1mA
D
1mA
–
e1
3
D1
11
R5
2kΩ
10
–
11
8
3
10
8
5
–
eIN
+
R6
51Ω
RS
XTR101
+
e1
–
0.01µF
eIN
6
4
XTR101
+
+
–
24V
RL
eL
–
Thermocouple
TTC
+
7
+
R4
e2
RT
V4
7
4
+
–
+
e2
–
+
e'2
0.01µF
VTC
+
–
V4
R4
–
R2
2.5kΩ
Temperature T2 = TD
Temperature T1
0.01µF
FIGURE 10. Thermocouple Input Circuit with Two
Temperature Regions and Diode (D) Cold
Junction Compensation.
FIGURE 9. Circuit for Example 1.
EXAMPLE 2
With eIN = 0 and VTC = –1.28mV,
V4 = e1 + eIN – VTC
Thermocouple Transducer shown in Figure 10.
Given a process with temperature (T1) limits of 0°C and
+1000°C, configure the XTR101 to measure the temperature
with a type J thermocouple that produces a 58mV change for
1000°C change. Use a semiconductor diode for a cold
junction compensation to make the measurement relative to
0°C. This is accomplished by supplying a compensating
voltage, VR6, equal to that normally produced by the thermo-
couple with its “cold junction” (T2) at ambient. At a typical
ambient of +25°C this is 1.28mV (obtained from standard
thermocouple tables with reference junction of 0°C). Trans-
mit 4mA for T1 = 0°C and 20mA for T1 = +1000°C. Note:
eIN = e2 – e1 indicates that T1 is relative to T2.
= 14.9mV + 0V – (–1.28mV)
1mA (R4) = 16.18mV
R4 = 16.18Ω
COLD JUNCTION COMPENSATION:
The temperature reference circuit is shown in Figure 11.
The diode voltage has the form
IDIODE
KT
q
VD
=
ln
ISAT
ESTABLISHING RS:
Typically at T2 = +25°C, VD = 0.6V and ∆VD/∆T =
–2mV/°C. R5 and R6 form a voltage divider for the diode
voltage VD. The divider values are selected so that the
gradient ∆VD/∆T equals the gradient of the thermocouple at
the reference temperature. At +25°C this is approximately
52µV/°C (obtained from standard thermocouple table);
therefore,
The input full scale span is 58mV (∆eINFS = 58mV).
RS is found from equation (1)
40
RS =
Ω
∆IO/∆eIN – 0.016
R6
(2)
40
40
∆TC/∆T = ∆VD/∆T
=
=
Ω
= 153.9Ω
R5 + R6
16mA/58mV – 0.016
0.2599
R6
52µV/°C = 2000µV/°C
SELECTING R4:
R4 is chosen to make the output 4mA at TTC = 0°C (VTC
R5 + R6
=
–1.28mV) and TD = +25°C (VD = 0.6V). A circuit is shown
in Figure 10.
R5 is chosen as 2kΩ to be much larger than the resistance of
the diode. Solving for R6 yields 51Ω.
VTC will be –1.28mV when TTC = 0°C and the reference
junction is at +25°C. e1 must be computed for the condition
of TD = +25°C to make eIN = 0V.
THERMOCOUPLE BURN-OUT INDICATION
In process control applications it is desirable to detect when
a thermocouple has burned out. This is typically done by
forcing the two-wire transmitter current to either limit when
VD 25°C = 600mV
e1 25°C = 600mV (51/2051) = 14.9mV
eIN = e2 – e1 = VTC + V4 – e1
®
XTR101
10
BB [ BURR-BROWN CORPORATION ]
