This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS114S08B: Noise Issues

Part Number: ADS114S08B

Hello,

We are using the ADS114S08B to measure a servo's feedback pot. Frequently, whenever the servo limit switch open, it will cause the DRDY pin to stop going low and the internal 2.5V reference will go to 0V. The noise could be coming through the input to the A/D or through the AC/DC switch mode power supply that is fed with the same 120V that powers the servo. The schematic for the A/D input circuit is below. We've tried to mitigate the noise on the AC lines and we used shielded cabling for the feedback pot wires and the A/D is still dropping the 2.5V reference and not dropping the DRDY pin.

Any help is appreciated,

Thanks,

Jake

  • Hi Jake,

    It sounds like there is an event that is resetting the ADS114S08B.  This can happen if an absolute maximum rating is exceeded (often from a transient) or if a supply should collapse in a black out or brown out condition.  If the cause is from a transient, you may need to add a TVS diode on the analog inputs to help stabilize the input voltage.

    There was one thing I should mention relative to your schematic and that is the REFOUT of the ADC.  The internal reference is 2.5V and you have it referenced to AGND.  Normally you would reference this to AVSS so that there is sufficient headroom from the bipolar supplies.  If AVDD is 2.5V and the reference is 2.5V, you would want to make sure that the reference remains stable at 2.5V even if AVDD drops to 2.45V.  This will not happen with this configuration.  I suppose you want to use the REFOUT to drive a sensor bias.  In this case I would recommend that you use +2.5V from you AVDD supply instead of REFOUT.  I would also recommend that you set REFCOM to AVSS instead of AGND.  I see no downside in doing this and the upside would be to have a stable internal reference voltage of 2.5V even if the AVDD supply should dip or drift slightly.

    Best regards,

    Bob B

  • Bob,

    Thanks for the response. Unfortunately schottky clamp diodes on the input did not solve the issue.

    We have an interrupt whenever the DRDY pin drops low that gets all the values from the A/D. So whenever the DRDY stops dropping low we stop getting data. We have a timeout in our software so that if the convertor stops dropping the DRDY pin then we drop the reset pin to reset the chip. I did see in the datasheet that the chip has a SPI timeout bit. When that times out will it start to resend the data instead of us manually resetting the A/D?

    Thanks,

    Jake

  • Hi Jake,

    As I said previously, from your description something is resetting the ADC either by a transient or collapse of the supplies.  When the device resets, the registers will reset to the default values and the ADC will stop converting.  You can verify if the device has reset by reading back the registers to see if they are the default values.

    The SPI timeout feature is used for situations where the CS is tied low permanently.  The timeout clears the SPI bus similar to toggling CS.  The timeout will not restart the ADC or correct any register settings.

    You said you used a 'Schottky clamp' diode.  What is the part number of the device you were using?  And where precisely did you place the diode(s)?

    Best regards,

    Bob B

  • Bob,

    The part number for the diodes are B160-13-F.. They were on another board I have lying around so I just tried them. I placed one from "Ain1-1_IN" to AGND and the other from "Ain1-1_IN" to +2.5V.

    Thanks,

    Jake

  • Hi Jake,

    Basically you are making an ESD type array to the supplies and it is possible that the supplies are not robust enough to handle the transient voltage/current.  If there is an EMI event, it is possible that the other inputs may also be affected too.  I would suggest probing around the board to see what is actually happening.  I would start with an oscilloscope on the supplies to see if they dip.

    Best regards,

    Bob B