Specifications

Section 4. Sensor Support
4-5
AutoRange is recommended for a signal that occasionally exceeds a particular
range, for example, a Type J thermocouple measuring a temperature usually
less than 476 °C (±25 mV range) but occasionally as high as 500 °C (±250 mV
range). AutoRange should not be used for rapidly fluctuating signals,
particularly signals traversing several voltage ranges rapidly. The possibility
exists that the signal can change ranges between the range check and the actual
measurement.
Open Input Detect / Pull into Common Mode
The CR1000 can check for an open measurement circuit or input, as can occur
with a broken sensor wire. Simultaneously, the CR1000 will pull the signal
into common mode range. Range codes ending with “C” enable these features.
Refer to TABLE 4.2-2 for limitations.
Op
en input detect works by connecting the voltage input to a 300 mV internal
source for 50 µs. A differential voltage input has the high side connected to
300 mV and the low side connected to ground. After disconnecting, the input
is allowed to settle, and the voltage measurement is made. If the sensor is open
(inputs not connected and “floating”) the inputs will remain floating near the
voltage they were connected to; a measurement on the ±2.5 mV, ±7.5 mV,
±25 mV, or the ±250 mV voltage range will over range and return NAN (Not-
A-Number). If the sensor is good, the signal from the sensor will drive the
inputs to the correct value.
Briefly connecting the inputs to the internal CR1000 voltages also serves to
pull a floating differential voltage into the CR1000 common mode (Section 7.2
C
ommon Mode Range). This voltage range option should be used for making
differential voltage measurements of thermocouples (TCDiff) and for other
sensors with floating differential output (e.g., solar radiation sensors).
Open input detect on the ± 2500 mV input range (mV2500C) is available with
some differences from it use on ±2.5 mV, ±7.5 mV, ±25 mV, or the ±250 mV
voltage ranges. With the ± 2500 mV input range, the high side of the input is
internally connected to a voltage that is greater than 2500 mV, but not large
enough to over-range. To detect an open bridge, program If ... Then logic in
the CRBASIC program to determine if the resulting measurement exceeds
2500 mV. 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. An result of X > 1 indicates an open input for the
V1 measurement.
4.2.3 Offset Voltage Compensation
Analog measurement circuitry in the CR1000 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 2500 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. A secondary source of
offset voltage are return currents incident to powering external devices through
the CR1000. Return currents create voltage drop at the ground terminals that
may be used as signal references.