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Diagnostic Technique
Detects Open and Short
Circuits in Wiring Harnesses
By Don Nisbett
As a vital part of modern cars, wiring harnesses containing
thousands of assembly components connect various electronic
systems, enabling them to work together. A single failure in any
harness can affect the entire system. Nevertheless, to accommodate
the growing demand for in-car electronics, the complexity of
automotive wiring harnesses continues to grow, increasing the
need to detect broken or shorted wires quickly and easily. Wire
diagnostics are important throughout the entire life of the car.
Starting with the installation phase, diagnosing and repairing
wiring faults can cause extensive manufacturing delays. During
the operational phase, diagnosing and repairing wiring faults can
cause prolonged visits to the repair shop, adding significant costs
to manufacturers in the form of warranty repairs.
Active safety systems, including lane detection and parking
assist (front and rearview cameras)—and infotainment systems,
including navigation and rear seat entertainment—are some
of the more highly sought automotive electronics systems. For
these systems to be effective, video data transmitted via cable
from all corners of the car must reliably get to the driver and
passengers. Cable health is crucial for maintaining proper
operation of these systems.
ENABLE
(INPUT)
This article offers a circuit idea that provides a robust, costeffective technique for implementing wire diagnostics on the video
and audio transmission lines in automotive applications.
The circuit shown in Figure 1 can effectively detect short-tobattery (STB), short-to-ground (STG), open-circuit, and shortcircuit faults. The circuit uses an ADA4433-1 (U1) fully integrated
video reconstruction filter as part of the video transmission signal
chain and an ADA4830-1 (U2) high-speed difference amplifier
as the detection circuit. The ADA4433-1 features a high-order
filter with a −3-dB cutoff frequency of 10 MHz, 45-dB rejection
at 27 MHz, and an internally fixed gain of 2 V/V. It has excellent
video specifications, overvoltage protection (STB) and overcurrent
protection (STG) on its outputs, and low power consumption.
The ADA4830-1 provides an attenuating gain of 0.50 V/V and
a fault detection output flag that can indicate the presence of an
overvoltage condition on its inputs. It features input overvoltage
protection of up to 18 V, a wide common-mode input voltage range,
and excellent ESD robustness.
In the example circuit shown in Figure 1, U1 represents the
differential output buffer that transmits the video signal from a
rearview camera or engine control unit (ECU) to the receiver.
The input would typically be driven by a CMOS imager or
video encoder. The primary function of U1 is to provide the
active filtering function (reconstruction) and to drive the
video signal through the cable to the display. The inputs of
U2 are connected across the outputs of U1 to provide the
fault detection features listed in Table 1 and described in the
following paragraphs.
VS
ENA
STB FLAG
(OUTPUT)
+VS
STB
ADA4433-1 (U1)
DAC
OUTPUT
+IN
LPF
300
STB
VS
37.5
+OUT
37.5
75
TWISTED
PAIR
INP
75
AGND
STB
7.5k
0.1F
–OUT
1.33k
–IN
INN
LPF
AGND
RECEIVER
GND
AGND
VS
STB FLAG
OUTPUT TO GPIO
ENABLE
(INPUT)
4.7k
STB
+VS
VS
ENA
0.1F
+VS
ADA4830-1 (U2)
+
2.2F
VREF
4.7k
INP
TO MICROCONTROLLER
ADC INPUT
VOUT
INN
GND
Figure 1. Wire diagnostic circuit using the ADA4433-1 (U1) and ADA4830-1 (U2).
Analog Dialogue 46-07 Back Burner, July (2012)
www.analog.com/analogdialogue
1
Short-to-Battery Fault Detection
Short to Adjacent Output
Both U1 and U2 have integrated short-to-battery detection and
an STB output flag. During a short-to-battery event, the output
flag of U2 will signal a logic low that can be easily read by a microcontroller’s general-purpose input/output (GPIO) port.
Set the positive input (INP) of U1 to 0 V. The differential output
between +OUT and −OUT should be approximately 1 V. If both
outputs are shorted together, the differential voltage at the output
of U2 will be approximately 0 V.
Short-to-Ground Fault Detection (Single Output)
Normal Operation (No Cable Faults)
Connect the positive input (INP) of U1 to the negative input
(INN). The differential output between +OUT and −OUT
should be 0 V. If either output is shorted to ground, the differential voltage at the output of U2 will be greater than 500 mV.
Short-to-Ground Fault Detection (Both Outputs)
Set the positive input (INP) of U1 to 0 V. The differential output
between +OUT and −OUT should be approximately 1 V. If both
outputs are shorted to ground, the differential voltage at the
output of U2 will be approximately 0 V.
Open Circuit
Set the positive input (INP) of U1 to 0 V. The differential output
between +OUT and −OUT should be approximately 1 V. If there
is an open connection, the resulting differential voltage at the
output of U2 will be approximately 500 mV.
Set the positive input (INP) of U1 to 0 V. The resulting differential
output between +OUT and −OUT should be approximately
1 V. The resulting differential voltage at the output of U2 will be
approximately 250 mV.
Author
Don Nisbett [[email protected]] is a
marketing engineer in the High Speed Signal
Conditioning Group. Prior to his current
position, he held product engineering and
applications engineering responsibilities,
respectively. He has worked at Analog Devices
since 2002, following his graduation from
Worcester Polytechnic Institute with a Bachelor of Science
degree in Electrical Engineering.
Table 1. Summary of Diagnostic Output Indicators
Fault Condition
U1 Input Configuration
U2 Output Indicator
Voltage Level at Indicator1
Pin 5
85 mV
Short to Battery
Short to Ground
(Single Output)
INP = INN
Pin 6
530 mV
Short to Ground
(Both Outputs)
INP ≠ INN
Pin 6
10 mV
Open Circuit
INP ≠ INN
Pin 6
500 mV
Short to Adjacent Output
INP ≠ INN
Pin 6
0 mV
Pin 6
250 mV
Normal Operation
(No Cable Faults)
1
2
All voltage levels are approximate and should be characterized for a particular design.
Analog Dialogue 46-07 Back Burner, July (2012)
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