Data Sheet OP177
Rev. H | Page 9 of 16
APPLICATIONS INFORMATION
GAIN LINEARITY
The actual open-loop gain of most monolithic operational
amplifiers varies at different output voltages. This nonlinearity
causes errors in high closed-loop gain circuits.
It is important to know that the manufacturer’s A
VO
specifica-
tion is only a part of the solution because all automated testers
use endpoint testing and, therefore, show only the average gain.
For example, Figure 24 shows a typical precision operational
amplifier with a respectable open-loop gain of 650 V/mV.
However, the gain is not constant through the output voltage
range, causing nonlinear errors. An ideal operational amplifier
shows a horizontal scope trace.
Figure 25 shows the OP177 output gain linearity trace with the
truly impressive average A
VO
of 12,000 V/mV. The output trace
is virtually horizontal at all points, assuring extremely high gain
accuracy. Analog Devices, Inc., also performs additional testing
to ensure consistent high open-loop gain at various output
voltages. Figure 26 is a simple open-loop gain test circuit.
A
VO
≥ 650V/mV
R
L
= 2kΩ
V
X
–10V 0V +10V
00289-023
Figure 24. Typical Precision Operational amplifier
V
Y
V
X
–10V
0V
+10V
00289-024
A
VO
≥ 12000V/mV
R
L
= 2kΩ
Figure 25. Output Gain Linearity Trace
–
+
V
Y
V
X
10kΩ10kΩ
1MΩ
10Ω
R
L
V
IN
= ±10V
OP177
00289-025
Figure 26. Open-Loop Gain Linearity Test Circuit
THERMOCOUPLE AMPLIFIER WITH COLD-
JUNCTION COMPENSATION
An example of a precision circuit is a thermocouple amplifier
that must accurately amplify very low level signals without
introducing linearity and offset errors to the circuit. In this
circuit, an S-type thermocouple with a Seebeck coefficient of
10.3 μV/°C produces 10.3 mV of output voltage at a temperature
of 1000°C. The amplifier gain is set at 973.16, thus, it produces
an output voltage of 10.024 V. Extended temperature ranges
beyond 1500°C are accomplished by reducing the amplifier
gain. The circuit uses a low cost diode to sense the temperature
at the terminating junctions and, in turn, compensates for any
ambient temperature change. The OP177, with the high open-
loop gain plus low offset voltage and drift, combines to yield a
precise temperature sensing circuit. Circuit values for other
thermocouple types are listed in Table 5.
Table 5.
Thermocouple
Type
Seebeck
Coefficient
R1 R2 R7 R9
K 39.2 μV/°C 110 Ω 5.76 kΩ
102 kΩ 269 kΩ
J 50.2 μV/°C 100 Ω 4.02 kΩ
80.6 kΩ
200 kΩ
S 10.3 μV/°C 100 Ω 20.5 kΩ
392 kΩ 1.07 MΩ
V
OUT
–15V
10µF
0.1µF
+15V
10µF
0.1µF
R
4
50Ω
1%
R
5
100Ω
(ZERO
ADJUST-
MENT)
ANALOG
GROUND
ANALOG
GROUND
10µF
R
8
1.0kΩ
0.05%
+
10µF
COPPER
COPPER
ISOTHERMAL
BLOCK
COLD-JUNCTION
COMPENSATION
REF01
2.2µF
+
+15V
6
4
2
10.000V
–
+
TYPES
ISOTHERMAL
COLD-
JUNCTIONS
–
+
OP177
R
1
100Ω
1%
R
2
20.5kΩ
1%
R
3
47kΩ
1%
R
7
392kΩ
1%
R
9
1.07MΩ
0.05%
00289-026
Figure 27. Thermocouple Amplifier with Cold Junction Compensation
