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Section 3. CR800 Measurement Details

3-2

two measurements. The first measurement determines the range to use. It is

made with the 250 µs integration on the ±2500 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 ±25

mV range is “mV25”, the code for this range with open circuit detect is

“mV25C”

The open circuit detect works by briefly (50 microseconds) connecting the

voltage input to 300 mV within the CR800. 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 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 ±250 mV range and cannot be used for higher voltages. If AutorangeC

was allowed to switch to the ±2500 mV range it would not be able to detect

open circuits.

Briefly connecting the inputs to the internal CR800 voltages also serves to pull

a floating differential voltage into the CR800 common mode (Section 3.2).

This 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).

The open circuit detect does not work on the ±2500 mV or ±5000 mV ranges.

However, the mV2500C and mV5000C ranges can be used to pull a floating

differential signal within common mode range. There is no reason to use these

ranges for single-ended measurements.

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