Circuit configuration of differential circuits

Target processing voltage:This refers to the collection and processing of voltages, such as the collection and processing of bus voltages in a system, as well as the collection and processing of AC voltages.

Differential in-phase/out-of-phase voltage divider resistors:To obtain a voltage suitable for operational amplifier processing, high voltage signals need to be divided. As shown in Figure 1, the voltages across V1 and V2 are processed through voltage division, ultimately yielding voltages Vin+ and Vin- suitable for operational amplifier processing.

Differential amplifier circuit:

Feedback, for operational amplifier circuits, the operational amplifier operates in the linear region, so there must be negative feedback here. Without feedback (open loop) or with positive feedback, it is a comparator circuit rather than an amplifier circuit. At this point, the operational amplifier operates in the saturation region or nonlinear working region, and because of saturation, the output is the amplitude of the power supply voltage.

Figure 2 shows an operational amplifier circuit with positive feedback, which can no longer be called an operational amplifier circuit because the ideal open-loop gain of the operational amplifier is infinite. Of course, in practice, it cannot be infinite, so the structure shown is a hysteresis voltage comparator, with the operational amplifier operating in the nonlinear or saturation region.

Figure 3 is still a voltage comparator structure. As mentioned above, the open-loop gain of the operational amplifier is very large, and without negative feedback, it operates as a nonlinear region, used as a voltage comparator.

Operational amplifier, when 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 for all signals is the power supply, so the output cannot exceed the power supply amplitude), the function achieved is to amplify the signal. Connecting to the non-inverting terminal "+" is positive feedback, and the circuit function is a voltage comparator. In practice, we do not advocate using operational amplifiers as voltage comparators, but rather selecting dedicated comparators such as LM339, LM393, LM211, etc., because the working states of internal components in comparators and operational amplifiers are still different.

The comparator is connected with current-limiting resistors—"R74, R77". This is because when the comparator switches amplitude, the fast rising or falling edge charges and discharges the subsequent capacitive load. This charging and discharging current comes from this active device—the comparator, so the purpose of adding current-limiting resistors is to prevent current surges.

RC filtering:Can be adjusted as appropriate, with the aim of preventing output overshoot and other signal distortion issues.

In the circuit of Figure 4, for convenience in calculation, we specify each resistance value.

Another characteristic of differential circuits is symmetry, R40=R56 and R47=R55, and the resistors in the two branches of the differential voltage divider are also equal.

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

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

First, using the "virtual short" concept for the non-inverting pin 5 and inverting pin 6 of the operational amplifier, where the coefficient 6 refers to six 100k resistors, to simplify the expression:

Then, through the voltage division relationship, we obtain Vin+:

Again, through the voltage division relationship, we obtain Vin-:

Thus, we get the value of Vin+ minus Vin-.

In fact,there is another simple method.To obtain the value of Vin+ minus Vin-, using the virtual short characteristics of the operational amplifier, the circuit can be equivalently represented as:

Therefore, calculating the value of Vin+ minus Vin- becomes very easy; it is just a simple voltage divider circuit, as calculated below:

The differential voltage input value is 0.84V.

Calculations for the differential amplifier circuit.

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

In practical application circuits, to simplify calculations, we also use the simplest method for calculations. The commonly used circuit is also the above circuit, letting the resistors be equal to simplify calculations.

"Offset calculation" of the amplifier circuit.

Why do we need to offset the output voltage?This is because when collecting negative values, our sampling chips and MCUs almost do not support negative value sampling, so you must perform an offset to ensure that the output is always positive.

Offset circuit, as shown in Figure 8, where we connect a voltage value at the original non-inverting terminal resistor grounded to GND, which is usually also called offset voltage. So what is the final expression?

By applying the superposition theorem, we finally obtain:

The validity of this formula ensures that R64=R72 and R73=R57, so the final offset formula adds a voltage offset of 2.5V_Ref to the original basis:

As long as the appropriate offset value is selected based on actual applications, the output will always be a positive value.

For example, in the circuit of Figure 9, if the input voltage changes to -100V, then the final output voltage will be:

This offsets the negative voltage to a positive voltage, meeting the processing requirements of the processor. The offset circuit is widely used in the collection of AC power and in control circuits with negative DC voltages.