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AD8428 not working with transducer connected

Category: Hardware
Product Number: AD8428
Software Version: N/A


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

  • Hello Mahesh,

    the AD8428 has an input bias current of up to 200nA, and an input offset current of up to 50nA. With the signal generator and without AC-coupling this current ist driven by your source and makes no big voltage drop.

    With AC-coupling caps the current has to run over R212 and R213, which are both 1 MOhm. 50nA over 1MOhm gives a voltage drop of 50mV, and this is what interprete as input offset of the AD8428.

    Reduce R212 and R213 e.g. by a factor of 10 (i.e. use 100kOhm), and the offset will reduce accordingly.

    best regards


  • Thank you very much Achim, this worked for me on the board, I went down to 10kohm resistors and also verified this with simulation.  Thank you again!

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