AN-858
APPLICATION NOTE
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AD7142 Sensor Board In-Line Production Test Procedure
by Wayne Palmer
INTRODUCTION
PRINCIPLE OPERATION
This application note describes the in-line testing procedure for
the AD7142 sensor board.
Figure 1 shows an example circuit diagram for a 3-button
sensor board application to be tested. In this example, the
sensor buttons (S1, S2, and S3) are connected to STAGE0,
STAGE1, and STAGE 2, respectively, as shown in Figure 2.
Use this procedure to check that the AD7142 sensor board is
properly connected to the system host controller board and that
the AD7142 sensor board and sensor elements are operating as
expected.
Once the AD7142 sensor board is powered up and configured,
the S1, S2, and S3 sensor output values can be measured via the
ADC_RESULTS_Sx registers for each sensor. By monitoring
these values, with and without the sensor excitation source
(SRC) enabled, the system processor determines if the AD7142
sensor board and sensors are outputting the expected values.
AD7142 SENSOR BOARD
VDRIVE
AVCC, DVCC
AD7142 SENSOR BOARD
TOP LAYER
S2
INT 25
GPIO 26
TEST 27
VREF+ 28
VREF– 29
INT
CS 24
1
CIN3
2
CIN4
3
CIN5
SDI 22
4
CIN6
SDO 21
5
CIN7
VDRIVE 20
6
CIN8
DGND2 19
7
CIN9
DGND1 18
8
CIN10
SS
SCLK 23
AD7142
HOST
PROCESSOR
WITH SPI
INTERFACE
SCK
J1
S1
CIN0 30
AD7142 SENSOR BOARD
BOTTOM LAYER
CIN1 31
CIN2 32
2.2kΩ
MOSI
MISO
VHOST
CIN13
CSHIELD
AVCC
AGND
SRC
11
12
13
14
15
SRC
CIN12
10
DVCC 17
16
CIN11
9
1.8V
AVCC, DVCC
2.7V TO 3.6V
1µF TO 10µF
(OPTIONAL)
0.1µF
10nF
06283-001
S3
Figure 1. Example of a 3-Button (S1, S2, and S3) Circuit Application
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AN-858
S1
CIN9
SRC
ADC_RESULTS_S1
(ADDR 0x00C)
+
STAGE2
–
ADC_RESULTS_S2
(ADDR 0x00D)
SRC ENABLED
ON VALUE
35500
250kHz
EXCITATION
SOURCE
EXCITATION_SOURCE
(ADDR 0x000)
25500
20500
15500
EXCITATION_SOURCE = 1
SRC ENABLED OFF VALUE
15000
0
06283-002
AD7142
35000
20
40
60
80
100
TIME (ms)
06283-003
S3
+
STAGE1
–
EXCITATION_SOURCE = 0
CIN7
ADC_RESULTS_S0
(ADDR 0x00B)
SERIAL
WRITE
(ADDR 0x000)
EXCITATION_SOURCE = 0
S2
+
STAGE0
–
ADC_RESULTS_S0 VALUE (Codes)
CIN4
Figure 3. Disabling SRC While Measuring Sensor S0 CDC Output
Figure 2. AD7142 3-Button Configuration Example
SENSOR CHARACTERIZATION
IN-LINE PRODUCTION TEST PROCEDURE
The ADC_RESULTS registers are measured while enabling and
disabling the SRC. This information is used to determine if the
AD7142 sensor board is operating as expected. Therefore, a
precharacterization of each sensor on several sensor boards is
necessary to obtain an expected range of sensor values.
1.
Install the AD7142 sensor board, connecting it to the host
processor.
2.
Power up and configure the AD7142 device.
3.
Set the EXCITATION_SOURCE register, located at
Register Address 0x000, to 0 to enable the SRC source.
4.
Measure the ADC_RESULTS_Sx register values for each
sensor and compare this value to the mean SRC Enabled
On value obtained in Step 3.
Sensor Characterization Procedure
The following steps comprise the recommended sensor
characterization procedure. Perform this procedure on several
known good sensor boards.
Continue to Step 5 if the value is within the expected range.
1.
Power up and configure the AD7142 device.
2.
Set the EXCITATION_SOURCE register, located at
Register Address 0x000, to 0 to enable the SRC source.
3.
Measure and record the ADC_RESULTS_Sx register values
for each sensor and store the values as SRC Enabled On.
4.
Set the EXCITATION_SOURCE register, located at
Register Address 0x000, to 1 to disable the SRC source.
5.
Measure and record the ADC_RESULTS_Sx register values
for each sensor and store them as SRC Enabled Off.
6.
Calculate the mean and standard deviation for the SRC
Enabled On and SRC Enabled Off values.
Otherwise, troubleshoot the sensor board to identify why
the value is out of range. Then proceed to Step 5.
5.
Set the EXCITATION_SOURCE register, located at
Register Address 0x000, to 1 to disable the SRC source.
6.
Measure the ADC_RESULTS_Sx register values for each
sensor and compare this value to the mean SRC Enabled
Off value obtained in Step 5.
If the value is within the expected range, the test is
complete.
Otherwise, troubleshoot the sensor board to identify why
the value is out of range.
These mean values are used in the in-line production test
procedure as described in the In-Line Production Test
Procedure section.
Figure 3 shows an example of the ADC_RESULTS_S0 register
while enabling and disabling the EXCITATION_SOURCE for
Sensor Button S1.
©2006 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
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