OP27GU Datasheet, PDF (16/22 Pages) Texas Instruments – LOW-NOISE HIGH-SPEED PRECISION OPERATIONAL AMPLIFIERS

OP27A, OP27C
LOW-NOISE HIGH-SPEED PRECISION OPERATIONAL-AMPLIFIER
SLOS100E − FEBRUARY 1989 − REVISED FEBRUARY 2010
APPLICATION INFORMATION
noise testing (continued)
When measuring noise on a large number of units, a noise-voltage density test is recommended. A 10-Hz
noise-voltage density measurement correlates well with a 0.1-Hz to 10-Hz peak-to-peak noise reading since
both results are determined by the white noise and the location of the 1/f corner frequency.
Figure 27 shows a circuit measuring current noise and the formula for calculating current noise.
10kΩ
100 Ω
500 kΩ
−
500 kΩ
+
V_no
I_n =
[V_no2 − (130 nV)²]^1/2
1 MΩ × 100
Figure 27. Current Noise Test Circuit and Formula
offset voltage adjustment
The input offset voltage and temperature coefficient of the OP27 are permanently trimmed to a low level at wafer
testing. However, if further adjustment of V_IO is necessary, using a 10-kΩ nulling potentiometer as shown in
Figure 28 does not degrade the temperature coefficient α_VIO. Trimming to a value other than zero creates an
α_VIO of V_IO/300 μV/°C. For example, if V_IO is adjusted to 300 μV, the change in α_VIO is 1 μV/°C.
The adjustment range with a 10-kΩ potentiometer is approximately ± 2.5 mV. If a smaller adjustment range is
needed, the sensitivity and resolution of the nulling can be improved by using a smaller potentiometer in
conjunction with fixed resistors. The example in Figure 29 has an approximate null range of ± 200 μV.
10 kΩ
15 V
Input
2 ¹⁸
−
76
3
+
4
Output
−15 V
Figure 28. Standard Input Offset
Voltage Adjustment
4.7 kΩ
1 kΩ
15 V
4.7 kΩ
Input
2 ¹⁸
3
7 _{6 Output}
4
−15 V
Figure 29. Input Offset Voltage Adjustment With
Improved Sensitivity
offset voltage and drift
Unless proper care is exercised, thermoelectric effects caused by temperature gradients across dissimilar
metals at the contacts to the input terminals can exceed the inherent temperature coefficient ∝V_IO of the
amplifier. Air currents should be minimized, package leads should be short, and the two input leads should be
close together and at the same temperature.
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