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ads1246 design choices

Ron Ying
Intellectual 285 points
Other Parts Discussed in Thread: ADS1246

Hi, there,

I wonder if you could help me on the following design choices:

1) have Vref close to the lower limit 0.5v (but above 0.5v) with PGA=2;

2) have Vref at about 2.4V (AVDD=5V, AVSS=0V) with PGA=16.

I see that that the PGA tends to drift more over temperature as PGA value increases; is case 1) a better design than case 2)? better = more accurate and more stable over a range of temperatures.

However I am concerned that Vref close to the lower limit, which might have some adverse effect as well.

Would you please comment over these two choices?

Thank you,

Ron

  • 0
    TI__Guru** 113765 points

    Hi Ron,

    I'm sorry for the delayed response.  Apparently your post was moved from one forum to this one and I missed seeing it.  Your question is not that easy to answer because noise is another factor.  Generally speaking the noise (SNR) improves with increased PGA gains.  Another improvement can be realized if the measurement can be made ratiometricaly.  These considerations are dependent on the type of sensor, the sensor range, sensor excitation, operating temperature, desired resolution and data rate.  The transfer function for the output code is inversely proportional to the reference.  As the ADS1246 requires an external reference, the reference source will most likely be the dominating factor for stability and noise.

    As far as the ADS1246 device performance is concerned, it will operate well within the reference region specified within the datasheet of 0.5V to (AVDD-AVSS)-1V.  As you have mentioned there are some trade-offs in operation using one method over another.  From a noise perspective I would use a higher reference voltage with increased gain.  However, there may be some increased drift as you've mentioned.  Also, depending on the actual sensor input voltage, you may gave some issues with the common mode input range restriction using higher gains.

    Best regards,

    Bob B