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  • TI Thinks Resolved

INA129: CMRR problem with a buffer on VIN+

Part Number: INA129

Customer's circuit is as bellow and they care the CMRR most.

The test input is VCM=10V@10Hz, VDIFF=100uV@10Hz.

While input from INS+ and INS-, CMRR>119dB;    but from IN+ and IN-, CMRR<80dB.

Customer's requirement is how to make CMRR>110dB while input from IN+ & IN-.

Thank you and happy new year!

Marc

  • Hi Marc,
    Why has the customer added U119 into the signal path ahead of INA129? It's possible that this op amp is either creating a source impedance mismatch (and converting common mode signals to differential signals at the inputs of the INA), or that the added phase shift from U119 is converting common mode to differential and degrading CMRR.

    I'd recommend the customer either bypass the OPA or add a similar buffer to the INS+ input.

    Regards,

    Alex Davis

    Precision Amplifier Applications - TI Tucson

  • In reply to Alexander Davis:

    Hi Alex,

    Customer use IN- to drive 16 INS-, the U119 is for the drive ability.

    I will recommend customer to add a similar buffer to the INS+ input to see the result.

    Thank you and best regards,

    Marc

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