Campbell Scientific CR3000 Micrologger - Manual (Page 287)

Section 8. Operation
287
mV and -1090 mV. The CR3000 indicates a measurement over-range by returning
a NAN (not a number) for the measurement.
8.1.2.5.3 Common Mode Null / Open Input Detect
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
( mV20C , for example) enables the nulling of the common-mode voltage prior to a
differential measurement. 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 (see table Analog
Voltage Input Ranges with CMN / OID
(p. 286)
).
On the ±20 mV, ±50 mV, and ±200 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. The ±1000-mV and ±5000-mV ranges are treated in the
same way, but with the applied excitation being approximately 1200 mV and
5200 mV respectively. 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 ±20 mV, ±50 mV, and ±200 mV input ranges. Similarly, an open
input will result in a voltage of approximately 1200 mV for the ±1000 mV range
and approximately 5200 mV for the ±5000 mV range, with a resulting
NAN for
those 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.
8.1.2.6 Offset Voltage Compensation
Analog measurement circuitry in the CR3000 may introduce a small offset
voltage to a measurement. Depending on the magnitude of the signal, this offset
voltage may introduce significant error. For example, an offset of 3 μV on a 1000-
mV signal introduces an error of only 0.00012%; however, the same offset on a
0.25-mV signal introduces an error of 1.2%.
The primary source of offset voltage is the Seebeck effect, which arises at the
junctions of differing metals in electronic circuits. Secondary sources of offset
voltages are return currents incident to powering external devices through the
CR3000. Return currents create voltage drop at the ground terminals that may be
used as signal references.
CR3000 measurement instructions incorporate techniques to cancel these
unwanted offsets. The table Analog Measurement Instructions and Offset Voltage
Compensation Options
(p. 288)
lists available options.