Circuit analysis: "offset calculation" of differential operational amplifier circuit

The differential operational amplifier circuit effectively suppresses the common mode signal, but only amplifies the differential signal, so it is widely used.

Circuit configuration of differential circuit


Figure 1 Differential circuit

Target processing voltage: It is the acquisition and processing voltage, such as the acquisition and processing of bus voltage in the system, and the acquisition and processing of AC voltage.
Differential non-inverting/inverting voltage divider resistance: In order to obtain a voltage suitable for op amp processing, the high-voltage signal needs to be divided into voltage processing. As shown in Figure 1, the voltages at both ends of V1 and v2 are divided to obtain a voltage suitable for op amp processing. Vin+ and Vin-.

Differential amplifier circuit:
Feedback, for the operational amplifier circuit, the operational amplifier works in the linear region, so here must be negative feedback, there is no feedback (open loop) or positive feedback, that is the comparator circuit rather than the amplifier circuit, this time the operational amplifier is working in saturation Area or called non-linear working area, because of saturation, the output is the amplitude of the power supply voltage.

Figure 2 is an op amp circuit with positive feedback. It cannot be called an op amp circuit here, because the ideal open-loop magnification of the op amp is infinite, of course, it is impossible to be infinite in practice, so the following structure is the hysteresis voltage comparison The operational amplifier works in the non-linear region or saturation region.


figure 2

Figure 3 is still the voltage comparator structure. As mentioned above, the operational amplifier has a large open loop gain without negative feedback. It works like a non-linear region and is used as a voltage comparator.


image 3

For operational amplifiers, the feedback resistance from the output to the inverting terminal "-" is negative feedback. Of course, when the output signal does not exceed the power supply voltage (note: the energy source of all signals is the power supply, of course, the output cannot exceed the power supply amplitude). The function of the signal is to amplify the signal; when connected to the non-inverting terminal "+", it is positive feedback, and the circuit function is a voltage comparator. Of course, in practice, we do not advocate using op amps as voltage comparators, but use dedicated comparators, such as LM339, LM393, LM211, etc., because the working status of the internal components of the comparator and the op amp is still different in practice. of.

The comparator is connected to the current-limiting resistor—"R74, R77". This is because when the amplitude is switched, the comparator charges and discharges the subsequent capacitive load with a fast rising or falling edge. This charging and discharging current does come from this active device. —Comparator, so the purpose of adding a current-limiting resistor is to prevent current impact.

RC filter: It can be adjusted as appropriate to prevent signal distortion problems such as output overshoot
Calculation of differential input voltage
In the circuit of Fig. 4, in order to facilitate the calculation, we give each resistance value.
Another feature of the differential circuit is symmetry, R40=R56 and R47=R55, the resistance of the two branches of the differential voltage divider is also equal.


Figure 4

How are the values of Vin+ and Vin- calculated?
We first get it through cumbersome calculations, and then simplify the calculations.

First of all, the non-inverting terminal 5 pins and the inverting terminal 6 pins of the op amp are obtained by using "virtual short", where the coefficient 6 refers to 6 100k resistors, which is convenient to simplify the formula:



Then Vin+ is obtained through the partial pressure relationship:



Vin- is obtained through the partial pressure relationship again:





Then we get the value of Vin+ minus Vin-.

In fact, there is a simple way to get the value of Vin+ minus Vin-. Using the virtual short characteristic of the op amp, the circuit can be equivalent to:


Figure 5


Figure 6

So it is very easy to calculate the value of Vin+ minus Vin-, it is just a simple voltage divider circuit, calculated as follows:





The differential voltage input value is 0.84V.

Calculation of differential amplifier circuit


Figure 7

The calculation formula derivation still follows the virtual short and virtual break characteristics of the op amp. When R56=R40, R47=R55, the difference calculation can be simplified as:



In the actual application circuit, in order to simplify the calculation, we also use the simplest method to calculate. The circuit that is often used is also the above circuit, so that the resistance is equal to the relationship, and the calculation is simplified.

"Offset calculation" of amplifier circuit
Why should the output voltage be offset? This is because when collecting negative values, our sampling chip and MCU hardly support negative value sampling, you have to offset so that the output is always positive.

The offset circuit, as shown in Figure 8, where the original non-inverting end resistance is grounded to GND, we connect a voltage value, which is usually called an offset voltage. So what is the final expression?


Figure 8

Through the superposition theorem, we finally get:



The formula here is established, to ensure that R64=R72, R73=R57, then the final offset formula is to add a voltage offset 2.5V_Ref on the original basis:



As long as the appropriate offset is selected according to the actual application, the output will always be a positive value.


Picture 9

For example, in the circuit of Figure 9, the input voltage becomes -100V, then the final output voltage is:

In this way, the negative voltage is shifted to a positive voltage, and the processor meets the processing requirements of the processor. The offset circuit is widely used in collecting, for example, alternating current and control circuits with negative direct current voltage.

Please read the Chinese version for the details and the pictures.