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ADS1261: Measure in unipolar mode and pga amplification

Stefano Bizzari
Prodigy 10 points
Part Number: ADS1261

I've configured my ADS1261 (on the ADS1261EVM board) in this way:
-) REFP: AVDD (internal) ; REFN: AVSS(internal)
-) MUXP: AIN3; MUXN: AINCOM

I've plugged a sensor that has a signal that goes from 0 to 5V between AIN3 and AINCOM. I can read ADC points from the evaluation software but I've 2 questions:

1) How can I reach 24 bit resolution? I see that the ADC values goes from 0 to 8388608; it seems it is using only 23bits; as far as I can understand, if I set that the reference has to go from AVSS (0V) to AVDD (5V) I think that the converter has to give me values referring to these 2 points.

2) When my signal is at (for example) 1 V I read around 1660000 ADC points which seems correct if we refer to the 23 bits above. But if I change the PGA gain to 2V/V I see that the points goes up to around 2210000. I figured out that they had to double (around 3300000 for example).

What am I doing wrong?

  • 0
    TI__Mastermind 45715 points

    Hi Stefano,

    I guess your sensor is floating and there is no connection between AINCOM and the ground or external DC bias voltage. 

    1. If so, you need a level-shift voltage. You can turn on the switch to enable the level-shift voltage on AINCOM by setting the VBIAS bit 4 of INPBIAS to 1. You should be able to see the code up to 16777216. Also, the internal PGA should be bypassed so that the input voltage range can be extended to the analog supplies.

    2. Since your input configuration is differential +/-5V input, so you got 166xxxx code which is correct corresponding to your configuration. When you change the PGA's gain from the default 1 to 2, the full-scale input range is reduced from +/-5V to +/-2.5V, so your conversion code will be double to 33xxxxx code, this is a correct output according to your configuration.

    Regards,

    Dale