CIO-EXP16 User's Guide
Functional Details
Cold junction compensation
The CIO-EXP16 has a semiconductor temperature sensor to measure the temperature of the board around the
screw terminals—this is called the
cold
junction
temperature
. This sensor provides a stable, accurate and linear
output which is temperature-dependent. The cold junction temperature is required to accurately calculate the
thermocouple temperature. This is known as
cold
junction
compensation
(CJC). You can map the sensor output
to one of the A/D board's analog inputs using the
CJC output to DAS
jumper block.
The CJC reference is required for accurate temperature readings
The CJC temperature reference is universally used by software to compensate for the voltage induced at the
cold junction (the area around the screw terminals). If you don't supply a CJC reference your temperature
readings may be inaccurate.
Open thermocouple detect
You enable open thermocouple detect (OTD) by closing the channel’s associated 'V' pad with a solder bridge.
OTD applies a full-scale negative voltage to the high side of the thermocouple signal. If a thermocouple opens,
the OTD voltage drives the signal on that channel to negative full-scale.
Design your software program to recognize when a negative full-scale condition exists, since the input is pulled
to negative full-scale if the thermocouple wire is broken or disconnected. Most software is set up to alarm for an
open thermocouple when a temperature falls to negative full-scale.
The CIO-EXP16 will accurately measure thermocouples when the 'V' pad is open, but without OTD protection
enabled.
Low pass filter
A low pass filter is implemented by closing the 'C' pad. When bridged, a 1
µ
F capacitor is connected across the
signal’s high and low inputs, forming a low-pass filter. The bandwidth of this filter is 7 Hz. Signals changing at
rates greater than 7 times per second are damped. The low pass filter helps to reject noise picked up along the
thermocouple wire. Since temperature measurement is generally a ‘low frequency’ application, the filter does
not affect the temperature measurement.
The CIO-EXP16 will accurately measure thermocouples when the 'C' pad is open, but the measurements may
fluctuate due to noise present on the thermocouple wire.
Ground reference
The CIO-EXP16 inputs are fully differential, which help reject noise on thermocouple wires. Close the ‘G’ pad
thermocouples to work properly. The 'G' pad provides a reference from ground to the analog low input via a 100
K resistor. When closed, enough current passes through the resistor to provide a reference to ground. The
analog high and low inputs are still able to float within the common mode range.
Input impedance
When you leave the V and G pads open, the input impedance is greater than 100 M ohms. When you short the
V and G pads, the input impedance is 100 k ohms.
Amplification and thermocouples
The voltage from a thermocouple is very low and must be amplified to take advantage of the A/D board’s full
resolution. Use the
GAIN
switch to set the amplification of the CIO-EXP16 board's 16 analog inputs (see
The following table lists the theoretical amplifier output voltage at maximum temperature for each gain. In most
cases, the range on the A/D board is set to ±5 V. The maximum voltage for some thermocouple/gain
combinations can exceed the maximum A/D input range (typically 5 V), or even the maximum output voltage
of the CIO-EXP16 board (±10 V).
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