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INA828: CMRR vs Frequency, datasheet fig 17, confirmation requested

Fabio BARONE
Prodigy 20 points
Part Number: INA828

Tool/software:

Dear Sir Madam,

The datasheet (SBOS792A –AUGUST 2017–REVISED JANUARY 2018) Fig 17 (copied below) shows CMRR vs Frequency for four (4) values of gain. 

The traces show that CMRR is lowest for G=1 (eg: 100dB @1kHz), and highest for G=1000 (~120dB @1kHz).  This would seem counter-intuitive, so I am requesting confirmation that this is indeed correct, ie: that CMRR improves (is higher) as gain increases, and not vice-versa.    

Many thanks.

  • 0
    TI__Guru 63050 points

    Fabio,

    The graph of INA828 CMRR vs frequency is correct. Since the DC CMRR is dominated by the resistor mismatch, its output error gets divided by the gain of the configuration.  Thus, for a given change in Vos caused by change of Vcm in G=1 (CMRR=(change_Vcm)/(change_Vos/Gain) = 100dB), Vos gets divided by 10 for G=10 becoming 120dB and gets divided by 100 for G=100 becoming 140dB, etc. However, for higher gain it gets limited to around 145dB by the output noise of INA828.