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What are the input impedance and input pin leakage specs for this part?

Jerome Villapando
Intellectual 300 points
Part Number: TLA2528

Tool/software:

I cannot find it on the datasheet. I plan to use an ADC input to measure a somewhat sensitive signal. Wondering if I truly need a buffer to interface it before the TLA2528 input.

  • 0
    TI__Genius 14270 points

    Hi Jerome,

    The need for a buffer depends on a few things, such as the sampling speed and how fast the input signal changes. If you are sampling voltage rails temperature measurements from a thermistor, these are slow moving signals, and you can usually get away not having a buffer if the sampling speed is also low.

    I recommend watching the "Driving SAR ADC without amplifiers" TI Precision labs video, as well as other videos in the "SAR ADC input driver design" series for more specific recommendations, and guidance to simulating your circuit. Let me know if they are helpful, or if I can provide any more information.

    Regards,
    Joel 

  • 0 in reply to Joel Meraz
    Intellectual 300 points

    I am sampling a very large resistance voltage divider (R1 = 20Mohm, R2=0 to 20 Mohm). I was just wondering if the TLA2528 input impedance or input leakage currents could cause an offset error in the sensor. We care about accuracy of being able to know what the resistance of R2 to some extent.

  • 0 in reply to Jerome Villapando
    TI__Genius 14270 points

    From the perspective of the ADC, there can definitely be some voltage drop across the input resistance due to input leakage/bias currents. Due to the internal protection diodes, 1uA is the maximum expected leakage current at 125°C, which could potentially contribute to a large error. 

    At a maximum 10Mohm max source impedance (R1 || R2), I am almost certain you will need a buffer unless you sample at very low speeds. This will introduce some offset error, but it can be calibrated for in general. Otherwise, settling to the full accuracy of the ADC will be difficult, and again there is the potential error due to leakage currents. 

    Regards,
    Joel