OP27G データシートの表示（PDF） - Analog Devices

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OP27G

Low Noise, Precision Operational Amplifier

Analog Devices 

OP27

For reference, typical source resistances of some signal sources

are listed in Table 7.

Table 7.

Device

Strain Gauge

Magnetic

Tape Head

Magnetic

Phonograph

Cartridges

Linear

Variable

Differential

Transformer

Source

Impedance

<500 Ω

<1500 Ω

<1500 Ω

<1500 Ω

Comments

Typically used in low frequency

applications.

Low is very important to reduce

self-magnetization problems

when direct coupling is used.

OP27 IB can be neglected.

Similar need for low IB in direct

coupled applications. OP27 does

not introduce any self-

magnetization problems.

Used in rugged servo-feedback

applications. Bandwidth of

interest is 400 Hz to 5 kHz.

Table 8. Open-Loop Gain

Frequency

OP07

At 3 Hz

100 dB

At 10 Hz

100 dB

At 30 Hz

90 dB

OP27

124 dB

120 dB

110 dB

OP37

125 dB

125 dB

124 dB

AUDIO APPLICATIONS

Figure 41 is an example of a phono pre-amplifier circuit using the

OP27 for A1; R1-R2-C1-C2 form a very accurate RIAA network

with standard component values. The popular method to

accomplish RIAA phono equalization is to employ frequency

dependent feedback around a high quality gain block. Properly

chosen, an RC network can provide the three necessary time

constants of 3180 µs, 318 µs, and 75 µs.

For initial equalization accuracy and stability, precision metal

film resistors and film capacitors of polystyrene or polypro-

pylene are recommended because they have low voltage

coefficients, dissipation factors, and dielectric absorption.

(High-k ceramic capacitors should be avoided here, though

low-k ceramics, such as NPO types that have excellent

dissipation factors and somewhat lower dielectric absorption,

can be considered for small values.)

Data Sheet

C4 (2)

+220µF +

R5

NŸ

MOVING MAGNET

CARTRIDGE INPUT

3

RA

CA

150pF

A1

OP27

6

NŸ

2

LF ROLLOFF

C3

OUT IN

0.47µF

R1

NŸ

R4

NŸ

C1

0.03µF

OUTPUT

R2

NŸ

C2

0.01µF

R3

Ÿ

G = 1kHz GAIN

= 0.101 ( 1 + R1 )

R3

= 98.677 (39.9dB) AS SHOWN

Figure 41. Phono Preamplifier Circuit

The OP27 brings a 3.2 nV/√Hz voltage noise and 0.45 pA/√Hz

current noise to this circuit. To minimize noise from other

sources, R3 is set to a value of 100 Ω, generating a voltage noise

of 1.3 nV/√Hz. The noise increases the 3.2 nV/√Hz of the

amplifier by only 0.7 dB. With a 1 kΩ source, the circuit noise

measures 63 dB below a 1 mV reference level, unweighted, in a

20 kHz noise bandwidth.

Gain (G) of the circuit at 1 kHz can be calculated by the

expression:

G

0.101 ¨©§1



R1

R3

¸¹·

For the values shown, the gain is just under 100 (or 40 dB).

Lower gains can be accommodated by increasing R3, but gains

higher than 40 dB show more equalization errors because of the

8 MHz gain bandwidth of the OP27.

This circuit is capable of very low distortion over its entire

range, generally below 0.01% at levels up to 7 V rms. At 3 V

output levels, it produces less than 0.03% total harmonic

distortion at frequencies up to 20 kHz.

Capacitor C3 and Resistor R4 form a simple −6 dB per octave

rumble filter, with a corner at 22 Hz. As an option, the switch

selected Shunt Capacitor C4, a nonpolarized electrolytic,

bypasses the low frequency roll-off. Placing the rumble filter’s

high-pass action after the preamplifier has the desirable result

of discriminating against the RIAA-amplified low frequency

noise components and pickup produced low frequency

disturbances.

A preamplifier for NAB tape playback is similar to an RIAA

phono preamplifier, though more gain is typically demanded,

along with equalization requiring a heavy low frequency boost.

The circuit in Figure 41 can be readily modified for tape use, as

shown by Figure 42.

Rev. H | Page 16 of 21

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