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ADC122S101: IN1 makes noise on IN2

Part Number: ADC122S101

Dear BU,

When my customer give IN1 a DC input and give IN2 nothing, there will always be a 60mV DC voltage on IN2 input.

And when I give IN1 nothing, the DC voltage on IN2 disappear.

Could you please give me some advice on this situation?

Voltage waveform of IN2 when there is DC voltage on IN1:


  • By the way, this situation only happens when converter works. When CS is high or there is no digital clock input, the noise disappear.

  • Hello,

    My first suspicion is cross-talk between the channels. Is this measurement done in fully laid out PCB board, or are there cables and wires being used to provide inputs and connect boards?  Using a clean board and connection will help eliminate any signals interacting with each other. 

    What is Ch2 connected to during this test? is it left floating or is it connected to GND or an input source?

    Regards

    Cynthia 

  • Hi  Cynthia,

    Please refer to the  schematic diagram.

    In customer's test, for Ch1, R74 is connected to 3.3V DC voltage by a wire.  For CH2, R71  is connected to GND by a wire. Then there is a 60mV noise on CH2.

    When R74 is also connected to GND, the noise disappear.

    And when CS footprint is high or there is no digital clock input, the noise also disappear. From this situation, it seems the ADC itself cause the noise?

  • Hi Cynthia,

    Could you please give some suggestions?

  • Hello,

    Apologies for the late response. I would like to confirm where the noise is coming from,if in fact from the ADC or from other noise sources. 

    From the scope shots you provided, it seems there is some periodic signal appearing. there is a dip in the first and a spike in the second scope shot. 

    Do you know where this is coming from?

    To make sure the noise is not coming from the other input, is it possible to remove the 10Kohm connected to the ADC input. This well help to allow to measure the source output with a precision multimeter, as well as see if the interaction to the original channel is gone. As part of this, the diode might also be causing the unexpected voltage. Depending on the diode leakage current, this with the 10Kohm resistors could be the cause. You can try changing both the 10Kohms to 5kOhms. If the 60mV seen drops by half, then this helps to determine that the diode leakage current is the cause

    Regards

    Cynthia