AN1711 datasheet(18/40 Pages) STMICROELECTRONICS

18 / 40 page

18/40

SOFTWARE TECHNIQUES FOR COMPENSATING ST7 ADC ERRORS

Practical measurements

You can test how this works in practice using a hardware setup like that shown in Figure 8.

The objective is to eliminate the errors resulting from noise in the analog input pin of the mi-

crocontroller. The loop takes 16 conversions and finds if the values received are within the

range. If the values are out of range, a new conversion cycle starts, ignoring the results re-

ceived and resetting all the variables.

A switch connected to port pin can be used to trigger the software to send the conversion re-

sults to the PC using RS232 communication. The results can be displayed on the PC screen

using the HyperTerminal as shown in Figure 11.

You can use a function generator to introduce noise in the pin next to the analog signal being

converted and to generate conversion errors.

Figure 11. Illustration of noise filtering

Comments

1. The code eliminates the outlier values even if the first conversion in the loop is noisy.

2. Note that the conversion loop is restarted when a noisy value is received, so if the environ-

ment is very noisy then it is very important to define the proper range for filtering the results.

3. In noisy environments, the conversion loop could take too much time, making endless con-

versions and rejecting values. You can avoid this by putting a timeout variable in your pro-

gram.

4. This technique filters the same way as the histogram technique but it is more efficient in

terms of RAM usage and execution.

5. This technique is more accurate than averaging because it takes care of outlier results,

however it uses more RAM and takes more execution time for the same number of conver-

sions.

Noisy conversion

Maximum

converted

value

Minimum

converted

value

Average

Rejected

value of

loop

AN1711 Datasheet(PDF) 18 Page - STMicroelectronics