know how to use an op-amp. Perhaps you know about op amps, you know that differential amplification amplifies differential signals, and you even know about the common mode rejection ratio. So, today let’s talk about what common mode level means for an op-amp and for a signal system.
Here is a typical signal transmission system.
We can see that the important point in this system is the reference potential of the subsystem, the common mode level of the op-amp is also a matter of reference voltage. The common-mode voltage of the op-amp is defined as
The formula Vcm=(Va+Vb)/2,can be interpreted as the middle potential point of the signal at the differential ends of the differential op-amp.
So what is the significance of the common mode level of the op-amp? This can be traced back to the amplification circuit of a triode: the
Transistor amplification requires the configuration of a static operating point, which means that the bias voltage needs to be set to keep the amplifier in an amplified state rather than saturated or cut-off. Therefore we need to provide a bias voltage for AC signal amplification, and for dual-supply op amps, the common mode voltage 0V is also its common mode voltage. Therefore, we can see that we do not need to deal with the common mode voltage amplification, the role of the common mode level is to provide a “platform”.
Common mode voltage and noise.
In fact, the first diagram shows the problem of common mode voltage interference. Differences in reference potentials can lead to differences in common mode levels, and for signal detection systems, the sources of interference are capacitive coupling from nearby electric fields, inductive coupling from nearby magnetic fields, and electromagnetic coupling from space radiation signals. The diagram below shows several characteristics of interference in signal cables, and we can use twisted pairs and shielding to deal with some of the interference.
Here is how to suppress common mode interference for transmission lines: using suppression devices as well as isolation.
For example, when we deal with AC signal amplification, we often isolate the DC level by means of capacitors to avoid affecting the common mode input of the rear stage.
Common-mode interference from reference potentials can also be isolated by means of power supplies, creating two relatively independent power supply systems.
Finally, we introduce the detection system for high common mode voltage, which is more often used in BMS. We need to monitor the voltage of series-parallel connected battery packs, and there will be high common mode voltage up to hundreds of V appear. The common mode voltage input of the general op-amp is also just smaller than the power supply.
A dedicated differential detection op-amp is described here, using the AD629 as an example.
The internal structure of the op-amp is relatively simple: the
It seems that we can actually do it directly with ordinary op-amps, but in fact we can see that for hundreds of V common mode signals for differential detection, if the feedback resistor does not match, it will lead to common mode to differential mode output, which greatly affects the output voltage, in fact, it is the problem of common mode rejection. In particular, the detection current is very small in the case of output voltage in mv, and the integrated chip internal resistance because of the semiconductor manufacturing process can maintain a good match.
The following graph shows the common mode rejection performance.
Summary: To put it simply, the common-mode voltage of an op-amp is just a form of op-amp input, what is important is that we need to understand its impact and role.
