Hi
I am using ADS1281 ADC for my project. For analog inputs I am using differential signal with a DC offset of 1.25V. My reference voltage values are 2.5V and -2.5V. My expectation is getting the numeric value of the difference of the input signals. However, I encountered at the output with a high DC offset which I did not understand. Did I miss some points while using the device?
Hello Osman,
In general, you're correct. My guess is that your differential voltage source has offset but we'll have to step through the logic and see if that's true.
V_IN is defined as V_IN = V_AINP - V_AINN. If there is a common mode voltage, V_CM, (which I assume you've defined as DC offset) on each of the differential pin, then they will cancel out.
With that said, there are some limitations to what the common mode can be, and mostly we need to ensure the absolute voltage on the pins do not exceed the supply voltages (AVSS and AVDD). Most people put the common mode at mid supply so they can make use of the full dynamic range.
Because your VREFP and VREFN are 2.5V and -2.5V, I assume AVSS and AVDD are -2.5V and 2.5V, respectively. So, V_CM = 1.25V should be okay.
The ADS1281's offset error specification is shown below:
You did not say what the offset but if you average out your sample set, to attain the DC offset, what do you get? Converting that averaged code value to voltage should fall within the +/-200uV spec above. If it does not, then we'll need to verify the ADC's offset.
You can verify this by disconnecting the differential signal, tying AINP and AINN together and to mid supply (which I assume is 0V in your case) and taking a significant number of samples and averaging together. Assuming all noise sources are uncorrelated and random, you should get an averaged output code that falls within the spec range. If you don't we'll have to do some other investigations, but if it does, then you can be sure that your differential voltage source is the one that is providing some offset.
Best,
-Cole
