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ADS1220: Vref doesn't match the calculation

Anveil
Prodigy 40 points

Part Number: ADS1220

Tool/software: 

Here are my PCB document and register configuration:

#define ADS1220_CONFIG_REG0_PT1000 0x60 // AINP=AIN0, AINN=AIN1, PGA=1, PGA ON
#define ADS1220_CONFIG_REG1_PT1000 0x04 // DR=20SPS, NORMAL MODE, CONTINUOUS CONVERSION
#define ADS1220_CONFIG_REG2_PT1000 0x56 // EXTERNAL REFERENCE (REFP0, REFN0), 50Hz/60Hz REJECTION, IDAC=1000μA
#define ADS1220_CONFIG_REG3_PT1000 0x70 // IDAC1=AIN2, IDAC2=AIN3

I used the formula Vref = 2 · I_IDAC1 · R_REF to calculate the reference voltage, and the result is 2V. However, when I measured both sides of R43 with a multimeter, the reading was 1.519V.
Despite this discrepancy, using the measured 1.519V as the reference voltage for ADC conversion yielded accurate results.

  • 0
    TI__Intellectual 2410 points

    Hi Anveil, 

    Try decreasing the IDAC magnitude to the next lower setting (500uA).  Your current configuration violates the recommended operating conditions of the IDACs Compliance Voltage of AVDD -0.9V. This could explain the delta in the measured voltage of 1.519V to the expected value of 2V.

    Regards, 

    Andrew

  • 0 in reply to Andrew Kalthoff
    TI__Intellectual 2410 points

    Hi Anveil, 

    I took another look at your schematic, and I found a few more notes to add:

    1. C40 should be 10x the value of C39/C41. This will help prevent the common mode noise conversion to differential noise on the analog input. 

    2. Diodes D12 and D13 can be replace with series resistors. These can limit any current that would flow back into the pins. (Optional)

    3. You could short R39 and remove the Common Mode capacitors(C44 and C42) on the reference input. (Optional)

    Regards, 

    Andrew

  • 0 in reply to Andrew Kalthoff
    Prodigy 40 points

    Hi Andrew,

    Just wanted to let you know that I tried reducing the IDAC current, and it worked. Thanks for the great suggestion.
    I will also follow your advice to make the corresponding circuit adjustments.

    Best regards,
    Anveil