REV. A

OP37

–11–

APPLICATIONS INFORMATION

OP37 Series units may be inserted directly into 725 and OP07

sockets with or without removal of external compensation or

nulling components. Additionally, the OP37 may be fitted to

unnulled 741type sockets; however, if conventional 741 nulling

circuitry is in use, it should be modified or removed to ensure

correct OP37 operation. OP37 offset voltage may be nulled to

zero (or other desired setting) using a potentiometer (see offset

nulling circuit).

The OP37 provides stable operation with load capacitances of

up to 1000 pF and

±10 V swings; larger capacitances should be

decoupled with a 50

Ω resistor inside the feedback loop. Closed

loop gain must be at least five. For closed loop gain between five

to ten, the designer should consider both the OP27 and the OP37.

For gains above ten, the OP37 has a clear advantage over the

unity stable OP27.

Thermoelectric voltages generated by dissimilar metals at the input

terminal contacts can degrade the drift performance. Best

operation will be obtained when both input contacts are main-

tained at the same temperature.

10k

OP37

V+

OUTPUT

V–

+

–

Figure 1. Offset Nulling Circuit

Offset Voltage Adjustment

The input offset voltage of the OP37 is trimmed at wafer level.

Ω trim

nulling circuit). Other potentiometer values from 1 k

Ω to 1 MΩ

can be used with a slight degradation (0.1

µV/°C to 0.2 µV/°C) of

will be 0.33

adjustment range with a 10 k

Ω potentiometer is ±4 mV. If smaller

adjustment range is required, the nulling sensitivity can be reduced

by using a smaller pot in conjunction with fixed resistors. For

example, the network below will have a

±280 µV adjustment range.

1

8

4.7k

4.7k

1k

POT

V+

Figure 2. TBD

OP37

–18V

+18V

Figure 3. Burn-In Circuit

Noise Measurements

To measure the 80 nV peak-to-peak noise specification of the

OP37 in the 0.1 Hz to 10 Hz range, the following precautions

must be observed:

• The device has to be warmed-up forat least five minutes. As

shown in the warm-up drift curve, the offset voltage typically

changes 4

µV due to increasing chip temperature after power up.

In the ten second measurement interval, these temperature-

induced effects can exceed tens of nanovolts.

• For similar reasons, the device has to be well-shielded from

air currents. Shielding minimizes thermocouple effects.

• Sudden motion in the vicinity of the device can also

“feedthrough” to increase the observed noise.

• The test time to measure 0.1 Hz to l0 Hz noise should not

exceed 10 seconds. As shown in the noise-tester frequency

response curve, the 0.1 Hz corner is defined by only one zero.

The test time of ten seconds acts as an additional zero to eliminate

noise contributions from the frequency band below 0.1 Hz.

• A noise-voltage-density test is recommended when measuring

noise on a large number of units. A 10 Hz noise-voltage-density

measurement will correlate 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.

Optimizing Linearity

Best linearity will be obtained by designing for the minimum

output current required for the application. High gain and

excellent linearity can be achieved by operating the op amp with

a peak output current of less than

±10 mA.

Instrumentation Amplifier

A three-op-amp instrumentation amplifier provides high gain and

wide bandwidth. The input noise of the circuit below is 4.9 nV/

√Hz.

The gain of the input stage is set at 25 and the gain of the second

stage is 40; overall gain is 1000. The amplifier bandwidth of

800 kHz is extraordinarily good for a precision instrumentation

amplifier. Set to a gain of 1000, this yields a gain bandwidth

product of 800 MHz. The full-power bandwidth for a 20 V p-p

output is 250 kHz. Potentiometer R7 provides quadrature

trimming to optimize the instrumentation amplifier’s ac common-

mode rejection.

R7

100k

C1

100pF

R1

5k

0.1%

R3

390

R2

100

R4

5k

0.1%

INPUT (+)

INPUT (–)

R5

500

0.1%

R6

500

0.1%

R8

20k

0.1%

R9

19.8k

R10

500

NOTES:

TRIM R10 FOR dc CMRR

OP37

+

–

OP37

+

–

OP37

Figure 4a. TBD