OP177 Datasheet, PDF (8/12 Pages) Analog Devices – Ultraprecision Operational Amplifier

OP177
APPLICATIONS 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 specifi-
cation is only a part of the solution, since all automated testers
use endpoint testing and, therefore, only show the average gain.
For example, Figure 22 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
nonlinear errors. An ideal op amp would show a horizontal
scope trace.
Figure 22. Typical Precision Op Amp
THERMOCOUPLE AMPLIFIER WITH COLD-JUNCTION
COMPENSATION
An example of a precision circuit is a thermocouple amplifier
that must amplify very low level signals accurately without
introducing linearity and offset errors to the circuit. In this
circuit, an S-type thermocouple, which has a Seebeck coefficient
of 10.3 µV/°C, produces 10.3 mV of output voltage at a
temperature of 1,000°C. The amplifier gain is set at 973.16.
Thus, it will produce an output voltage of 10.024 V. Extended
temperature ranges to beyond 1,500°C can be 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 very precision temperature sensing circuit. Cir-
cuit values for other thermocouple types are shown in Table I.
Table I.
Thermo- Seebeck
couple Type 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
Figure 23. OP177’s Output Gain Linearity Trace
Figure 24. Open-Loop Gain Linearity Test Circuit
Figure 23 shows the OP177’s output gain linearity trace with its
truly impressive average A_VO of 12000 V/mV. The output trace
is virtually horizontal at all points, assuring extremely high gain
accuracy. PMI also performs additional testing to insure
consistent high open-loop gain at various output voltages.
Figure 24 is a simple open-loop gain test circuit for your own
evaluation.
Figure 25. Thermocouple Amplifier with Cold Junction
Compensation
PRECISION HIGH GAIN DIFFERENTIAL AMPLIFIER
The high gain, gain linearity, CMRR, and low TCV_OS of the
OP177 make it possible to obtain performance not previously
available in single stage, very high-gain amplifier applications.
See Figure 26.
R1
R3
For best CMR, must equal . In this example, with a
R2
R4
10 mV differential signal, the maximum errors are as listed in
Table II.