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ADS7828-Q1: AIN reads as though reference is changing from from 2.5V to 3.3V

Ryan Alford
Prodigy 20 points
Part Number: ADS7828-Q1

We are using the ADS7828-Q1 in single ended mode. All 8 channels are used on our design. We are using the internal reference, and the chip is powered with +3.3V.  On the REFIN/REFOUT pin, we have a 0.1uF capacitor.  (Schematic snippet attached.) Our I2C bus is configured for 100kHz. 

We are testing by inputting a 1.03V signal into the inputs.  With the 2.5V reference, the signal is reading correctly at a 1.03V over 2.5V scale, or 41.2%

After many back to back reads, suddenly we are reading 31.1%. It is as though we are reading the same 1.03V voltage over a 3.3V scale.

Converter PD1,PD0 settings have been verified many times. We are leaving the converter on continuously and using the internal Ref left on continuously.

The issue seems difficult to reproduce. As a next step we will attempt to closely monitor the VDD pin and REFIN/REFOUT pin to see what they are doing when the issue occurs.

Is there anything that would cause the chip to suddenly change it's internal reference (or scale) from 2.5V to 3.3V?
Thank you,

Ryan

  • 0
    TI__Mastermind 22950 points

    Hi Ryan,

    When using the internal reference, it should be at 2.5V and unless you are providing an external reference ( which you are not) so I don't see how it can change. I would be a little worried about the reference not being settled when performing a measurement but from your post, you are leaving the reference on at all times. Is this correct? If you are turning the reference off, then there would need to be adequate time for the reference to settle after turning it on. 

    Have you been able to probe the REFOUT pin and monitor that? Is there anything else connected to REFOUT or VDD that is not shown in the schematic?

    I think monitoring VDD is also a good point and I would be interested in seeing the behavior of VDD when the measurements are skewed. 

    Regards,
    Aaron Estrada

  • 0 in reply to Aaron Estrada
    Prodigy 20 points

    Aaron, thank you for your reply.   We have since done some monitoring of the REFIN/REFOUT along with +3.3V.  We are getting noise on our +3.3V power supply and that is causing REFIN/REFOUT to jump as high as +3.2V!!!!   (Even briefly).   During these events we have monitored, the data read is just fine.  We suspect the times where data scales to a +3.3V range that the event is substantially larger.

    We are searching for the source of the +3.3V noise.  In the future we plan to add an external reference.   In the near term, we plan to power down the ADC between conversions.  Currently, we never power it down.   The chip seems to be operating very robustly considering the noise we are seeing is easily reproduced.

    At this time I do not have further questions so I will mark this issue as resolved.

    Thank you for your input, and have a nice day!