The time constants of the resistor R 16 and the capacitor C9 connected to the non-inverting terminal 3 of the second-stage operational amplifier U6 are too large, and it becomes an integrating circuit to integrate the signal voltage with sudden changes of high and low cuff pressures into a slowly changing DC signal.

Solution: Reduce the time constants of R 16 and capacitor C9, and cancel capacitor C6 and C 10.

Measure the output pin 6 of the first-stage noninverting operational amplifier U3 with an oscilloscope. When the cuff deflates slowly, observe whether the pressure signal changes, so as to check whether the gain adjustment resistor R9 of the operational amplifier U3 is adjusted correctly, and then check whether the output voltage of the second-stage and third-stage noninverting operational amplifiers causes the signal voltage change of cuff pressure change.