AN2719

ADC errors

11/22

2.5.2

Reference voltage source

One of the biggest potential sources of errors in an ADC is the reference voltage. It is

important to look at three reference voltage specifications: temperature drift, voltage noise,

and load regulation (input resistance of reference input on STM8 ADC converter is >60 k

Ω).

2.5.3

Temperature influence

Temperature has a big effect on ADC precision. Mainly two specifications are important:

offset drift and gain drift. Those errors can be compensated in microcontroller firmware. One

method is to fully characterize the offset and gain drift and provide a lookup table in memory

to correct measurement due to temperature change. The ADC in each MCU device must

then be compensated individually - therefore this calibration takes additional cost and time.

The second method is to recalibrate the ADC when temperature change reaches given

values. The effect of temperature on ADC accuracy is usually not specified seperately for

microcontrollers because it is lower than other effects. It is included in the values for the

other error types.

2.5.4

AC performance

Usually ADC performance is good only if input signals are from DC level up to Nyquist

frequency. This is because an ADC design contains not only resistors but also capacitors

(and parasitic RLC elements). Therefore frequency response degrades ADC performance if

frequency increases. In case of AC measurement, the ADC frequency parameters must be

studied; mainly signal-to-noise ratio (SNR), total harmonic distortion (THD) and spurious-

free dynamic range (SFDR). Additional parameters can be signal-to-noise and distortion

ratio (SINAD) and effective number of bits (ENOB).