mdot Analog in question
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Leon Lindenfelser.
Bob GiffordHi There, using pin 15 as an analog input on the Mdot to read our battery voltage. I have an error in the value we read and I am attempting to understand it. Our battery can go up to 5.25volts so we used a divider to cut the voltage in half at pin 15. I have the MDot in a 25 second cycle where it reads and reports the voltage over LORA every 25 seconds. All of the data comes across just fine. If I look at the pin with a volt meter I see the value decrease 100mv during the read conversion. I looked at the data sheet for the processor and it indicates that the input would like less then a 50K impeadence looking out. I had originally used two 100K resistors to keep my current consumption low. I changed them to 10K and I still see the voltage drop 100mv on pin 15 during the read. Whne I set the node at 2v (Vbat at 4) I end up with 1.885. Seems like the conversion is loading the pin 15 node. I tried adding a small cap and got the same response. Anyone else have issues with accuracy when reading DC voltages this way?
Leon LindenfelserHi,
I would expect the voltage to drop during a read as that will add impedance. So your numbers with the 10k resistors sound pretty close with a 50K internal impedance. With the 100K resistors I would expect a greater drop on read. That difference is what’s puzzling to me but the drop is expected.
Kind regards,
Leon