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ADS8668: False Positive Alarm on Low Threshold for Channel 7

Candelaria Figliolo Cajal
Prodigy 10 points
Part Number: ADS8668

Tool/software:

Hello,
I am developing a monitoring system using multiple ADC channels (0, 3, 4, 5, 6, 7).
I’m utilizing the alarm pin and associated registers to notify me when any channel value falls outside the defined lower and upper thresholds.

This setup works correctly for all channels except channel 7. I’m encountering an issue where the low threshold for channel 7 always triggers when the value written to the low threshold register exceeds 0x01F, even though the actual ADC value is well above the intended threshold.
The threshold I want to set is 0x71F. However, even setting it to 0x02F causes the alarm to trigger prematurely. I’ve read the ADC counts from the register, and they are within a range that should not activate the alarm.
Has there been any known issue reported regarding channel 7 or any other alarm related issue? What could be causing this behavior, and is there a recommended solution?

Best regards,
Candelaria Figliolo Cajal

  • 0
    TI__Expert 3820 points

    Hello Candelaria, 

    Welcome to TI's E2E Forum! Thank you for your question! 

    Just to make sure that I understand, you configure the lower alarm threshold for channel 7 to 0x71F, but the alarm does not trigger until the ADC output reaches 0x01F, correct? 

    Are you configuring any value for the Hysteresis? 

    Could you please share the register configurations you have for the channel 7 alarm, both the low and high threshold as well as the hysteresis? If you could also share the values of a functional channel that would also be helpful

    Are you noticing the alarm being triggered through the register read back or through the alarm pin? 

    Best regards, 

    Yolanda 

  • 0 in reply to Yolanda Gonzalez
    Prodigy 10 points

    Hello Yolanda,
    thank you for your reply. Here are more details of my problem.

    What a notice is a false positive. So the alarm triggers even though it should not trigger.
    I am configuring the hysteresis to be 0.

    The value I read from the ADC is 2067(0x813) and the value I configured in the adc low threshold is 1823(0x071F). And the behavior I notice is that it always triggers the low threshold as long as the low threshold is higher than 0x01F, (0x01F comes from me experimenting putting down the threshold until it stops triggering). 

    Note: The error counter shows an internal calculation of how many times I think that it should have triggered, this is not related to the alarm registers reading. The alarm registers are in the next image.
    value_counts is what I read from the ADC. lower_tolerance_counts is the lower threshold configured for the alarm. upper_tolerance_counts is the higher threshold I configure for the alarm.

    I notice the alarm being triggered both in the alarm pin and in the alarm registers.



    For reference I show you the other channels which are not triggering and are working correctly:





    I send you in the following image the spi communication for configuring the channel 7 alarms:


    In the following image is the reading of the ADC channel 6 counts, with the command for next receiving channel 7:


    And the following is the reading of the channel 7:



    Best regards,
    Candelaria

  • 0 in reply to Candelaria Figliolo Cajal
    TI__Expert 3820 points

    Hello Candelaria, 

    Thank you for sharing the screenshots, that is very helpful.  

    It helped understand that even though 0x71F is being configured as the lower threshold, it appears to be responding to 0x1F.  

    Just to make sure, all channels are configured to the same range or at least the appropriate range for the expected input, correct? 

    Could you please also share a schematic of the ADC?  I would like to confirm the grounding configuration of the device as well as the channels. Would it be possible to probe the Ch7 ground pin? or do a differential measurement between AIN_7P & AIN_7GND? is the ~2.5V seen at the input too?

    Best regards, 

    Yolanda