I'm using the fixed gain of 2000 AD8428 part to amplify a small piezoelectric transducer signal (range of 50uV to 500uV). The AD8428 works fine when I have a signal generator but doesn't work with the transducer connected. The main reason I believe is the DC offset of the transducer is too high (~30mV range). To address this I added ac-coupling caps in place of 50ohm series resistors (R203 and R204 in schematic), the ac-coupling caps are of value 10nF. When my board is not powered, the AC coupling caps do the job and transducer signal goes down to ~0mV retaining the AC waveform. However with board and amplifier powered, the AD8428 doesn't work with the ac-coupling. I'm not able to get amplified output, I have tried the following configurations:
1) No ac-coupling caps, series 50 ohm resistors --> DC offset too high and output goes to max (~3.3V). Supply voltage is +/- 4V
2) With ac-coupling caps in place of 50 ohm resistors --> amplifier output goes to reference voltage of 0.55V (using OP1177) but no amplification of signal.
3) Tried grounding negative input and no improvement in performance.
4) Tried series 50 ohm resistors and external ac-coupling caps (leaded caps outside board) but still no success.
Again, signal generator with no dc offset seems to work so I believe amplifier is not defective.
Also, added 6800pF caps to ground on each input to address EMI interference issue (pulse train appearing), these caps fixed this issue. Per datasheet recommendation.
Also, have LT spice simulation for this circuit, but not able to simulate with ac-coupling caps.
Please advise any thoughts, schematic snippit attached, thanks, Mahesh