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Original question:

[FAQ] ADS129x: How do I calculate the DC gain of the RLD or BIAS amplifier?

ADS1298R: Affect of PGA Gain on RLD

Kevin Kreger
Expert 1020 points
Part Number: ADS1298R

Hello Everyone,

We are simulating RLD using TINA TI and we started with a simulation ECG_external.VSC file provided by Matt Hann of TI here:

https://e2e.ti.com/support/amplifiers/f/14/t/117436

Matt's work also involved writing an article for EE Times which is archived here:

https://archive.eetasia.com/www.eetasia.com/STATIC/PDF/201111/EEOL_2011NOV01_AMP_TEST_TA_01.pdf?SOURCES=DOWNLOAD

Our basic issue is calculation of the inputs to the RLD amp. We already have adjusted our simulation to account for the 220k
output resistors as per the post we are related to. We are only two using 'positive' channel inputs to RLD, LA and LL, so our input resistance
in our simulation drops to 110k.

Here is what we want to confirm:

Our PGA gain is 12. It seems that because the opamps outputs are wire or'd (shorted) that we do not use
a gain of 12, but a gain of 1. 

Here is a pic illustrating how we made the conclusion illustrating two P channel switches engaged:

Thanks in advance,

Kevin

  • 0
    TI__Guru 66446 points

    Hi Kevin,

    Thank you for your post and for sharing that article! I've known Matt for several years, he has a lot of expertise in this field.

    In the ADS1298R, the PGA gain is differential as the amplifiers in each channel are connected in an instrumentation amp configuration. The common-mode signal, however, (i.e. the individual P and N amplifier outputs) will see a gain of 1. Therefore, the common-mode signals applied to the inverting input of the RLD amplifier will be summed together with a gain of Rf / (220k || 220k || ...), just like you described. Of course, this is only the gain at DC. The gain at the frequencies of interest (i.e. 50 Hz and 60 Hz) will depend on the R-C cutoff in the RLD feedback network.

    Best regards,

  • 0 in reply to Ryan Andrews
    Expert 1020 points

    Thanks Ryan for the prompt reply!

    This resolve my doubts/issue.

    Best,

    Kevin