Here are some questions about the differential signal amplifier circuit

西里古1992

Here are some questions about the differential signal amplifier circuit [Copy link]

 

Because the op amp is powered by a single power supply, a bias voltage VCM1 of tens of mV must be added (why?). I see that the classic differential amplifier circuits are all grounded? Also, I used a breadboard to build a peripheral circuit, and 5V was divided by R1 and R2 to simulate an Io+Io- signal. The master said that because the input impedance of the differential signal input end is small, the resistance values of R1 and R2 cannot be too large, tens of Ω level instead of kΩ level. I don’t understand this small input impedance and small resistance value selection? ? ?

LeoMe

Regarding single-power op amps, we have to mention the concept of "rail-to-rail". The bias voltage should be used to eliminate the problem that the output is not 0V when the input is 0V. In this way, after the circuit is built, the output can be adjusted to 0V when the input is 0V by adjusting VCM1. Is the second sentence written in reverse? According to my understanding, R52 and 57 should be larger. Let's see if there are any experts who continue to reply. Pay attention

maychang

"Because the op amp is powered by a single power supply, a bias voltage VCM1 of several tens of mV must be added (why?)" "Bias voltage" is probably a mistake for "bias voltage".

maychang

"Because the op amp is powered by a single power supply, a bias voltage VCM1 of several tens of mV must be added (why?)" The 5V voltage is divided by R1 and R2 to give a bias voltage, and its value is obviously 5V multiplied by the voltage division ratio of R1 and R2. As for the Vcm in the figure, I don't know what its purpose is.

maychang

R1 and R2 cannot be too large, because the op amp input current will cause shunting and change the R1R2 voltage divider ratio. But R1R2 does not need to be as small as tens of ohms, which will consume too much power. The principle of R1R2 value is to be much smaller than the op amp input impedance, and it is reasonable to use thousands of ohms.

maychang

"I built a peripheral circuit with a breadboard, and 5V was divided by R1 and R2 to simulate an Io+Io- signal." This kind of use requires that the op amp input is allowed to reach the negative end of the power supply.

西里古1992

LeoMe posted on 2019-5-29 08:59 Regarding single-power op amps, we have to mention the concept of "rail-to-rail". The bias voltage should be used to eliminate the problem that the output is not 0V when the input is 0V...

The resistance values of R1 and R2 should not be too large, not R57R52

西里古1992

maychang posted on 2019-5-29 09:39 R1 and R2 cannot be too large, because the op amp input current will cause shunting and change the R1R2 voltage divider ratio. But R1R2 does not need to be as small as tens of ohms, as that would consume too much power...

Teacher, are the op amp input impedances R52 and R57? In that diagram, they are already tens to tens of kilo-ohms.

maychang

Sirigu1992 posted on 2019-6-3 10:25 Teacher, are the op amp input impedances R52 and R57? In that diagram, they are already tens to tens of kilo-ohms

The op amp input resistances are R52 and R57. "It doesn't have to be as small as tens of ohms" refers to R1 and R2.

LeoMe

Sirigu1992 posted on 2019-6-3 10:23 The resistance values of R1 and R2 should not be too large, not R57R52

R57 and R52 are the input resistors of the op amp (let's call them that). Because the negative end of the differential is grounded, it is actually a common-phase amplifier with reference to the ground plane. One difference between common-phase amplifier and inverting amplifier is that the input impedance of the common-phase amplifier is determined by the input resistor R52, while the input impedance of the inverting amplifier is determined by the op amp itself, so the input impedance of the inverting amplifier is generally large.

Therefore, R52 may not be able to reach tens or hundreds of M due to thermal noise or other reasons. Therefore, the output impedance of R1 R2 as the signal output terminal cannot be too large.

In layman's terms (not necessarily rigorous), R1 R2 is used to divide the voltage, and the voltage division is accurate only when all the current flows through R1 R2. Now R1 R2 is much larger than R52, and as a result, a lot of current flows into the op amp's non-inverting terminal from R52, and less current flows through R2, so the voltage division is inaccurate.

For a circuit like this that takes the voltage divider value, if there is a requirement for accuracy, you can consider making a follower after the voltage divider to at least ensure that the input impedance of the signal amplification part is high enough, and then talk about amplification and filtering later.

The above humble opinion

西里古1992

LeoMe posted on 2019-6-13 09:00 R57 and R52 are op amp input resistors (let's call them that), because the negative end of the differential is grounded, it is actually a common-mode amplifier referenced to the ground plane...

Yes, yes