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Original question:

ADS122C04: High Input impedance value

ADS122C04: Input impedance >10 Megaohm

Jonathan Navor
Expert 7805 points
Part Number: ADS122C04

Hi Experts,

Good day! We have a concern from the customer regarding the high impedance. 

Could you help us to quantify the input impedance of this ADC? I know this depends on different variables like the temperature, the PGA, etc. Nevertheless, I do not know precisely how to calculate an input impedance from the datasheet. Is there perhaps a formula that I can use to calculate the input impedance for a specific setting? This would allow me to calculate a range of input impedance.  To measure the voltage correctly in my setting I need an input impedance of >10 Megaohm. However, in one of your posts, I read that the PGA acts like an instrumentation amplifier with high input impedance. It seems that with instruction amplifiers there is no need for impedance matching, does this also hold for the ADS?

Or could you possibly recommend another ADC that still has high resolution and appropriate input impedance for our application?

Thank you in advance for your support.

Best regards,

Jonathan

  • 0
    TI__Guru** 112725 points

    Hi Jonathan,

    The impedance is quite high, but it is somewhat dynamic based on input voltage and modulator sampling rate.  It was decided that instead of giving a specific value it would be better to show what happens over temperature and input voltage with respect to actual input current flow.  The input current is demonstrated in the datasheet using graphs in figures 4 through 11. The graphs show both absolute current and differential input current.

    The input current is the highest closest to the supply voltages which is primarily due to ESD diode leakage.  During a normal conversion cycle (where the input is selected and ADC is converting) the primary current is from input bias currents as there is an input chopping stage to lower offset.  There are also some differences depending on whether the PGA is enabled or disabled and the graphs are shown for both conditions.

    Also, the electrical characteristics show typical input current at the top of section 6.5.  If the input current is around +/- 1nV the impedance is well above your target.

    Best regards,

    Bob B