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ADS7828-Q1: ADS7828-Q1: Unexpected voltage offset when sensor is connected to single-ended input

Part Number: ADS7828-Q1
Other Parts Discussed in Thread: ADS7823-28EVM, ADS7828

I am experiencing a strange behavior on my own design with ADS7828EIPWRQ1, and have been able to replicate the issues using TI's evaluation board model ADS7823-28EVM.   I am trying to understand if the behavior is expected, or if there is a problem with how I am connecting the ADS7828 and also the EVM.

Description of problem:  We have a sensor which requires power and ground.  The sensor then has an analog output (0 to 2.5V) and a corresponding signal ground. When the sensor is powered ON and NOT connected to the ADS7823-28EVM, it will read 1.029V.  (This is the value we are expecting for this sensor in our controlled environment.)   However, when the analog output and signal ground are connected to the EVM, the voltage we are seeing with a multimeter is now 1.086V.

Some details for you:

  • The sensor is connected in single ended mode.  So, we are connecting it to CH0 and GND (on the EVM this would be pins 1 + 2 of connector J1).

  • We have tried powering the EVM and sensor are both powered from the same bench supply. 

  • When I disconnect the signal ground between the sensor and EVM, the offset goes away and the multimeter reads the correct value of 1.029V.  (This makes it seem like we are dealing with a grounding issue.)

  • I have tried various combinations where earth ground is connected only the EVM, only to the sensor, and also to both.  No matter what I do, I cannot eliminate the offset.

  • When testing with our my design, firmware is involved.  We are powering up the sensor and taking readings, then powering it off when not used.  When using the EVM, there is no controller connected so the ADC is in its default state.  Nothing is connected to the I2C bus because it is just the EVM.

I am wondering if the strange offset on the EVM is because the ADC has not been configured yet.  What might be happening with the GND or CH0-CH7 pins when the device is off that could cause such an offset to occur? 

Our design and the EVM both have the COM pin tied to GND. Should we be using the COM pin in a different way?

Thank you for any insight you can provide.

  • Hi Ryan,

    It is strange that you are seeing this. Are you able to provide details on what exact sensor you are using? I too, have a suspicion on whether is may be caused by the device not being configured yet. I recommend configuring the device for single ended inputs to see if this behavior goes away. 

    As for the COM pin, this is typically connected to GND so I don't think this would be an issue. 

    Aaron Estrada

  • Aaron,

    Thank you for your reply.

    I have done further testing and can conclude we are seeing a DC offset as a result of long cable lengths between our sensor and the EVM.  When I use short cables, our DC offset goes away.  I do not believe we are having any issue at all with the EVM board.

    Have a great day,