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Original question:

ADS1235: ADS1235

ADS1235: Reading a defined out of range value from the AD1235

user6228055
Prodigy 90 points
Part Number: ADS1235
Other Parts Discussed in Thread: ADS1261

Hi

First I try to describe our planned configuration of the AD1235. We have an external +2.5V reference on REFP0/REFN0 and also connect this +2.5V to AIN0 as reference input for a single-end measurement.
The input voltage range of our application (single-ended) is 0.35 .. 4.7V and we use the internal PGA with gain=1.

Now my question: Is there a way to get an out of range value (range of our application) back from the AD1235 (e.g. 0x000000 or something else)? We think about a diagnostic function of the ADC to read a defined value from the ADC which is out of the range of our application.
What happens if I connect 0V to one of the inputs and measure it referenced to my +2.5V (e.g. 0V @ AIN5 / +2.5V @ AIN0)? So the differential input voltage would be -2.5V, that means it's out of the PGA range (+0.3V .. +2.2V). what is the result of such a conversion?
I think with a bypassed PGA we could do that, but with the included PGA?

Best regards
Oliver

  • 0
    TI__Mastermind 44680 points

    Hi Oliver,

    With a 2.5-V reference and PGA gain of 1 V/V, the ADS1235 can measure differential voltages of +/- 2.5V. However, with a 5-V unipolar supply (AVDD = 5V, AVSS = 0V), the PGA will only allow absolute input voltages between 0.3V and 4.7V before the outputs of the PGA begin to saturate. Therefore, if you were to input 0V (or 5V) and measure it with respect to 2.5V, the PGA output would be saturated and outputting less than 2.5V differential, which is not outside of the ADC's output code range, but is outside of the PGA's linear range.

    If you wanted to test the ADC, you could first try shorting the inputs to the 2.5V signal on AIN0. The ADC should output a very small voltage (which can be positive or negative, depending on the noise and the offset). Ignoring offset you might see +/- 20 codes or so around zero, so output codes could range anywhere between 0xFFFFEC (2's complement for -20) to 0x000014 (+20 in decimal).

    Additionally, if you wanted to over-range the ADC and see the output code clip to either +/- full-scale (i.e. 0x7FFFFF or 0x800000), you could measure your input signal with an input voltage close to 0V or close to 5V (with respect to the +2.5V signal on AIN0) and then increase the PGA gain to get the output code clip.

    FYI: There is a PGA common-mode range calculator and code conversion calculator for the related ADS1261 device, which is equally applicable to the ADS1235, whcich you can download from here: http://www.ti.com/lit/zip/SBAC200. The main difference between the ADS1261 PGA and ADS1235 PGA are the available PGA gains.

  • 0 in reply to Christopher Hall
    Prodigy 90 points

    Hi Chris,

    The mentioned Excel Tool is very helpful, thank you very much.
    Beside the +2.5V reference connected to AIN0, we plan to connect AGND to AIN5. So if we measure between AIN0 & AIN5 (U_diff 0 +2.5V) and set gain to 128, the PGA output should be clipped to its maximum value. This value is definetely out of our application range, so we can check the ADC on that way.
    Thank you!