CR1000 Manual - Campbell Scientific

Section 8. Operation
8.1.2.5.2 Fixed Voltage Ranges
8.1.2.5.3 Common Mode Null / Open Input Detect
An approximate 9% range overhead exists on fixed input voltage ranges. For
example, over-range on the ±2500 mV-input range occurs at approximately +2725
mV and -2725 mV. The CR1000 indicates a measurement over-range by returning
a NAN (not a number) for the measurement.
For floating differential sensors, such as thermocouples, nulling of any residual
common-mode voltage prior to measurement pulls the H and L input amplifier
(IA) inputs within the ±5-V Input Limits. Appending a C to the range code
(mV2_5C, for example) enables the nulling of the common-mode voltage prior to a
differential measurement on the ±2.5-mV, ±7.5-mV, ±25-mV, and ±250-mV input
ranges. Another useful feature for both SE and DIFF measurements is the
detection of open inputs due to a broken or disconnected sensor wire, to prevent
otherwise undetectable measurement errors. Range codes ending with C also
enable open detect for all input ranges, except the ±5000 mV input range (see
table Analog Voltage Input Ranges with CMN / OID (p. 280) ).
On the ±2.5-mV, ±7.5-mV, ±25-mV, and ±250-mV input ranges, the C range code
option results in a 50-µs internal connection of the H and L inputs of the IA to 300
mV and ground, respectively, while also connected to the sensor to be measured.
The resulting internal common-mode voltage is ±150 mV, which is well within
the ±5-V Input Limits. Upon disconnecting the internal 300-mV and ground
connections, the associated input is allowed to settle to the sensor voltage and the
voltage measurement is made. If the associated input is open (floating), the input
voltages will remain near 300 mV and ground, resulting in an over-range output
(NAN) on the ±2.5-mV, ±7.5-mV, ±25-mV, and ±250-mV input ranges. If the
associated sensor is connected and functioning properly, a valid measured voltage
will result after the input settling associated with open input detect.
On the ±2500-mV input range, the C option (measurement instruction argument is
mV2500C) can be used for open input detect with some limitations, as an internal
voltage large enough to cause measurement over range is not available. The C
option for a voltage measurement on the ±2500-mV input range (mV2500C, for
example), results in the H input being briefly connected to a voltage greater than
2500 mV, while the L input is connected to ground. The resulting common-mode
voltage is > 1.25 V, which is not very helpful in nulling residual common-mode
voltage. However, open input detect is still possible by including an If / Then /
Else statement in the CRBasic program to test the measured results. For example,
the result of a voltage measurement on the ±2500-mV input range with the C
option could be tested for > 2500 mV to indicate an open input. For bridge
measurements, the returned value X being > 1 would indicate an open input. For
example, the BrHalf() instruction returns the value X defined as V1/Vx, where
V1 is the measured single-ended voltage and Vx is the user-defined excitation
voltage having a 2500-mV maximum value. For a BrHalf() measurement,
utilizing the C option on the ±2500-mV input range (measurement instruction
argument is mV2500C), a result of X > 1 indicates an open input for the V1
measurement. The C option is not available on the ±5000-mV input range.
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