AN2834 datasheet(23/45 Pages) STMICROELECTRONICS

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AN2834

How to get the best ADC accuracy

The solution is to add noise or some signal change (with uniform signal distribution e.g.

triangular sweep) to the input signal which pushes its level across 1-bit ADC level (so that

the signal level changes below 0x14A and above 0x14B level). This causes the ADC results

to vary. Applying software averaging to the different ADC results, produces the mean value

of the original input signal.

Example of implementation of this method is done by using a triangular generator with RC

coupling to the input signal (white noise generation is more complicated). Care must be

taken not to modify the mean value of the original input signal (so, capacitive coupling must

be used).

A very simple implementation of the quasi-triangular source which is generated directly by

the STM32x microcontroller is on Figure 22.

Figure 22. Simple quasi-triangular source using a microcontroller output

3.2.5

Matching the ADC dynamic range to the maximum signal amplitude

This method improves accuracy by a proper selection of the reference voltage or by using a

preamplifier stage to obtain the maximum possible resolution using the full ADC output

range.

Selecting a reference voltage (method for devices delivered in 100-/144-pin

packages only)

The reference voltage is selected in the expected range of the signal to be measured. If the

measured signal has an offset, then the reference voltage should also have a similar offset.

If the measured signal has a defined maximum amplitude, then the reference voltage should

also have a similar maximum value. By matching this reference voltage to the measurement

signal range, we obtain the maximum possible resolution using the full ADC output range.

In STM32x devices delivered in 100- and 144-pin packages, the ADC reference voltage is

connected to the external VREF+ and VREF- pins that should be tied to ground. This makes it

possible to match the reference voltage and the measured signal range.

For example, if the measured signal varies between 0 V and 2.5 V, it is recommended to

choose VREF+ = 2.5 V, possibly using a reference voltage like LM235 (see LM235 datasheet

for more details). Figure 23 illustrates these conditions.

Vin

MCU

Ain

OUT

VDD

Vin

Uin

Uout

ai170803

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