Zero compensation problem of Wheatstone bridge

傅里叶的猫

Zero compensation problem of Wheatstone bridge [Copy link]

I have been studying the relevant knowledge of Wheatstone bridge recently. This circuit diagram is the schematic diagram of the module attached to the module sold on Taobao. The document explains that RP1 is used to adjust the output zero point. The zero point output of the bridge is not zero because the initial resistance of the bridge arm resistors is different. It may be that the temperature coefficients of the bridge arm resistors are different, or the bridge arm resistors have different resistance values. According to my understanding, his compensation method is to adjust the output zero point by adjusting RP1 and adding a DC component to the circuit. Personally, I think the principle is wrong, and a tricky method is used to achieve the purpose of adjusting the zero point. After my MULTISIM simulation, adjusting RP1 cannot change the output DC bias. The second op amp looks like an inverting amplifier, but I don’t quite understand the voltage of the in-phase terminal. Thank you again for your guidance.

maychang

"The second op amp is like an inverting amplifier."

It's not "quite similar", but "exactly".

maychang

Note that this circuit uses a single power supply for both op amps.

If there is no RP1 (the non-inverting input of the second op amp is grounded), then the output of the first op amp must be positive because it uses a single power supply. The second op amp is an inverting amplifier and also uses a single power supply. The inverting input is positive, and its output must be negatively saturated, that is, the output is zero. The circuit cannot work at all.

maychang

Therefore, the second op amp must be biased with an appropriate positive voltage so that the potential at the in-phase input of the second op amp is equal to the output of the first op amp when the strain gauge is not stressed (note: the bridge is not balanced at this time). Only then can the second op amp be in the linear working area and work normally.

maychang

To understand this circuit, note that the Wheatstone bridge composed of the three 348-ohm resistors R4, R5, and R13 and the strain gauge is not in equilibrium when the strain gauge is not subjected to force. Instead, the potential of the first op amp's non-inverting input terminal is slightly higher than that of the inverting input terminal.

This circuit may be followed by an AD converter, and its zero point is not the ground potential, but an intermediate value between VCC and GND.

傅里叶的猫

maychang published on 2021-3-27 09:45 Note that both op amps in this circuit use a single power supply. If there is no RP1 (the second op amp's non-inverting input is grounded), then the first op amp uses a single power supply...

After the teacher's analysis, I suddenly understood. In the future, I will pay more attention to the power supply when looking at circuits. Thank you!

Gen_X

It is best to connect a 7-8K resistor before the 3 terminals of UA1, otherwise the common mode will be a bit off-adjusted and it will be difficult to adjust the effect you need.