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ads1235: capacitor across CAPP and CAPN

Alexander R
Prodigy 140 points
Other Parts Discussed in Thread: ADS1235

Hello. Can I change capacitance (4.7nF C0G) across CAPP and CAPN? I have capacitor 3.3nF, 10nF and 33nF C0G.

  • 0
    TI__Guru 50381 points

    Hi Alexander,

    Thanks for your post and welcome to the forum! 

    Sorry for the delay in response. I am not the direct support for this device, but I imagine using the 3.3nF or 10nF cap would be fine. 

  • 0 in reply to Alexander Smith
    TI__Mastermind 44620 points

    Hi Alexander,

    I would expect that using a slightly different value capacitor probably wouldn't be an issue; however, please note that all of the datasheet specifications are based off of the recommended 4.7 nF capacitor.

    I think Alex's recommendation to avoid using 33 nF is probably for the best, since that's almost an order of magnitude off of the recommended value...

    If the capacitive load on the PGA is increased too much then it is possible for the PGA to become unstable. However, for smaller capacitance changes, you may see some trade-off in the noise performance vs setting time. Unfortunately, I don't have any data with different capacitor values on the CAPP/CAPN pins to be able to provide additional guidance on this.

  • 0
    Prodigy 140 points

    Hi.

    This answer of Jonathan Geronga Texas Instruments Customer Support from case CS0086475:

    "Hi Alexander, Thank you for contacting TI Customer Support center.  Regarding your inquiry on your plan to change the capacitance value of the capacitor (4.7nF C0G) across CAPP and CAPN. The 4.7nF value is required to make sure that the ADC Antialias filter frequency on the PGA monitor will be 60 kHz. changing the capacitance value may change the Antialias filter frequency."

    What you think about it (Antialias filter frequency)?

  • 0 in reply to Alexander R
    TI__Mastermind 44620 points

    Hi Alexander,

    Modifying the capacitor value will affect the RC filtering after the PGA...
    Increasing it may help to improve noise performance, but at the cost of additional settling time (and possibly stability). I would consider the anti-aliasing cutoff only after evaluating the stability and settling time of the PGA. Lowering the anti-aliasing cutoff is not all that important since this is an over-sampling ADC and aliasing frequencies will only occur at integer multiples of fMOD. Additionally, you can add an RC filter prior to the ADC inputs to reject aliasing frequencies without affecting the PGA performance.  

    If you do decide to change the capacitor value, then you'll need to evaluate the circuit performance for your application. We've thoroughly evaluated the ADS1235 performance with a 4.7 nF capacitor over all gains and over the entire operating temperature range, so that is the only capacitor value that I have data for and can recommend with high confidence.

  • 0
    Prodigy 140 points

    Thanks to All for detailed aswers.