OP177
APPLICATION INFORMATION
GAIN LINEARITY
The actual open-loop gain of most monolithic op amps 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 23 shows a typical precision op amp with a
respectable open-loop gain of 650 V/mV. However, the gain is
not constant through the output voltage range, causing non-
linear errors. An ideal op amp shows a horizontal scope trace.
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 also performs additional testing to
ensure consistent high open-loop gain at various output
voltages. Figure 25 is a simple open-loop gain 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 its 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
K
J
S
Seebeck
Coefficient
39.2 μV/°C
50.2 μV/°C
10.3 μV/°C
R1
110 Ω
100 Ω
100 Ω
R2
5.76 kΩ
4.02 kΩ
20.5 kΩ
R7
102 kΩ
80.6 kΩ
392 kΩ
R9
269 kΩ
200 kΩ
1.07 MΩ
V
X
–10V
0V
+10V
+15V
00289-023
2
REF01
4
6
R
3
47kΩ
1%
10.000V
R
7
392kΩ
1%
10µF
+
R
9
1.07MΩ
0.05%
+15V
0.1µF
10µF
A
VO
≥
650V/mV
R
L
= 2kΩ
2.2µF
+
Figure 23. Typical Precision Op Amp
V
Y
–
TYPES
ISOTHERMAL
COLD-
JUNCTIONS
R
2
20.5kΩ
1%
COPPER
R
8
1.0kΩ
0.05%
R
5
100Ω
(ZERO
ADJUST-
MENT)
R
4
50Ω
1%
–
V
X
–10V
0V
+10V
+
ISOTHERMAL
BLOCK
COPPER
OP177
+
10µF
10µF
0.1µF
V
OUT
A
VO
≥
12000V/mV
R
L
= 2kΩ
00289-024
COLD-JUNCTION
COMPENSATION
R
1
100Ω
1%
Figure 24. Output Gain Linearity Trace
V
Y
10kΩ
V
IN
= ±10V
ANALOG
GROUND
Figure 26. Thermocouple Amplifier with Cold Junction Compensation
10kΩ
1MΩ
–
10Ω
V
X
OP177
+
R
L
00289-025
Figure 25. Open-Loop Gain Linearity Test Circuit
Rev. E | Page 9 of 16
00289-026
–15V ANALOG
GROUND
AD [ ANALOG DEVICES ]
