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ADS127L11: Sampling voltage fluctuation

Colin Chen
Intellectual 685 points
Part Number: ADS127L11
Other Parts Discussed in Thread: REF6225,

Tool/software:

Dear Experts,

The design circuit diagram is as follows. During debugging, we found that the sampled voltage value will experience a jump, which is the actual voltage value or half of the actual value. The reference benchmark is 2.5V, and the chip used is REF6225. The value written in CONFIG1 is 0X3B, CONFIG2 is 0X08, CONFIG3 is 0x16, and CONFIG4 is 0X80. The input voltage has been set to 2 * REF, but the sampled data still jumps. We are currently unsure of the cause and hope you can help us find the reason. Thank you very much

Design schematic diagram：

Configure registers：

  • 0
    TI__Mastermind 48501 points

    Hi Chen Colin,

    Today is a holiday in the US, please expect a response to your question next week. Thanks for your patience

    -Bryan

  • 0
    TI__Mastermind 44670 points

    Hello Chen,

    I noticed that you have used 'AVSS' as the ADC ground and 'GND' as the input ground.  These grounds should be connected together using the 0Ohm R67 resistor to provide a proper bias path for the inputs.  Can you confirm that GND and AVSS are connected together in your system?

    Also, you state that the ADC reading is either the correct 'actual' value, or 1/2 of the actual (expected) value.  This could be caused by missing the MSB on the SPI bus, possibly caused by using the wrong SPI mode. Using an oscilloscope or logic analyzer, please capture one full SPI frame for the conversion data, including /DRDY, /CS, SCLK, and SDO.  The ADS127L11 launches the MSB on the first SCLK rising edge, and your MCU should capture SDO data on the SCLK falling edge.

    Regards,
    Keith Nicholas
    Precision ADC Applications