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ADS1278EVM-PDK: Constant Voltage on the input pins when no input provided

Vaishnavi Thakur
Prodigy 20 points
Part Number: ADS1278EVM-PDK

Hi

i have been using ADS1278EVM-PDK as standalone.

I have interfaced my board with EVM ( with no buffers selected for CH1 to CH4 i.e S4, S5, S7, S8 turned OFF) and no inputs connected except for the power supply of 5V, 3.3V and 1.8V.

Despite not providing input to any of the channels, I can see there is a constant voltage of 0.9V on input pin of Channel 3 i.e pin J3.4.

I have referred the schematics and it looks quite simple with no chance of anything going wrong or shorting with any other input.

May I know what can be the cause for this constant voltage on the input pin?

  • 0
    TI__Mastermind 36150 points

    Hello Vaishnavi,

    Welcome to the TI E2E Community.

    Internally, the input sampling capacitor is biased during the input acquisition phase.  If the inputs are floating, it is possible that you could measure a portion of this voltage.  I have not tried making this measurement, but I would guess with a very high input impedance DMM, you could see a small voltage in this range.  The other possibility is the CH3 input has been damaged, and there is an internal leakage current getting into the input.

    Questions/Suggestions:

    1.  Are you measuring this voltage with an external DMM, or is this the conversion result?

    2.  If the measurement is the conversion result, try adding 10Meg load resistance on the pins so they are not floating.  (You can use the higher voltage range of most DMMs, which typically have a 10Meg input resistance, as the load.)

    3.  If using an external DMM, is the CLK running?  If so, disable the CLK and retake the measurement.  

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Prodigy 20 points

    Hi Keith

    Thanks for your earliest reply. 

    We got to know about this issue when the conversion results displayed 0.9V on channel 3 , while rest all channels displayed 0, when no input connected.

    Then we measured the voltage at input pins i.e. at J3.4 with external DMM which displayed the same. I have also re-checked the input coming to this pin from our interface board ,but that seemed to be fine when checked isolating the ADS1278EVM. As you suggested , will check with CLK source disabled( here external CLK source).

    Thanks & Regards

    Vaishnavi Thakur