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OP177GPZ 查看數據表(PDF) - Analog Devices

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OP177GPZ
ADI
Analog Devices ADI
OP177GPZ Datasheet PDF : 16 Pages
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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
VX
+10V
AVO 650V/mV
RL = 2k
Figure 23. Typical Precision Op Amp
VY
–10V
0V
VX
+10V
AVO 12000V/mV
RL = 2k
Figure 24. Output Gain Linearity Trace
VIN = ±10V
10k
VY
10k
1M
10OP177
+
VX
RL
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
47k
1%
10.000V
10µF
+
R7
392k
1%
R9
1.07M
0.05%
+15V
0.1µF
TYPES
+
ISOTHERMAL
COLD-
JUNCTIONS
R2
20.5k
1%
COPPER
COPPER
ISOTHERMAL
BLOCK
COLD-JUNCTION
COMPENSATION
R1
100
1%
R8
1.0k
0.05%
R5
100
(ZERO
ADJUST-
MENT)
R4
50
1%
10µF
OP177
+ 10µF
10µF 0.1µF
VOUT
ANALOG
GROUND
–15V ANALOG
GROUND
Figure 26. Thermocouple Amplifier with Cold Junction Compensation
Rev. E | Page 9 of 16

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