charge amplifier

[Skyland]

I have designed a charge amplifier to convert the charge from the force sensor into voltage. The force of the sensor is dynamic from 0 to 100N which is modelled as a voltage source V3 with a sine wave oscillating from 0 to 1KHz. C2 is the sensor static capacitance. The current source B1 is differentiating the voltage from V3. The system need to be able to respond between 0.1 to 1KHz. The problem is as shown in the AC simulation result the circuit on the right hand side does not give a low pass filtered response with a cut off frequency of 1KHz. I have made the circuit on the left hand side with a current source just to confirm that the charge amplifier is designed properly and the response is is as expected so the problem seem to be coming from the charge source. What could be the problem?

[Marco]

Are you sure using DDT in AC simulations is kosher? (Also, Spice will almost certainly ignore this in AC ... but your opamp can't really put out 1000V.)

[mikerj]

That's exactly what I would expect.  Your current source has an output proportional to the differential of the voltage source, so the current output increases at 20dB/decade.  Your simple integrator rolls off at 20db/decade above the pole formed by 1k/100n  1/(2*pi*1k*100n)  = 1.5kHz.  When combined this gives a flat response up to a few hundred kHz.

Above this the op-amp is running out of gain/bandwidth, the output impedance increases and so the voltage at the inverting pin passes through the 100n cap to the output, and the output voltage starts increasing.