OP27GSZ datasheet(16/20 Pages) AD

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OP27

Rev. F | Page 16 of 20

For reference, typical source resistances of some signal sources

are listed in Table 7.

Table 7.

Device

Source

Impedance

Comments

Strain Gauge

<500 Ω

Typically used in low frequency

applications.

Magnetic

Tape Head

<1500 Ω

Low is very important to reduce

self-magnetization problems

when direct coupling is used.

OP27 IB can be neglected.

Magnetic

Phonograph

Cartridges

<1500 Ω

Similar need for low IB in direct

coupled applications. OP27 does

not introduce any self-

magnetization problems.

Linear

Variable

Differential

Transformer

<1500 Ω

Used in rugged servo-feedback

applications. Bandwidth of

interest is 400 Hz to 5 kHz.

Table 8. Open-Loop Gain

Frequency

OP07

OP27

OP37

@ 3 Hz

100 dB

124 dB

125 dB

@ 10 Hz

100 dB

120 dB

125 dB

@ 30 Hz

90 dB

110 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.)

150pF

OP27

47.5kΩ

MOVING MAGNET

CARTRIDGE INPUT

C4 (2)

220µF

0.03µF

0.01µF

0.47µF

LF ROLLOFF

OUT

OUTPUT

100kΩ

75kΩ

97.6kΩ

7.87kΩ

100Ω

G = 1kHz GAIN

= 0.101 ( 1 +

)

= 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:

⎟

⎠

⎞

⎜

⎝

⎛ +

101

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.

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