0.0047µF
1mA
R5
1mA
(1)
R3
11
3
–
+
V5
–
1mA
R1
+
VD
–
C2
D
XTR101
+
R6
V6
–
(1)
R4
4
13
+
12
C1
R2
FIGURE 11. Cold Junction Compensation Circuit.
NOTE: (1) R3 and R4 should be equal if used.
2
e2
e2
+
INPUT STAGE
the thermocouple impedance goes very high. The circuits of
Figures 16 and 17 inherently have down scale indication.
When the impedance of the thermocouple gets very large
(open) the bias current flowing into the + input (large
impedance) will cause IO to go to its lower range limit value
(about 3.8mA). If up scale indication is desired the circuit of
Figure 18 should be used. When the TC opens the output will
go to its upper range limit value (about 25mA or higher).
Internally eNOISE RTI
=
OUTPUT STAGE
Gain
FIGURE 12. Optional Filtering.
APPLICATION CIRCUITS
Voltage
Reference
OPTIONAL INPUT OFFSET VOLTAGE TRIM
MC1403A
+
The XTR101 has provisions for nulling the input offset
voltage associated with the input amplifiers. In many appli-
cations the already low offset voltages (30µV max for the B
grade, 60µV max for the A grade) will not need to be nulled
at all. The null adjustment can be done with a potentiometer
at pins 1, 2 and 14 as shown in Figures 5 and 6. Either of
these two circuits may be used. NOTE: It is not recom-
mended to use this input offset voltage nulling capability for
elevation or suppression. See the Signal Suppression and
Elevation section for the proper techniques.
VR = 2.5V
–
100pF
V+
XTR101
IO
OPA27
(4-20mA)
V–
R1
125Ω
R2
500Ω
IO(1) (0-20mA)
OPTIONAL BANDWIDTH CONTROL
Low-pass filtering is recommended where possible and can
be done by either one of two techniques shown in Figure 12.
C2 connected to pins 3 and 4 will reduce the bandwidth with
a cutoff frequency given by,
VR
R2
R1
NOTE: (1) IO = 1 +
IO
–
= 1.25 IO – 5mA
R2
Other conversions are readily achievable by
changing the reference and ratio of R1 to R2.
15.9
fCO
=
(R1 + R2 + R3 + R4) (C2 + 3pF)
FIGURE 13. 0-20mA Output Converter.
This method has the disadvantage of having fCO vary with
R1, R2, R3, R4, and it may require large values of R3 and R4.
The other method, using C1, will use smaller values of
capacitance and is not a function of the input resistors. It is,
however, more subject to nonlinear distortion caused by
slew rate limiting. This is normally not a problem with the
slow signals associated with most process control transduc-
ers. The relationship between C1 and fCO is shown in the
Typical Performance Curves.
®
11
XTR101
BB [ BURR-BROWN CORPORATION ]
