Useful Information | Analyzing the principles of differential circuits, why does the output voltage need to be offset?

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

Figure 1 Differential circuit

Target processing voltage: It is the collected and processed voltage, such as the collection and processing of bus voltage and AC voltage in the system.

Differential in-phase/anti-phase voltage-dividing resistors: In order to obtain a voltage suitable for the op amp to process, the high-voltage signal needs to be divided. For example, the voltages at both ends of V1 and V2 in Figure 1 are divided to obtain voltages Vin+ and Vin- suitable for the op amp to process.

Figure 2 is an op amp circuit with positive feedback. It cannot be called an operational amplifier circuit here because the open-loop gain of the op amp is ideally infinite. Of course, it is impossible to be infinite in practice. Therefore, the following structure is a hysteresis voltage comparator, and the op amp works in the nonlinear region or saturation region.

Figure 2 Op amp circuit with positive feedback

Figure 3 is still a voltage comparator structure. As mentioned above, the op amp has a large open-loop gain and no negative feedback. It works in the nonlinear region and is used as a voltage comparator.

Figure 3 Voltage comparator structure

For an operational amplifier, if the feedback resistor is connected from the output to the inverting terminal "-", it 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, and the output cannot exceed the power supply amplitude), the function achieved is to amplify the signal. If it is 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 recommend using op amps as voltage comparators, but instead choose dedicated comparators, such as LM339, LM393, LM211, etc., because the working states of the internal devices of comparators and op amps are actually different.

The comparator is connected to current limiting resistors - "R74, R77". This is because when the comparator switches the amplitude, it quickly rises or falls to charge and discharge the subsequent capacitive load. This charging and discharging current comes from the active device - the comparator. Therefore, the purpose of adding current limiting resistors is to prevent current shock.

RC filtering: It can be adjusted as appropriate to prevent signal distortion problems such as output overshoot.

Another feature of the differential circuit is symmetry. R40=R56 and R47=R55, and the resistances of the two branches of the differential voltage divider are also equal.

Figure 4

How are the values ​​of Vin+ and Vin- calculated?

We first obtain it through tedious calculations and then simplify the calculations.

First, the op amp's in-phase terminal 5 pin and inverting terminal 6 pin are obtained using "virtual short", where the coefficient 6 refers to 6 100k resistors, which is convenient for simplifying the formula:

Then Vin+ is obtained through the voltage division relationship:

Again, Vin- is obtained through the voltage division relationship:

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

In fact, there is another simple method to obtain the value of Vin+ minus Vin-. By using the virtual short characteristic of the op amp, the circuit can be equivalent to Figure 5.

Figure 5 Equivalent circuit

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

The differential voltage input value is 0.84V.

Figure 6

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

In actual application circuits, in order to simplify calculations, we also use the simplest method. The circuit often used is the circuit in Figure 6, which makes the resistances equal to simplify the calculations.

The offset circuit is shown in Figure 7. In the place where the original in-phase resistor is grounded to GND, we connect a voltage value, which is also commonly called the offset voltage. So what is the final expression?

Figure 7

Through the superposition theorem, we finally get:

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

As long as the appropriate offset is chosen for the application, the output will always be a positive value.

Figure 8

For example, in the circuit of Figure 8, if the input voltage becomes -100V, the final output voltage will be:

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

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