PIC A/D conversion offset voltage varies with Vref; why?
by Shahryiar
(Dhaka, Bangladesh)
I am facing a problem with A/D conversion for PIC MCU. For a wide span of reference voltage the offset seems to vary and so precision of measurement falls drastically. Can you help me with this?
Comments for
PIC A/D conversion offset voltage varies with Vref; why?
A few things to check are the stability of the voltage source used as a reference by the ADC module, the quality of signal coming out of any signal conditioner that is being used, and the linearity of the response from the sensor or transducer in use.
Reference voltage levels that are not stable will introduce errors as the ADC uses it in the conversion process. If it isn't the same for two consecutive readings of the same voltage at the input pin there will be two different conversion results. Try precision voltage chips on board if power supplies aren't up to the job.
Most signal conditioners are used to amplify small signals and filter out noise. If the gain of amplification isn't constant the output won't be. Rare, but it could happen. More likely for noise to be the problem. Compare the input and output waveforms on a scope to see what is going on.
Sensor response linearity can be found in the datasheets. Just multiplying/dividing the result from a curved response will introduce errors or offsets.
