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ADS1298: ADC data from ADS1298 through MSP430G2553

Mark Philip Felipe
Genius 10820 points
Part Number: ADS1298
Other Parts Discussed in Thread: MSP430G2553, , ENERGIA

Hi Team, we are seeking assistance on this matter.

Working on some project where we need high resolution of around 0.0001mV. We used the ADS1298 ECGFE Rev B board interfacing it with MSP430G2553.

We encounter some problem

1.) We are giving the differential input to the adc to channel number 2 with the two 10k multiturn potentiometers and using the IDE Energia to print the data on serial monitor. But the thing is that the raw value of adc doesn't go to zero instead when I give the differential input zero the adc raw value goes to negative. 
2.) And when we plot the data on Microsoft excel the data is too noisy also which I have somewhat cleared by applying input RC low pass filters with low cutoff frequency.
And side by side I am converting the raw adc data to millivolts by using the formula,
LSB = (2 x VREF)/ Gain / (2^24 - 1) 
Millivolts = LSB * (Raw adc data)
3.) Now when the differential voltage is of less millivolts the on board voltage difference and the data on the serial monitor matches but as the difference increases the on board actual voltage does not match the voltage on serial monitor.
Thank you very much in advance for your support.
Regards,
-Mark
  • 0
    TI__Genius 11550 points

    Hi Mark,

    Thank you for the post. Please see my inline comments below.

    1.) We are giving the differential input to the adc to channel number 2 with the two 10k multiturn potentiometers and using the IDE Energia to print the data on serial monitor. But the thing is that the raw value of adc doesn't go to zero instead when I give the differential input zero the adc raw value goes to negative. 
    >> When you short the input to the ADC (differential input zero), you will see the offset voltage on the ADC's output. The ADS1298 electrical characteristics table (page 13) shows the typical offset error of +/-500uV. 
     
    2.) And when we plot the data on Microsoft excel the data is too noisy also which I have somewhat cleared by applying input RC low pass filters with low cutoff frequency.
    And side by side I am converting the raw adc data to millivolts by using the formula,
    LSB = (2 x VREF)/ Gain / (2^24 - 1) 
    Millivolts = LSB * (Raw adc data)
    >> Table 1 to Table 4 summarize the noise performance for the ADS1298 with different PGA gain, output data rate, ADC mode (HR/LP), analog supply, and the reference voltage. You can use them as a reference to what to expect in your system.
     
    You will probably need some external gain stages and filtering to achieve the 0.1uV (0.0001mV) resolution that you need in your design. Without gain stages and filtering, your design will be limited by the noise performance of the ADS1298.
     
    3.) Now when the differential voltage is of less millivolts the on board voltage difference and the data on the serial monitor matches but as the difference increases the on board actual voltage does not match the voltage on serial monitor.
    >> You will see some gain error for full-scale input (Typ GE = +/-0.2% excluding voltage reference error). Can you please provide some of your results? 
     
     
    Thanks.
    -TC