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ADS122C04EVM: data value hunting occurrence according to SPS

YUEUN JEONG
Prodigy 20 points
Part Number: ADS122C04EVM
Other Parts Discussed in Thread: ADS122C04, ADS112C04

While developing the product through ADS122c04, there was DATA hunting according to the SPS, so the same test was conducted using the EVM board, and the same hunting occurred on the EVM board.

I think the same data range should come out even if the SPS changes, but it is not.

I used Delta-Sigma ADC Evaluation Software.

Why are the data results different? 

Below is my data.

1. 20SPS 

 

Register 0 = 0x08                MUX[3.0]=b0000(+:AIN0,-:AIN1), GAIN=b100(x16), PGA=b0(Enable)

Register 1 = 0x08                DR[2.0]=b000(Data Rate:20sps), MODE=b0(Normal), CM=b1(Continuous), VREF=b00(2.048V), TS=0(disable) 

Register 2 = 0x04                DRDY=b0, CNBT:b0, CRC:b00, BCS:b0, IDAC=b100(250uA) 

Register 3 = 0xB8                I1MUX[2.0]=b101(IDAC1=REFP), I2MUX[2.0]=b110(IDAC2=REFN), reserved=b00 

2) 45SPS


Register 0 = 0x08                MUX[3.0]=b0000(+:AIN0,-:AIN1), GAIN=b100(x16), PGA=b0(Enable)

Register 1 = 0x28                DR[2.0]=b001(Data Rate:45sps), MODE=b0(Normal), CM=b1(Continuous), VREF=b00(2.048V), TS=0(disable) 

Register 2 = 0x84                DRDY=b1, CNBT:b0, CRC:b00, BCS:b0, IDAC=b100(250uA) 

Register 3 = 0xB8                I1MUX[2.0]=b101(IDAC1=REFP), I2MUX[2.0]=b110(IDAC2=REFN), reserved=b00 

  • 0
    TI__Guru** 113765 points

    Hi YUEUN JEONG,

    I am not totally clear but what you mean as 'DATA hunting'.  Do you mean that the result is not a constant value?  You will not see a constant value due to noise.  Noise is inherent in any ADC and with higher resolution (smaller value of measurement) the noise appears to be greater.  For example, the ADS112C04 is a 16-bit version of the ADS122C04 which will appear to be more stable with the output code as compared to the ADS122C04 which is 24-bit.  But using a gain of 16 and the internal reference the ADS112C04 can only resolve to 3.9uV whereas the ADS122C04 can resolve to 15nV.

    If you look at the ADS122C04 datasheet you can see that the lowest possible noise is with shorted inputs.  This is where the reference noise and any external noise from the analog inputs is removed from the measurement and just ADC noise is present.  The noise tables appear in section 7.1 of the datasheet.  Table 1 shows the noise when using the internal reference, PGA enabled and various data rates.  Note that the noise changes with gain and data rate.  Also note that the noise tables are based on time (750ms) and not number of samples.  In this way all data collected will be absent from drift over time at the various data rates.  If you try to match the datasheet performance you would use 15 samples for 20sps and 34 samples for 45sps.

    Based on your results it appears that you are getting better than the typical performance from the ADS122C04 when compared to the datasheet tables.

    The other difference I see in the screen shots of the data is that the mean value is slightly different between 20sps and 45sps.  As it is unknown what you have connected to the inputs it is difficult to say what is causing that slight difference.  This could be related to PGA bandwidth changes or temperature drift.  It could also be related to slight external changes in input voltages relative to the circuit path.   I would suggest comparing the two data rates based on the mux selection of AINP and AINN shorted to (AVDD + AVSS) / 2

    Best regards,

    Bob B