SDK 15.2: Recommendation on accurate SAADC readings of moisture sensor over time

Question

Hi, 
 I am sampling a moisture sensor using one of the analog pins without issues. However, since my circuit is battery powered, I need to account for voltage drop of a 3V coin cell. The question is how do I compensate for the voltage drop while the battery is draining? Do I base the readings while taking the battery voltage in consideration? 
 
 Thanks

hmolesworth · Accepted Answer

If the moisture sensor voltage is derived directly from the coin voltage in that the moisture value falls proportionally with falling coin voltage then use Vdd as the reference to get a ratiometric measurement (reference falls in balance with moisture sensor output) which compensate to a first degree the falling coin voltage. 
 If the moisture sensor output is independent of the falling coin voltage use instead the fixed reference in the SAADC. Note even with a fixed reference there will be some small change in measured moisture value, so in both cases additionally sample Vdd (channel 9) and use a look-up table to provide additional compensation if necessary.

SDK 15.2: Recommendation on accurate SAADC readings of moisture sensor over time

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