OP37FP AD [Analog Devices], OP37FP Datasheet - Page 13

• Manufacturer Part Number: OP37FP
• Description: Low Noise, Precision, High Speed Operational Amplifier
• Manufacturer: AD [Analog Devices]

Device
Straln Gauge
Magnetic
Tapehead
Magnetic
Phonograph
Cartridges
Linear Variable <1500 Ω
Differential
Transformer
Audio Applications
The following applications information has been abstracted from
a PMI article in the 12/20/80 issue of Electronic Design magazine
and updated.
Figure 8 is an example of a phono pre-amplifier circuit using the
OP27 for A1; R1-R2-C1-C2 form a very accurate RIAA net-
work 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 polypropylene
are recommended since they have low voltage coefficients,
dissipation factors, and dielectric absorption.
capacitors should be avoided here, though low-K ceramics—
such as NPO types, which have excellent dissipation factors,
and somewhat lower dielectric absorption—can be considered
for small values or where space is at a premium.)
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, which generates a voltage noise of
1.3 nV/√Hz. The noise increases the 3.2 nV/√Hz of the amplifier
CARTRIDGE INPUT
MOVING MAGNET
Ra
47.5k
Ca
150pF
Source
Impedance Comments
<500 Ω
<1500 Ω
<1500 Ω
OP27
A1
Table I. TBD
is 400 Hz to 5 kHz.
Typically used in low-frequency
applications.
Low I
set-magnetization problems when
direct coupling is used. OP37
I
Similar need for low I
coupled applications. OP47 will not
introduce any self-magnetization
problem.
Used in rugged servo-feedback
applications. Bandwidth of interest
B
R3
100
R1
97.6k
can be neglected.
0.47 F
R2
7.87k
G = 1kHz GAIN
1
B
C3
= 0.101 (
= 98.677 (39.9dB) AS SHOWN
very important to reduce
+
C4 (2)
220 F
LF ROLLOFF
1 +
C1
0.03 F
C2
0.01 F
4
+
(High-K ceramic
R1
R3
75k
OUT
)
R4
B
100k
IN
in direct
R5
OUTPUT
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:
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 will 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 will produce less than 0.03% total harmonic distortion
at frequencies up to 20 kHz.
Capacitor C3 and resistor R4form 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 rolloff. Placing the rumble filter’s high-pass
action after the preamp 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 preamp, though more gain is typically demanded, along
with equalization requiring a heavy low-frequency boost. The
circuit In Figure 4 can be readily modified for tape use, as
shown by Figure 5.
While the tape-equalization requirement has a flat high frequency
gain above 3 kHz (t
for unity gain. The decompensated OP37 provides a greater
bandwidth and slew rate. For many applications, the idealized
time constants shown may require trimming of R
optimize frequency response for non ideal tape head perfor-
mance and other factors.
The network values of the configuration yield a 50 dB gain at 1 kHz,
and the dc gain is greater than 70 dB. Thus, the worst-case out-
put offset is just over 500 mV. A single 0.47 µF output capacitor
can block this level without affecting the dynamic range.
The tape head can be coupled directly to the amplifier input,
since the worst-case bias current of 85 nA with a 400 mH, 100 µin.
head (such as the PRB2H7K) will not be troublesome.
One potential tape-head problem is presented by amplifier bias-
current transients which can magnetize a head. The OP27 and
HEAD
TAPE
2
Ra
= 50 µs), the amplifier need not be stabilized
G
5
=
Ca
0 101 1
.
100k
5k
–
+
R2
OP37
33k



R1
0.01 F
+
R
R
1
3



0.47 F
T1 = 3180 s
T2 = 50 s
15k
A
and R2 to
OP37