ST AN1538 APPLICATION NOTE
AN1538
APPLICATION NOTE
Reducing Analog-Digital Conversion Error Using
ST10F269/ST 10F 280
By André ROGER
INTRODUCTION
The ST10F269/ST10F280 contains an Analog / Digital Converter with 10-bit resolution, 4.85µs
conversion time, a sample & hold circuit on-chip, ESD protected anal og inputs and a “Total Unadjusted
Error” of ± 2LSB across the whole automotive temperature range.
This application note identifies the causes of ADC error and gives solutions to optimize ADC
performance. An appendix reviews the meaning of the terms: resolution, accuracy and intrinsic error.
1/8March 2002
AN1538 - APPLICAT ION NOTE
TABLE OF CONTENTS PAGE
1 SOURCES OF ADC ERROR ..................................................................................... 3
1.1 ANALOG INPUT SIGNAL ERROR ............................................................................. 3
1.1.1 Source Internal Resistance Matching with ADC Input Parameters ... ......................... 3
1.1.2 Errors Due to High Frequencies from Input Signal ..................................................... 5
1.2 INPUT OVERLOAD ERRORS ....... ................ ................. ........................ ................ ... 5
1.3 REFERENCE VOLTAGE ERRORS ........................................................................... 5
1.3.1 Reference Voltage Accuracy and Absolute Tolerance on a Conversion Result ......... 5
1.3.2 Reference Voltage Accuracy and Differential Tolerance on Conversion Results . ...... 5
2 HOW TO MINIMIZE ERROR ...................................................................................... 6
2.1 OPTIMISE THE INPUT SIGNAL ................................................................................ 6
2.2 REDUCE INPUT OVERLOAD ERROR ...................................................................... 6
2.3 REFERENCE VOLTAGE ERROR REDUCTION ....................................................... 7
3 APPENDIX DEFINITIO N S ........................... ........................ ................ ................. ..... 8
4 REVISION HISTORY ......................... ................. ................. ................ ...................... 8
4.1 CREATION OF THE AN1538 ON THE 14TH OF MARCH 2002 ............................... 8
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AN1538 - APPLICAT ION NOTE
1 - SOURCES OF ADC ERROR
Sources of ADC accuracy error are classified into 3 categories:
– Analog input signal error
– Input overload error
– Reference voltage error
Each of these categories is describ ed in the following sections.
1.1 - Analog Input Signal Error
1.1.1 - Source Internal Resistance Matching with ADC Input Parameters
Analog input signal error can be created by poor matching of the source internal resistance with the ADC
input parameters, either caused by,
– Voltage drop in the voltage source resistance due to input leakage current,
– Or by poor charging of the ADC internal capacitance (Cin).
Analog input error can also be caused by noise from the analog input signal. This section described each
of these causes.
Figure 1 :
Source Internal Resistance Errors
SAMPLE
V
P5.x
IN
R1
I
C
IO
OZ1
Leakage
Current
C
IN
R
SOURCE
V
S
AGND AGND
I
(Input leakage current P ort5) : maximum ± 200 nA (test Condition: 0.45V<VIN<VDD)
OZ1
C
(Pin capacitance Port5): maximum 10 pF (test Condition: f = 1 MHz, TA = 25°C, guaranteed by design characterization)
IO
(ADC Internal capacitance): maximum 5 pF (guaranteed by design characterization)
C
IN
R1 (series input resistance): maximum 1.5kΩ (guaranteed by design characterization)
Refer to Figure 1 for a schematic of source internal resistance errors.
Static voltage drop in the source resistance:
The error generated by the voltage source internal
resistance is:
For example :
R
error
()
LSB
SOURCEI0Z1
---------------------------------------------- 1024×=
V
AREFVAGND
A source resistance of 15KΩ and a specified leakage current (I
×
–
) of ±500nA will cause
OZ1
a static voltage error of ±7.5mV or ±1.5LSB.
Refer the latest product datasheet for the value of
I
OZ1
.
Notes: 1. Input leakage c urrent is caused by parasitic current at input pin protection; this protection is
necessary to protect the device against ESD (Electrical Static Discharge) and against overload.
2. Adding a filter ing capaci tor at the ana log input pin do es not chan ge the static voltage drop; it
only improves the charging time of the internal capacitors (see after).
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