Measurement Computing CIO-EXP-GP - Manual (Page 21)

5.3.3 Adding a Ground Reference
The CIO-EXP-GP inputs are fully differential which helps reject noise on thermocouple wires. If
thermocouples connected to the CIO-EXP-GP inputs are to work properly, the 'G' pad must be closed on
any channel used for thermocouple measurement (see Figures 5-3 and 5-4). The 'G' pad provides a
reference from ground to the analog low input via a 10K resistor. Only 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.
NOTE: If you want to change the use of the input circuit to an RTD or bridge sensor, remove the solder
that closes the G pad (and the TC pad also).
5.4 Determining the Appropriate Gain
The voltage from a thermocouple must be amplified in order to take advantage of the A/D board's full
resolution. Without amplification, you would not get much resolution from thermocouples, as you can
see in the tables below. Typical gain settings for use with thermocouples are between X10 and X250.
Tables 5-3 and 5-4 below may be used to help determine the appropriate gain to use for the temperature
range and thermocouple type in use.
Table 5-3. Resolution vs Thermocouple Gain Settings for a ±5V, 12 bit A/D
0.35 1.4 3.49 34.9 1,450 0 7 R
0.35 1.4 3.49 34.9 1,450 0 7 S
0.03 0.16 0.39 3 900
−
200 62 E
0.06 0.24 0.61 6.1 350
−
200 40 T
0.06 0.24 0.61 6.1 1,250 -200 40 K
0.04 0.19 0.48 4.78 750 0 51 J
°C/bit
@X1000
°C/bit
@X250
°C/bit
@X100
°C/bit
@X10
Max °C Min °C Output
µV/°C
Type
A J-type thermocouple outputs 51mV per degree centigrade at 20°C. At a gain of 100, a 12 bit A/D on
the ±5V range resolves to 0.00002442 volts per bit (24.42µV/bit). With an output of 51mV/°C, that
represents about 0.5°C/bit. Look under the gain of 100 for a J-type and you will find 0.48°C/bit.
The table below shows the thermocouple output voltage at maximum temperature amplified by four
possible gain values. Where the output voltage exceeds 5V, the reading is clipped.
Table 5-4. Voltage Output @ Maximum Temperature
17 4.2 1.7 0.17 1,450 16.7 R
15 3.8 1.5 0.15 1,450 15mV S
69 17.2 6.9 0.69 900 68.8 E
18 4.5 1.8 0.18 350 17.8mV T
51 12.7 5.1 0.51 1,250 50.6mV K
42 10.6 4.2 0.42 750 42.3mV J
X1000 X250 X100 X10
Vout at Max Temp Max °C Output
@Max
Temp
Type
17