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ADS1299: CMRR of SRB2

John Fedak
Expert 6715 points
Part Number: ADS1299

Hi Team,

Good afternoon, my customer is interested to learn more about the CMRR when using the SRB2:

I see quite a degradation in CMRR (CMRR ~90dB or less) when using the SRB2 switches to route an input as a common reference.  I don't see any specifications for switch capacitance (ON or OFF) or ON resistance, etc.  Does anything like this exist?

Please let me know if you have any questions, thank you!

Regards,

~John

  • 0
    TI__Guru 57326 points
    Hi John,

    Thanks for supporting our ADS1299!

    In the past, I believe many customers have struggled to meet the CMRR performance targets when using the SRB1 or SRB2 pins. Though we don't spec their input impedance in the datasheet, you can assume that they will be close, but not perfectly matched to the to the other INxP or INxN inputs pins. Therefore, it does make sense that your customer sees worse CMRR when using this configuration.

    Is your customer also using the BIAS amplifier? Not only is this used to bias the patient's common-mode voltage, but it can also be used to sense the common-mode signals on the PGA outputs and drive an inverted common-mode cancellation signal to the body. This is often imperative in order to meet CMRR requirements in biopotential applications. See the Typical Applications section in the ADS1299 datasheet, specifically Figures 72 and 73.

    Best Regards,