AN1636 datasheet(7/42 Pages) STMICROELECTRONICS

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UNDERSTANDING AND MINIMISING ADC CONVERSION ERRORS

Figure 3. Electrical diagram of typical ADC application

Configuring the analog pin

Choose any I/O port that has analog input capability (AIN alternate function) and configure it

as floating input. You can do this by writing ‘0’ in the DDR and OR register bits of the corre-

sponding port. At reset, most of the ST7 IOs are configured by default as floating input.

The pin should NOT be configured as floating input with pull-up. This configuration reduces

the ADC accuracy. The reason being the potential divider formed between the pull-up resist-

ance and RADC. Also some current flows from VDD to the analog source. This current is drawn

from the VDD supply. Also there is a potential divider formed between VDD, RPU and RAIN,

where RAIN is the series impedance of the voltage source.

Figure 4. Analog input with pull-up

Configuring the analog input as floating input with pull-up ( instead of floating input ) will cause

more current to be drawn from the VDD supply.There is also an affect on the accuracy of the

ADC and the digital output converted by ADC may not be accurate.

AINx

VDD

±1

µA

0.6V

CADC

VAIN

RAIN

10-Bit A/D

Conversion

RADC

CAIN

\/\/\/\/\/\/

CADC

RADC

VSSA (Analog Ground)

VDD

RPU

NOT RECOMMENDED

VIN

Current

from VDD

RPU should not be enabled.

\/\/\/\/\/

RAIN

AN1636 Datasheet(PDF) 7 Page - STMicroelectronics