キーワード: self diagnostics, system diagnostics, auto-zero, output drift, high performance signal conditioner, low cost signal conditioner, MAX1457, MAX1478, MAX1452, MAX1455, MAX1463, signal conditioners conditioning
Adding Diagnostic Capability to MAX14XX Low Cost High Performance Signal-Conditioning Devices
Abstract: A cost-effective procedure for adding system diagnostic, product diagnostic, output drift check, auto-zeroing capability, etc., to your products which are based on the MAX1478, MAX1452, MAX1455, MAX1457, and MAX1463 family of high-performance signal conditioning products without changing the test system or adding significant time to the calibration process.
This approach takes advantage of built-in nonvolatile flash memory included in these low-cost signal conditioners. The concept is implemented by making an accurate measurement of the output voltage at predefined reference conditions and storing it in the flash memory for future reference.
This application note describes a simple and cost-effective
procedure for adding diagnostic checks to determine product functionality
and sensor output drift. This enhancement can be added to any MAX14XX
device with nonvolatile memory without changing the test system or adding
significant time to the calibration operation.
MAX14XX signal-conditioning devices are generally used in
conjunction with pressure sensors to correct their inherently large temperature
error. Although the same principle applies to other types of sensors,
this document refers solely to pressure-sensor applications for the sake
of simplicity.
Benefits of Adding Diagnostic Capability
Checking the functionality of the signal-conditioning device.
Indicating the level of error after calibration.
Indicating drift in the sensor output. This is desirable especially
in the case of absolute-pressure sensors, because loss of reference
vacuum can cause output drift, which may not be easily detectable after
calibration.
Verifying proper functioning and troubleshooting the test system.
Changes in output voltage in the reference condition could provide
hints of inaccurate pressure setting, incorrect VDD, malfunctioning
ASIC, etc.
Overview
Maxim's MAX14XX series of smart signal-conditioning
products include nonvolatile memory that can be used to store product-run
information and calibration values. Once the device is calibrated, the
output voltage (VCAL) of the device should be measured under reference
conditions and the value of VCAL written to memory. Reference temperature
and pressure conditions should be chosen to minimize the influence of
the test system on the reference conditions. For gage-pressure devices,
reference conditions are usually room temperature and atmospheric pressure.
For highly accurate products, room temperature may need to be controlled
and atmospheric pressure measured independently. When it comes to absolute-pressure
sensors, a repeatable reference pressure must be chosen (it may not be
atmospheric pressure).
Reference Temperature
Controlling the reference temperature is especially necessary
for devices with a large temperature coefficient. To minimize the overhead
associated with the generation of VCAL, the last calibration temperature
can be used as the reference. A reasonable choice is 25°C, because
it is easy to achieve and control and it is used as the reference for
most specifications.
Reference
Pressure
For gage-pressure sensors, use atmospheric pressure.
For absolute-pressure sensors, absolute (not gage) pressure must
be used and controlled to the desired accuracy. To minimize the effect
of any possible leak in the system, a pressure close to the atmospheric
pressure is recommended (100kPA).
For high-pressure sensors, atmospheric pressure can be used.
Procedure
for Setting Up Reference Voltage
Complete the compensation/calibration as this is normally done.
Apply the reference pressure and the temperature condition to the
device.
Read the output voltage to the desired accuracy.
Store the output voltage, VCAL, in the memory.
Procedure
for Using Reference Voltage
Apply the reference condition to the device.
Measure the output voltage, Vout.
Read VCAL from memory.
Compare the VOUTand VCAL values, dVOUT = VOUT- VCAL.
Take any specified action based on the value of dVOUT. For example,
shut down the system, call the operator, ignore the device output, etc.
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