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ADS8681EVM-PDK: Assistance Required for Evaluation Board ADS8681 - Reading Issue when No Connection Established

Part Number: ADS8681EVM-PDK
Other Parts Discussed in Thread: ADS8681

Dear Support,

I am writing to seek assistance regarding the evaluation board ADS8681. While conducting tests with the SPI command, I observed an issue related to the board's functionality. Specifically, when there is no connection established, the board reads a default value.

During my evaluation, I noticed that the evaluation board features several test points, with TP1 designated as the input. When I measured TP1 with multimeter, it displayed a value of 2.4 volts, which is roughly consistent with the readings obtained from the board itself.

Interestingly, when I apply a voltage of 1V, the board displays the correct value. Similarly, when I apply voltages such as 2.3V or 4V, it works accurately. However, the problem arises when nothing is connected, as the board consistently displays a reading of approximately 2.4 volts.

I would greatly appreciate your guidance and support in understanding this issue and finding a solution. Please let me know if you require any additional information or if there are any specific troubleshooting steps that I can follow to further investigate the problem.

Thank you for your attention to this matter. I look forward to your prompt response.

Best regards,


  • Hi Mehrdad,

    Welcome to our e2e forum!  By "No Connection Established" and based on your comments above, it sounds like you are asking why you see ~2.4V at TP1 with no input applied to the ADC.  This is actually not a problem - its an expected result if the ADS8681 is 'converting' while the analog input is essentially open circuit.  For SAR converters like the ADS8681, with an open circuit input, the internal switched capacitor of the sample/hold circuit is going to take on a value of approximately AVDD/2 or REF/2 depending on the internal working of the device.  So, its normal to see this - nothing to troubleshoot or debug.

  • Hi Tom,

    Thank you for your response and clarification regarding the behavior of the ADS8681 evaluation board. I understand that the approximately 2.4V reading at TP1 when no input is connected is an expected result due to the internal workings of the ADC.

    However, my concern arises when I am working with the ADS8681 on a custom board remotely, and I do not have physical access to the board. In such cases, it becomes difficult to determine whether an actual input is connected or if nothing is connected to the board.

    Is there any method or feature in the ADS8681 that can help me determine whether an input is connected, or if there is simply an open circuit condition resulting in the 2.4V reading at TP1? This information would be valuable for troubleshooting and ensuring the proper functioning of the ADC in remote scenarios.

    Thank you once again for your assistance. I look forward to your guidance on this matter.

    Best regards,


  • Hi Mehrdad,

    There are no provisions internal to the ADS8681 to determine if the input is 'open circuit' or not.  You could possibly use a weak pull down/up on the input to force FS+ or FS- in the event of an open circuit.