OPA27CZ Datasheet, PDF (9/22 Pages) Texas Instruments – Ultra-Low Noise, Precision OPERATIONAL AMPLIFIERS

APPLICATIONS INFORMATION
OFFSET VOLTAGE ADJUSTMENT
The OPA27 and OPA37 offset voltages are laser-trimmed
and require no further trim for most applications. Offset
voltage drift will not be degraded when the input offset is
nulled with a 10kΩ trim potentiometer. Other potentiometer
values from 1kΩ to 1MΩ can be used, but V_OS drift will be
degraded by an additional 0.1µV/°C to 0.2µV/°C. Nulling
large system offsets by use of the offset trim adjust will
degrade drift performance by approximately 3.3µV/°C per
millivolt of offset. Large system offsets can be nulled without
drift degradation by input summing.
The conventional offset voltage trim circuit is shown in Figure
3. For trimming very small offsets, the higher resolution
circuit shown in Figure 4 is recommended.
The OPA27 and OPA37 can replace 741-type operational
amplifiers by removing or modifying the trim circuit.
THERMOELECTRIC POTENTIALS
The OPA27 and OPA37 are laser-trimmed to microvolt-level
input offset voltages, and for very-low input offset voltage
drift.
Careful layout and circuit design techniques are necessary to
prevent offset and drift errors from external thermoelectric
potentials. Dissimilar metal junctions can generate small
EMFs if care is not taken to eliminate either their sources
(lead-to-PC, wiring, etc.) or their temperature difference (see
Figure 11).
Short, direct mounting of the OPA27 and OPA37 with close
spacing of the input pins is highly recommended. Poor layout
can result in circuit drifts and offsets which are an order of
magnitude greater than the operational amplifier alone.
0.1µF
100kΩ
10Ω
2kΩ
DUT
4.3kΩ
22µF
Voltage Gain
Total = 50,000
4.7µF
OPA111
100kΩ
2.2µF
Scope
x1
R_IN = 1MΩ
0.1µF
110kΩ
24.3kΩ
NOTE: All capacitor values are for nonpolarized capacitors only.
FIGURE 1. 0.1Hz to 10Hz Noise Test Circuit.
0.1Hz TO 10Hz NOISE
1s/div
40nv/div
FIGURE 2. Low Frequency Noise.
OPA27, OPA37
9
SBOS135C
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