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UCD90320: ADC equivalent input resistance versus recommended series input resistance

Mike Pettus
Prodigy 210 points
Part Number: UCD90320

The ADC equivalent input resistance in the data sheet shows 2.5k ohm max, which means it could even be lower.  We intend to drive the ADC inputs with either direct regulator voltage outputs, or with op-amp buffering from dividers.  But the data sheet also recommends a 200 ohm series resistor into the ADC input.  With a 2.5k ohm equivalent input resistance, and a series 200 ohm input resistance, doesn't that create a large analog error in the ADC reading?  What is the variation or range of the equivalent input resistance?

 

  • 0
    TI__Guru 66260 points

    Hello

    The 200 serial resistor is to help on the ESD latch up. The 2.5K ohm is a resistance between the ADC input pin and a high impedance ADC sample node. It does not impact your external voltage accuracy or divider circuity.

    Regards

    Yihe

  • 0 in reply to yihe
    Prodigy 210 points

    Yihe - Thank you for the information.   Is the high impedance at the sample node specified?  It would be good to know so we can calculate the loading effect on dividers.  Another way to ask this would be to know what the highest divider resistance value to ground could be to have less than 0.1% error?  Appreciated your help with this. 

    Mike

  • 0 in reply to Mike Pettus
    TI__Guru 66260 points

    Hello

    If accuracy is more important, I would recommend using external reference which has much better tolerance than the internal as described in the data sheet. 

    As for the external resistor divider,  1K-10K resistor with high accuracy shall be fine. 

    Regards

    Yihe