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ADS1291: Data Format convert to mV

Michael Liesenberg85
Mastermind 7560 points
Part Number: ADS1291

How do i calculate the output ADC data in mV from ADS1291?

On the datasheet page 28 "DATA FORMAT",   Vref/(2^23 -1) is the format

My ecg signal is around 8500 ADC Data and my Vref is 2.42V(internal reference and ADS1291 with 3V3 suppy).

Vecg= 2.42V*8500/8388607 = 2.45mV and what about the Gain? I am using Gain 12 so i would go and divide it like this Vecg/Gain.

Vecg/gain=2.45mV/12=0.2mV

Is this correct?

  • 0
    TI__Guru 55312 points
    Hello Michael,

    Thank you for your post and interest in the ADS1291.

    Your calculation is correct, however keep in mind that the equation only works for positive values.

    A more complete equation you can use is:

    1LSB = (Full Scale Range (2*VRef))/2^n = (2*2.42)/2^24 = 2.45mV

    The above equation provides the voltage seen by the input of the ADC, then divide by the gain of 12 to see the ECG output voltage.
  • 0 in reply to Alexander Smith
    Mastermind 7560 points

    Hi Alex,

    what about the external resistors connected to IN1P and IN1N.
    The values on the evaluationboard are 51kOhm. Does this effect the amplitude/Gain somehow?

  • 0 in reply to Michael Liesenberg85
    TI__Guru 55312 points
    Hi Michael,

    Like any resistor, the external 51kOhm resistors (as well as the resistance of the ECG leads) will have a voltage drop across them which has an effect on the amplitude of the voltage. In this case, the effect on the amplitude will be minimal.

    The impedance of the 51kOhm resistor is orders of magnitude smaller than the 100-1000MOhm input impedance of the ADS1291. When the two impedance's are compared, the 51kOhm external resistor can be regarded as negligible, as the 100-1000MOhm input impedance dominates the equation.