OP27GS datasheet(14/20 Pages) AD

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OP27

Rev. F | Page 14 of 20

APPLICATION INFORMATION

OP27 series units can be inserted directly into OP07 sockets

with or without removal of external compensation or nulling

components. Additionally, the OP27 can be fitted to unnulled

AD741-type sockets; however, if conventional AD741 nulling

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

correct OP27 operation. OP27 offset voltage can be nulled to

0 (or another desired setting) using a potentiometer (see

Figure 35).

The OP27 provides stable operation with load capacitances of

up to 2000 pF and ±10 V swings; larger capacitances should be

decoupled with a 50 Ω resistor inside the feedback loop. The

OP27 is unity-gain stable.

Thermoelectric voltages generated by dissimilar metals at the

input terminal contacts can degrade the drift performance.

Best operation is obtained when both input contacts are

maintained at the same temperature.

–-

OP27

V–

V+

OUTPUT

10k

Figure 35. Offset Nulling Circuit

OFFSET VOLTAGE ADJUSTMENT

The input offset voltage of the OP27 is trimmed at wafer level.

However, if further adjustment of VOS is necessary, a 10 kΩ trim

potentiometer can be used. TCVOS is not degraded (see Figure 35).

Other potentiometer values from 1 kΩ to 1 MΩ can be used

with a slight degradation (0.1 μV/°C to 0.2 μV/°C) of TCVOS.

Trimming to a value other than zero creates a drift of approxi-

mately (VOS/300) μV/°C. For example, the change in TCVOS is

0.33 μV/°C if VOS is adjusted to 100 μV. The offset voltage

adjustment range with a 10 kΩ potentiometer is ±4 mV. If smaller

adjustment range is required, the nulling sensitivity can be

reduced by using a smaller potentiometer in conjunction with

fixed resistors. For example, Figure 36 shows a network that has

a 280 μV adjustment range.

4.7k

Ω POTT

V+

Figure 36. Offset Voltage Adjustment

NOISE MEASUREMENTS

To measure the 80 nV p-p noise specification of the OP27 in

the 0.1 Hz to 10 Hz range, the following precautions must be

observed:

•

The device must be warmed up for at 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 10-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 10 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 10 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 correlates well with a 0.1 Hz to

10 Hz p-p noise reading, since both results are determined

by the white noise and the location of the 1/f corner

frequency.

UNITY-GAIN BUFFER APPLICATIONS

When Rf ≤ 100 Ω and the input is driven with a fast, large

signal pulse (>1 V), the output waveform looks as shown in the

pulsed operation diagram (see Figure 37).

During the fast feedthrough-like portion of the output, the

input protection diodes effectively short the output to the input,

and a current, limited only by the output short-circuit protect-

ion, is drawn by the signal generator. With Rf ≥ 500 Ω, the

output is capable of handling the current requirements (IL ≤ 20 mA

at 10 V); the amplifier stays in its active mode and a smooth

transition occurs.

When Rf > 2 kΩ, a pole is created with Rf and the amplifier’s

input capacitance (8 pF) that creates additional phase shift and

reduces phase margin. A small capacitor (20 pF to 50 pF) in

parallel with Rf eliminates this problem.

–

OP27

2.8V/

μs

Figure 37. Pulsed Operation

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