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Original question:

ADS1298 WCT external capacitance

ADS1298R: WCT Capacitor 100pF or 1000pF?

Kevin Kreger
Expert 1020 points
Part Number: ADS1298R
Other Parts Discussed in Thread: ADS1298, , ADS1198, ADS1298ECGFE-PDK

Hey Folks,

We see the ADS EVM is using 100pF for the WCT feedback cap, and we found a thread here which discusses using a 100pF 

cap as well on the ADS1298 here: https://e2e.ti.com/support/data-converters/f/73/p/496049/1792960?pi318173=1

However figs. 107 and 108 in the ADS1298R data sheet show a 1nF (1000pF) capacitor.

I usually assume that the EVM is the more refined design and use that. In addition, we use the ADS1198 in a prior design

and used the 100pF.

So, do we stick with the old ADS1198's 100pF cap or use the 1000pF cap on the ADS1298R?

Thanks in advance,

Kevin

  • 0
    TI__Guru 61826 points

    Hello Kevin,

    Thank you for your post.

    There seems to be a discrepancy between the ADS1198 and ADS1298 data sheets. The text and figures in the ADS1198 data sheet agree that a 100-pF capacitor should be used. The ADS1298 data sheet mentions the same 100 pF capacitor in the text, but the figures you referenced show 1000 pF. To further confuse things, the ADS1298ECGFE-PDK and the ADS1298RECGFE-PDK show different capacitors as well.

    Let me talk with a designer to understand what was the original recommendation.

    Best regards,

  • 0 in reply to Ryan Andrews
    Expert 1020 points

    Thanks Ryan. We are standing by.

    Best,

    Kevin

  • 0 in reply to Kevin Kreger
    TI__Guru 61826 points

    Hi Kevin,

    Please use a 100-pF capacitor to bypass the WCT pin. This will be plenty of capacitance. The main intention of this cap is to help filter the noise of the summing junction resistors and the WCT amplifiers. 1 nF was found to be excessive and may introduce too much phase shift in the WCT output vs. the chest electrodes.

    Best regards,