AN1830 Datasheet, PDF (8/15 Pages) STMicroelectronics – ACHIEVING HIGH ACCURACY AND 13-BIT RESOLUTION WITH ST7LITE2 ADC

ACHIEVING HIGH ACCURACY AND 13-BIT RESOLUTION WITH ST7LITE2 ADC
3 TECHNIQUES TO IMPROVE THE ACCURACY
3.1 AMPLIFIER OFFSET COMPENSATION
The ADC must be offset-compensated because the output of the amplifier corresponding to
0V input is non-zero (equal to V_OFFSET). This output corresponding to 0V input can be ob-
served without having to apply 0V at the input. This is done by using the control signal AM-
PCAL.
The Amplifier takes the input voltage from the analog pad (after being multiplexed) and ampli-
fies it by a factor of 8. In addition to this, it also introduces an offset at the output, which varies
from one part to another due to manufacturing process variations.
For this reason the ADC should be calibrated by configuring 0V once, storing it as an offset
and subtracting it from all amplified-and-converted digital codes.
Thus compensated ADC output = ADC Read data - OFFSET CNT (when AMPCAL=1)
Refer to Table 5 to understand how it is used.
3.2 SOFTWARE AVERAGING
Software averaging is a method used to reduce the effect of erroneous ADC output because
of noise or wrong conversion for fixed analog input.
As we take the average of several readings, these readings must correspond to the same an-
alog input voltage. Take care that the analog input remains at the same voltage during the
time period when the conversions are done. Otherwise digital values corresponding to dif-
ferent analog inputs will get added and introduce errors, i.e. the analog input should not
change between the different readings considered for the averaging.
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