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ADS127L18: AINP and AINN Input Impedance

Dylan Kwong
Prodigy 90 points
Part Number: ADS127L18

Tool/software:

Hello, 

What is the input impedance of AINP and AINN when the input buffers are on?

I see in the datasheet (Section 5.5) that there is 2uA input current for Vref = 4.096V, but I'm assuming different input voltages affect input current. My closest working guess is that R_in = Vref/2uA = 2MOhm but I'd like some confirmation if there is an upper or lower bound documented from internal testing. 

Thanks, 

Dylan

  • 0
    TI__Mastermind 46430 points

    Hello Dylan,

    Input impedance is closer to 8Meg; most of the input current is common mode that does not change with input voltage when using the internal buffers.  There is no upper/lower bound for this specification, but you can expect input current to vary by +/-20% from part to part, lot to lot, due to process variation.  Also, keep in mind that the input current is an average value due to the internal switched sampling capacitor.  Peak input current per modulator period (78ns using 25.6MHz clock in high-speed) will be on the order of 100's of uA.  Using the recommended input capacitor (1nF to 10nF) delivers most of this AC current and averages down to the typical spec of +/-2uA provided in the datasheet.

    Regards,
    Keith Nicholas
    Precision ADC Applications