Section 8. Operation
Open-Input Detect
Note The information in this section is highly technical. It is not
necessary for the routine operation of the CR800.
Summary
• An option to detect an open-input, such as a broken sensor or loose
connection, is available in the CR800.
• The option is selected by appending a C to the Range code.
• Using this option, the result of a measurement on an open connection
will be NAN (not a number).
A useful option available to single-ended and differential measurements is the
detection of open inputs due to a broken or disconnected sensor wire. This
prevents otherwise undetectable measurement errors. Range codes appended
with C enable open-input detect for all input ranges except the ±5000 mV input
range. See TABLE: Analog Input Voltage Ranges and Options
(p. 346).
Appending the Range code with a C results in a 50 µs internal connection of the
V+ input of the PGIA to a large over-voltage. The V– input is connected to
ground. Upon disconnecting the inputs, the true input signal is allowed to settle
and the measurement is made normally. If the associated sensor is connected, the
signal voltage is measured. If the input is open (floating), the measurement will
over-range since the injected over-voltage will still be present on the input, with
NAN as the result.
Range codes and applicable over-voltage magnitudes are found in TABLE: Range
Code Option C Over-Voltages
(p. 323).
The C option may not work, or may not work well, in the following applications:
• If the input is not a truly open circuit, such as might occur on a wet cut
cable end, the open circuit may not be detected because the input
capacitor discharges through external leakage to ground to a normal
voltage within the settling time of the measurement. This problem is
worse when a long settling time is selected, as more time is given for the
input capacitors to discharge to a "normal" level.
• If the open circuit is at the end of a very long cable, the test pulse (300
mV) may not charge the cable (with its high capacitance) up to a voltage
that generates NAN or a distinct error voltage. The cable may even act as
an aerial and inject noise which also might not read as an error voltage.
• The sensor may "object" to the test pulse being connected to its output,
even for 100 µs. There is little or no risk of damage, but the sensor
output may be caused to temporarily oscillate. Programming a longer
settling time in the CRBasic measurement instruction to allow
oscillations to decay before the A-to-D conversion may mitigate the
problem.
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