OP27A, OP27C

LOW-NOISE HIGH-SPEED PRECISION OPERATIONAL-AMPLIFIER

SLOS100E − FEBRUARY 1989 − REVISED FEBRUARY 2010

16

POST OFFICE BOX 655303

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.

+

−

10k

Ω

100

Ω

500 k

Ω

500 k

Ω

1 M

Ω × 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

Figure 28 does not degrade the temperature coefficient

The adjustment range with a 10-k

Ω 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.

+

−

−15 V

Output

2

3

7

8

4

1

Input

6

15 V

10 k

Ω

−15 V

Output

2

3

7

8

4

1

Input

6

4.7 k

Ω

Figure 28. Standard Input Offset

Voltage Adjustment

Figure 29. Input Offset Voltage Adjustment With

Improved Sensitivity

15 V

1 k

Ω

4.7 k

Ω

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

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.