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How to get the best ADC accuracy

AN2834

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DocID15067 Rev 2

3.2.2

Reference voltage / Power-supply regulation

The power supply should have good line and load regulation since the ADC uses VREF+ or

VDDA as the analog reference and the digital value is the ratio of the analog input signal to

this voltage reference. VREF+ must thus remain stable at different loads.

Whenever the load is increased by switching on a part of the circuit, the increase in current

must not cause the voltage to decrease. If the voltage remains stable over a wide current

range, the power supply has good load regulation.

For example: for the LD1086D2M33 voltage regulator, the line regulation is 0.035% typical

when VIN varies from 2.8 V to 16.5 V (when Iload = 10 mA), and the load regulation is 0.2%

when Iload varies from 0 to 1.5 A (please refer to the LD1086 series datasheet for details).

The lower the line regulation value, the better the regulation. Similarly, the lower the load

regulation value, the better the regulation and the stability of the voltage output.

It is also possible to use a reference voltage for VREF+, for instance the LM236, which is a

voltage reference diode of 2.5 V (refer to LM236 datasheet for more details).

3.2.3

Analog-input signal noise elimination

Averaging method

Averaging is a simple technique where you sample an analog input several times and take

the average of the results by software. This technique is helpful to eliminate the effect of

noise on the analog input in case of an analog voltage that does not change often.

The average has to be made on several readings that all correspond to the same analog

input voltage. Make sure that the analog input remains at the same voltage during the time

period when the conversions are done, otherwise you will add up digital values

corresponding to different analog inputs, and you will introduce errors.

Adding an external filter

Adding an external RC filter eliminates the high frequency. An expensive filter is not needed

to deal with a signal that has frequency components above the frequency range of interest.

In this case, a relatively simple lowpass filter with a cutoff frequency fC just above the

frequency range of interest will suffice to limit noise and aliasing. A sampling rate consistent

with the highest frequency of interest will suffice, typically 2 to 5 times fC.

Note:

The R and C that form the external filter should have values that match the conditions

described in Section 3.2.4 and Section 3.2.7.

3.2.4

Adding white noise or triangular sweep to improve resolution

This method combines hardware and software techniques to improve precision. From a

software point of view, this method uses averaging (oversampling) and from a hardware

point of view, it uses signal modification/spreading/dithering.

Averaging can be used in cases where the input signal is noisy (some signal change is

necessary in order to be able to calculate an average) and the requirement is to obtain the

mean value of a signal. A problem appears when the input signal is a very stable voltage

without noise. In this case, when the input signal is measured, each data sample is the

same. This is because the input signal level is somewhere between two ADC word levels

(e.g. between 0x14A and 0x14B). Therefore it is not possible to determine the input voltage

level more precisely (e.g. if the level is near to 0x14A or near to 0x14B level).

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