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Difference Amp CMRR: G = 0.1 vs G = 10

Damien Dusha
Prodigy 190 points
Other Parts Discussed in Thread: INA143, INA146

Hi All,

I am looking at a problem where I need to attenuate a large (+/-24V) signal for conversion,  The INA143 looks to be a good choice for my problem.

I am looking at the effect of CMRR. The part has a minimum specified CMRR of 86dB.   I have taken a look at this blog post [1] where there is a spreadsheet which performs the calculation.  With a configuration of G = 10 and 0.01% resistor matching, the CMRR comes in at 88dB, essentially matching the datasheet. Fair enough.

When changing the configuration to G = 0.1 (my desired case) then the CMRR drops to 68dB, or about 360uV . This is quite a drop!   Is there some difference in how the numbers are calculated vs the datasheet? What sort of voltage offset am I really looking at due to CMRR in this circumstance (e.g. 12Vcm because of the +24V - 0V difference)?  How does this change compared to the G = 10 case?


Thanks in advance,

Damien.

[1] e2e.ti.com/.../what-you-need-to-know-about-cmrr-the-instrumentation-amplifier-part-2

  • 0
    TI__Mastermind 25795 points
    Hello Damien,

    Yes, as the differential gain decreases, so does the CMRR performance as you indicated.

    However, there is another device that may be of interest. The INA146 is a difference amplifier with G=0.1V/V followed by a non-inverting gain stage. You can set the gain of the output stage to 1V/V thereby ensuring a differential gain of 0.1V/V. The minimum CMRR specification for the device in such a gain configuration is 70dB (typical is 80dB), which is a little better than the calculated CMRR performance of the INA143 in G=0.1V/V.