Other Parts Discussed in Thread: LMC6041, OPA130, , TL081
I am using the circuit below to monitor the output of a piezoelectric diaphragm, represented by a sine wave generator and capacitor C3.
The circuit is powered from a split supply (+5V/-5V) derived from a linear power supply (230V, 50Hz). I need the full rail-to-rail dynamic range (or as close as possible).
The functions of the amplifiers are as follows:
- U1 - charge amplifier, frequency range of interest: 1Hz-60Hz
- U2 - Sallen Key filter, low pass, fc = 110Hz
- U3 - scale and bias amplifier to convert U2 output to 0V min, +3V max, suitable for ADC.
The circuit works in simulation, but in practice I find that a high amplitude mechanical impulse on the piezoelectric disk causes U3 to oscillate'. (The signal seems to output narrow pulses which jump to the +5V rail at a frequency of 50Hz). The oscillation will eventually disappear (after about 30-40 seconds) if the piezo is left untouched. I checked the power supply rails - they are clean with little ripple, even when the oscillation is happening.
If I disconnect U3 and monitor the output of U2, the problem does not occur. I can "cure" the oscillation in the circuit by putting a voltage follower amplifier between U2 and U3.
My question is: Does U3 cause a problem for the stability of U2 at high frequencies? The impedance of C101 will be low for a high frequency impulse and U2 is trying to drive current to support this as well as maintaining the correct current through R5 to keep the input voltages on pin 2 at around 1V d.c.
Perhaps there's a better way to offset the output signal?