ADS1261-Q1: Pseudo-Code for ADS126x single ended measurements

Hi Dimitri,

Measuring voltages close to to AVSS is always a bit of a challenge. If you have the internal PGA of the ADS1261 or ADS1261-Q1 enabled, then neither of the positive or negative inputs can accept voltages lower than "AVSS + 0.3 V", or else the PGA outputs will saturate and you'll get an erroneous measurement result.

I'd recommend looking at the ADS1261 PGA common-mode range calculator, which you can download from here: http://www.ti.com/tool/ADS1261-EXCEL-CALC-TOOL 

To measure a 0.3V or smaller signal (with respect to ground) on the ADS1261 there are a few options:

1. Bypass the internal PGA
   By bypassing the PGA, your single-ended input voltage can swing between AVSS and AVDD without any issues. However, when the PGA is bypassed the input source is connected directly to the input of the delta-sigma modulator which has a much lower input impedance. If the signal source does not have a low output impedance, then you may see a gain error in the measurement due to the loading on the source.
    
   
   
2. Use a bipolar analog supply voltage
   The AVDD and AVSS voltages on the ADS1261 can be configured to bias the input signal to a mid-supply voltage. Typically +/- 2.5V supplies rails are used, but if you need measure a voltage between 0.25V and 3V, then you might consider configuring AVDD to be 4V and AVSS to -1V to allow the input signal to swing between -0.7V and 3.3V, for example). The disadvantage to this approach is the additional circuitry required to generate these supplies, if acceptable supply voltages are not already available in your system.
   
   In the case of a traditional +/- 2.5V bipolar supply, you could take advantage of the LM27762 to generate both supplies. However, if you only need a slightly negative voltage for AVSS, such as in your case, then the LM7705 might be a good option for generating a -230 mV AVSS supply rail. The ADS1261 allows for up to a 5.25V analog supply, so you could keep AVDD at 5V and connect AVSS to the -230 mV supply.
   
   
   
3. Use an external amplifier to level-shift the input signal to a desired common-mode voltage
   Refer to https://e2e.ti.com/blogs_/b/analogwire/archive/2016/09/20/three-ways-to-scale-an-analog-input-signal. There are various circuit configurations that can level shift the input voltage. Most require some kind of reference voltage; however, you could most likely use the internal 2.5V reference of the ADS1261 for this purpose. The disadvantage to this approach is that, again it is yet another component, and the noise of the op-amp will add to the overall noise in the measurement result, slightly degrading the overall system resolution.
   
   

If you configure your circuit so AINP swings between 2.5V and 3V, and AINN is fixed to VREF (if AVSS = 0V, then VREF = AVSS + 2.5V = 2.5V), then the differential voltage (AINP - AINN) will always be less than 2.5V and you can utilize the internal 2.5V reference for this measurement.

However, if your input signal is somehow proportional to the 5V analog supply voltage (for example if you were measuring a bridge voltage that uses the 5V analog supply as an excitation voltage), then I would definitely recommend using the 5V supply voltage as your reference voltage to make the measurement ratiometric.

Let me know if that helped answer your questions.