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SOFTWARE TECHNIQUES FOR COMPENSATING ST7 ADC ERRORS

4.2.2 Use of external DAC

A precise external DAC (Digital to Analog Converter) can be used to provide a software con-

trolled voltage reference. The output received can then be used by the ADC to get the digital

value.

The comparison between the expected and actual value will provide the calibration constant.

Using the DAC creates a closed loop system and hence this technique is very effective.

Disadvantages

1. The disadvantage of this method is the cost of the extra device which is generally high.

2. The DAC will have its own errors which will effect the calibration, therefore a DAC with very

good accuracy is required.

Figure 17. Hardware setup for ADC calibration using DAC

4.2.3 Maintaining a Lookup table

You can maintain a lookup table to correlate the ADC conversion results with the correct re-

sult. This requires converting each voltage step with ADC and storing the digital results for

each digital code in non volatile (program) memory. This will use lot of program memory.

For example to maintain the lookup table for all 1024 digital codes would require program

memory equal to 1024 words. This is equivalent to 2KB memory.

For an 8-bit ADC, the program memory required for the lookup table would be 256 bytes.

Comments:

1. This is the fastest method of getting the correct values for any analog input voltage.

2. It is a very time consuming process to take the readings for all 1023 steps and then maintain

the table in the software.

DAC

Microcontroller

Control

ADC

AN1711 데이터시트(PDF) 27 Page - STMicroelectronics