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TPS659037: Input impedance of GPADC_IN2

Patrick Ruf
Genius 4965 points
Part Number: TPS659037

Hi,

I use the TPS6590379 in one of our designs.
At the GPADC_IN2 I have a voltage devider with RT=10kOhm and RB=68kOhm connected to VANA (LDOVANA_OUT). I measure VANA as 2.094V.
The theoretic output voltage of the divider should be 1.825V but I only measure 1.45VDC at GPADC_IN2.

Can you please tell me the input impedance of the GPADC_IN2 input. It looks like GPADC_IN2 has a resistor of around 33kOhm to GND.

Thanks and best regards,

Patrick

  • 0
    TI__Prodigy 370 points

    Hi Patrick,

    The maximum voltage range for the GPADC_IN2 pin is 1.215-1.250V as well as a maximum current capability of 200uA. These values can be found in Table 4.12 Electrical Characteristics: 12-bit Sigma-Delta ADC in the TPS659037 datasheet. I have a few follow up questions to try and understand your design:

    1) What is the purpose of using the VANA, is that for regulation?

    2) How are you measuring the voltage on the GPADC_IN2 pin, with software or a probe?

    Thank you,

    Jadyn

  • 0 in reply to Jadyn
    Genius 4965 points

    Jaydyn,

    I'm quite shure that GPADC_IN2 has a 0...2.5V input range with a divider of 2. Please see Table 5-3 (below) or chapter 4.3.
    I use GPADC_IN2 for hardware version identification. (please see picture below). The voltage is measured with a probe.
    When I go through all those values for R393 I can see that there must be an input impedance of around 33kOhm at the GPADC_IN2 pin.


    Regards,

    Patrick

  • 0 in reply to Patrick Ruf
    TI__Prodigy 370 points

    Hi Patrick,

    You are correct GPADC_IN2 can have a maximum input of 2.5V since it has a scaler of two, sorry for the earlier confusion. As for the input impedance you are seeing, after looking through device documentation there is no specific call out for the internal impedance of the GPADC_IN2 pin so if you are seeing 33k consistently for your tests then using that value should be ok. 

    Thank you,

    Jadyn