AN672
APPLICATION NOTE
OPTIMIZING THE ST6 A/D CONVERTER ACCURACY
INTRODUCTION
When using the internal Analog to Digital Converter of the ST62 family and maximum A/D converter accuracy is required, it is desirable to filter out any noise present on the analog input,
but also noise present on the ground and V
voltage reference of the A/D converter. Good decoupling must be made with capacitors on the
analog input and between V
state while the conversion is in progress, so as to minimize noise injected into V
eration of the micro-controller itself.
Finally, when enough time is available, it is highly recommended to make several successive
A/D conversions and take an average of the results. This is the most effective way to get the
most accuracy out of the ST6 family A/D converter.
and ground. It is also recommended to put the MCU in wait
CC
supply lines of the MCU as VCC is also the
CC
by the op-
CC
The following code fragment demonstrates a burst of 256 successive measurements, after
which the average is put into the accumulator. The whole routine takes approximately 30 milliseconds with an 8 MHz clock. When less time is available, it is of course possible to reduce the
number of conversions: 8, 16 or 32 conversions also give good results, although the most conversions give the best results.
Rev. 2.0
AN672/1104 1/3
1
Optimizing The St6 A/d Converter Accuracy
;******************** SUBROUTINE AVERAGE ***********************
;description: measures ADC input 256 times and stores average *
; of the 256 measures into accumulator *
;***************************************************************
average
lid ior,10h ; global enable interrupts
clr aver_lo ;aver_lo, aver_hi and count are RAM registers
clr aver_hi
ldi count,255
aver1
ldi adcc,10110000b ; start conversion with interrupt
wait
ld a,adc
;======================= two byte addition of adc to 16-bit word:
add a, aver_lo
jrnc aver2
inc aver_hi
aver2 ld aver_lo,a
;======================= end of two byte addition
ld a,count
jrz aver4
dec count
jp aver1 ;do it 256 times
aver4 ld a,aver_lo
cpi a,127 ;round to next value if decimal part >0.5
jrc aver3
inc aver_hi
aver3 ld a, aver_hi ;store high byte of result into accumulator,
ret
;*********************** interrupt service routine *************
adcint
ldi adcc,10h
reti
;the low byte is not significant
; set for 256 measurements
;*********************** interrupt vector **********************
.org 0ff0h
jp adcint
2/3
2
Optimizing The St6 A/d Converter Accuracy
“THE PRESENT NOTE WHICH IS FOR GUIDANCE ONLY AIMS AT PROVIDING CUSTOMERS WITH INFORMATION
REGARDING THEIR PRODUCTS IN ORDER FOR THEM TO SAVE TIME. AS A RESULT, STMICROELECTRONICS
SHALL NOT BE HELD LIABLE FOR ANY DIRECT, INDIRECT OR CONSEQUENTIAL DAMAGES WITH RESPECT TO
ANY CLAIMS ARISING FROM THE CONTENT OF SUCH A NOTE AND/OR THE USE MADE BY CUSTOMERS OF
THE INFORMATION CONTAINED HEREIN IN CONNECTION WITH THEIR PRODUCTS.”
Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences
of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted
by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject
to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not
authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics.
The ST logo is a registered trademark of STMicroelectronics.
All other names are the property of their respective owners
© 2004 STMicroelectronics - All rights reserved
STMicroelectronics group of companies
Australia – Belgium - Brazil - Canada - China – Czech Republic - Finland - France - Germany - Hong Kong - India - Israel - Italy - Japan -
Malaysia - Malta - Morocco - Singapore - Spain - Sweden - Switzerland - United Kingdom - United States of America
www.st.com
3/3
Loading...