OP177_06 Datasheet, PDF (9/16 Pages) Analog Devices – Ultraprecision Operational Amplifier

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 AVO 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.
Figure 24 shows the OP177 output gain linearity trace with its
truly impressive average AVO 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.
–10V
0V
V_X
+10V
A_VO ≥ 650V/mV
R_L = 2kΩ
Figure 23. Typical Precision Op Amp
V_Y
–10V
0V
V_X
+10V
A_VO ≥ 12000V/mV
R_L = 2kΩ
Figure 24. Output Gain Linearity Trace
V_IN = ±10V
10kΩ
V_Y
10kΩ
1MΩ
–
10Ω OP177
+
V_X
R_L
Figure 25. Open-Loop Gain Linearity Test Circuit
OP177
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
+15V
2 _REF01 6
2.2µF ⁴
+
R₃
47kΩ
1%
10.000V
10µF
+
R₇
392kΩ
1%
R₉
1.07MΩ
0.05%
+15V
0.1µF
–
TYPES
+
ISOTHERMAL
COLD-
JUNCTIONS
R₂
20.5kΩ
1%
COPPER
COPPER
ISOTHERMAL
BLOCK
COLD-JUNCTION
COMPENSATION
R₁
100Ω
1%
R₈
1.0kΩ
0.05%
R₅
100Ω
(ZERO
ADJUST-
MENT)
R₄
50Ω
1%
10µF
–
OP177
+ 10µF
10µF 0.1µF
V_OUT
ANALOG
GROUND
–15V ANALOG
GROUND
Figure 26. Thermocouple Amplifier with Cold Junction Compensation
Rev. E | Page 9 of 16