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OPA4187: OPA4187 vs INA826 in CMRR

Ming Liao
Intellectual 450 points
Part Number: OPA4187
Other Parts Discussed in Thread: INA826, , OPA187, TINA-TI

Hi,

According to spec, OPA187/OPA4187's CMRR is 140dB,typ, and INA826's is 95/115/130dB,typ with respect to Gain 1/10/100.

To get higher CMRR at 60Hz, use 3pcs of OPA187 (named OPA187_INA) as INA826 as attached figure.

Here is the CMRR simulation result (from Tina-TI) of both OPA187_INA and INA826 with Gain 33 and source resistance 10Kohm,

#1. OPA187_INA & INA826 CMRR at 60Hz: -192.1dB & -145.6dB if there is no source resistance imbalance (+IN/-IN: 10Kohm).

#2. OPA187_INA & INA826 CMRR at 60Hz: -137.5dB & -139.2dB if there is 1Kohm source resistance imbalance (+IN: 10Kohm, -IN: 9Kohm).

From Tina-TI simulation result, if no source resistance imbalance, OPA187_INA CMRR performance is very good, but if with 1Kohm imbalance, OPA187_INA CMRR has a big drop of 54.6dB compared to INA826. Is that big CMRR drop of OPA187_INA reasonable? To count in source imbalance, is INA826 the better solution than OPA187_INA? Please make comments.

Thanks.

Ming

  • 0
    TI__Guru 68541 points

    Low frequency CMRR in the instrumentation amplifier is a function of the resistors' matching and for that reason most of our INA's are laser trimmed at the wafer-level to 0.001% accuracy.  +/-1% mismatch results in min CMRR of 34dB, 20*log(0.02), while +/-0.1% mismatch yields min CMRR of 54dB, 20*log(0.002).  For that reason, in order to achieve 100dB CMRR, you would need to use resistors with 0.0005% matching, 20*log(.00001)=-100dB, which would be very expansive.  This is exactly the reason why instrumentation amplifiers are relatively costly.

  • 0 in reply to Marek Lis
    Intellectual 450 points

    Hi Marek,

    Thanks for your reply.

    By the way, from cost point of view, the price at 1k units, on TI online store, is 2.0 USD for OPA4187IPWR and 1.05 USD for INA826AIDRGR, which the instrumentation amplifier is cheaper.

    Thanks.

    Ming