OP176GS datasheet(9/21 Pages) AD

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OP176

REV. 0

–9–

Noise

The voltage noise density of the OP176 is below 6 nV/

√Hz from

30 Hz. This enables low noise designs to have good perfor-

mance throughout the full audio range. Figure 27 shows a

typical OP176 with a 1/f corner at 6 Hz.

Figure 27. 1/f Noise Corner

Noise Testing

For audio applications the noise density is usually the most

important noise parameter. For characterization the OP176 is

tested using an Audio Precision, System One. The input signal

to the Audio Precision must be amplified enough to measure

accurately. For the OP176 the noise is gained by approximately

1020 using the circuit shown in Figure 28. Any readings on the

Audio Precision must then be divided by the gain. In imple-

menting this test fixture, good supply bypassing is essential.

Figure 28. Noise Test

Upgrading “5534‘’ Sockets

The OP176 is a superior amplifier for upgrading existing

designs using the industry standard 5534. In most application

circuits, the OP176 can directly replace the 5534 without any

modifications to the surrounding circuitry. Like the 5534, the

OP176 follows the industry standard, single op amp pinout. The

difference between these two devices is the location of the null

pins and the 5534’s compensation capacitor.

The 5534 normally requires a 22 pF capacitor between Pins 5

and 8 for stable operation. Since the OP176 is internally

compensated for unity gain operation, it does not require

external compensation. Nevertheless, if the 5534 socket already

includes a capacitor, the OP176 can be inserted without

removing it. Since the OP176’s Pin 8 is a “NO CONNECT’’

pin, there is no internal connection to that pin. Thus, the 22 pF

capacitor would be electrically connected through Pin 5 to the

internal nulling circuitry. With the other end left open, the

capacitor should have no effect on the circuit. However, to

avoid altogether any possibility for noise injection, it is recom-

mended that the 22 pF capacitor be cut out of the circuit

entirely.

If the original 5534 socket includes offset nulling circuitry, one

would find a 10 k

Ω to 100 kΩ potentiometer connected between

Pins 1 and 8 with said potentiometer’s wiper arm connected to

V+. In order to upgrade the socket to the OP176, this circuit

should be removed before inserting the OP176 for its offset

nulling scheme uses Pins 1 and 5. Whereas the wiper arm of the

5534 trimming potentiometer is connected to the positive

supply, the OP176’s wiper arm is connected to the negative

supply. Directly substituting the OP176 into the original socket

would inject a large current imbalance into its input stage. In

this case, the potentiometer should be removed altogether, or, if

nulling is still required, the trimming potentiometer should be

rewired to match the nulling circuit as illustrated in Figure 29.

Figure 29. Offset Voltage Nulling Scheme

Input Overcurrent Protection

The maximum input differential voltage that can be applied to

the OP176 is determined by a pair of internal Zener diodes

connected across its inputs. They limit the maximum differen-

tial input voltage to

±7.5 V. This is to prevent emitter-base

junction breakdown from occurring in the input stage of the

OP176 when very large differential voltages are applied.

However, in order to preserve the OP176’s low input noise

voltage, internal resistances in series with the inputs were not

used to limit the current in the clamp diodes. In small signal

applications, this is not an issue; however, in applications where

large differential voltages can be inadvertently applied to the

device, large transient currents can flow through these diodes.

Although these diodes have been designed to carry a current of

±5 mA, external resistors as shown in Figure 30 should be used

in the event that the OP176’s differential voltage were to exceed

±7.5 V.

Figure 30. Input Overcurrent Protection

OP176

–VS

VOUT

ΩP1 = 10kΩ

VOS TRIM RANGE = ±2mV

+VS

OP176

1.4kΩ

–

50Hz /

300 mHz

\ 0 Hz

MKR:

5.4 Hz

10.0 µV /DIV

CH A: 80.0 µV FS

BW:

MKR: 15.9 µV/ Hz

OP176

OP37

OUTPUT

4.42k

Ω

909

Ω

909

Ω

100

Ω

490

Ω

100

Ω

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