I ordered 5 of AD734 and thinking about how to make a circuit for squaring the signal.
The "chopping" signal readout technique requires that we square the signals. Basically we need to (as rana suggested):
(1) square the signal from PD, (after 10100Hz bandpass) to convert it to power, and band pass it again.
(2) square the driving signal (might be varied from 0.1 1Hz.) This is illustrated in the diagram as doubling the frequency ("2 x freq" box.) The driving signal for PZT is offset. So the signal is V drive = A + B xsin (2pi fdrive t) with A > B. This ensures that the voltage on one end of the PZT is always higher than another end. We might need to high pass this signal first, to get a signal with only 2 fdrive frequency after we square it.
(3) multiply signal from (1) and (2) to demodulate the signal.
Basically, 3 multipliers are needed.
The first one is for (1), so the input frequency is ~ 10 100Hz, and the output is 20200 Hz.
The second multiplier is for (2), the signal is ~ 0.1  1 Hz, but this one might have large DC term after we square it.
The third one is for (3), this one has to multiply 2 low f signals together which is quite similar to (2), so the design can be the same.
I'll consult Frank and/or Koji again before finalize the multiplier circuit.
