Specifications

Section 3. CR3000 Measurement Details
made with the 250 µs integration on the ±5000 mV range. The second
measurement is made on the appropriate range using the integration specified
in the instruction. Both measurements use the settling time programmed in the
instruction. AutoRange optimizes resolution but takes longer than a
measurement on a fixed range, because of the two measurements required.
An AutoRange measurement will return Not-A-Number if the voltage exceeds
the range picked by the first measurement. To avoid problems with a signal on
the edge of a range, AutoRange selects the next larger range when the signal
exceeds 90% of a range.
AutoRange is very good for a signal that occasionally exceeds a particular
range, for example, a Type J thermocouple that most of the time will be less
than 476 °C (±25 mV range) but will occasionally see temperatures as high as
500 °C (±250 mV range, Table 3.4-2). AutoRange should not be used for
rapidly fluctuating signals, particularly those whose signal traverses several
voltage ranges rapidly because of the possibility that the signal could change
ranges between the range check and the actual measurement.
Open Circuit Detect / Pull into Common Mode
Another option selected with the voltage range code is to check for an open
circuit and at the same time pull the signal into common mode range. The
range codes for this option end in C. For example, the range code for the ±20
mV range is “mV20”, the code for this range with open circuit detect is
“mV20C”
The open circuit detect works by briefly (50 microseconds) connecting the
voltage input to 300 mV within the CR3000. 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 ±20 mV, ±50 mV, or the ±200 mV voltage range will
overrange and return Not-A-Number (NAN). If the sensor is good, the signal
from the sensor will drive the inputs to the correct value.
The autorange open circuit detect range (AutorangeC) will only autorange up
to the ±200 mV range and cannot be used for higher voltages. If AutorangeC
was allowed to switch to the ±1000 mV or ±5000 mV range it would not be
able to detect open circuits.
Briefly connecting the inputs to the internal CR3000 voltages also serves to
pull a floating differential voltage into the CR3000 common mode (Section
3.2). The open circuit detect voltage range option should be used for making
differential voltage measurements of thermocouples (TCDiff) and for other
sensors with floating differential outputs (e.g., Solar radiation sensors).
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