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LME49724: LME49724 ADC input buffer, unexpected VQ

Part Number: LME49724
Other Parts Discussed in Thread: OPA1632, TPS54202

We are attempting to prototype an LME49724 input buffer to the Cirrus Logic CS5361/81.  Our design is based on the OPA1632 and LME49724 input buffer examples from TI data sheets.

The VQ we are expecting from the Cirrus Logic chip is 2.5V which is what we see when the LME49724 is removed from the circuit.  However, when the LME49724 (or an OPA1632) is returned to the circuit, VQ drops to 0.23V.

We are expecting VQ to remain at 2.5V.  Further, when removing the voltage follower U6 and jumping pads 1 and 3, we still see 0.23V.  As a long shot we've also tried bridging pin 7 of the LME to ground which is in some of your examples.  No difference.

Perhaps this question is better suited to Cirrus Logic and we've reached out to them as well.  We just thought you might have some ideas.

Our op amp supplies are +/-10V and our -12V is supplied by a TPS54202.

  • Hi Aaron,

    From the schematic captures I don't see any AC coupling capacitors between the op amp output and the ADC input. Usually you have some AC coupling to avoid the bias from one stage to conflict with input bias from a next stage.

    This is a differential connection, DC bias should not affect but perhaps CS device still needs the DC blocking caps.

    I would just replace some of the resistors in series for capacitors and check if it works better.

    Best regards,
    -Ivan Salazar
    Applications Engineer