Datasheet OP27 (Analog Devices) - 17
| Manufacturer | Analog Devices |
| Description | Low Noise, Precision Operational Amplifier |
| Pages / Page | 21 / 17 — 0.47µF. OP27. TAPE. HEAD. 5mF. 0.01µF. T1 = 3180µs. LOW IMPEDANCE. OP27/. … |
| Revision | H |
| File Format / Size | PDF / 416 Kb |
| Document Language | English |
0.47µF. OP27. TAPE. HEAD. 5mF. 0.01µF. T1 = 3180µs. LOW IMPEDANCE. OP27/. T2 = 50µs. MICROPHONE INPUT. OUTPUT. OP37. = T2 . 3 4 -0 7. 1 3 0 0
Model Line for this Datasheet
Text Version of Document
Data Sheet OP27
+
sources is about 6 nV/√Hz, equivalent to 0.9 µV in a 20 kHz
0.47µF OP27
noise bandwidth, or nearly 61 dB below a 1 mV input signal.
TAPE R C A A HEAD – N
Measurements confirm this predicted performance.
R1 N C1 R1 R3 R6 5mF N N R2 N 0.01µF –
2
4
T1 = 3180µs LOW IMPEDANCE
-0
R OP27/ R7
7
P T2 = 50µs
1
MICROPHONE INPUT
3
N
0
N OUTPUT OP37
0
= T2 +
Figure 42. Tape Head Preamplifier
R2 R4 R3 R4
While the tape equalization requirement has a flat high
= N N R1 R2 3 4 -0 7
frequency gain above 3 kHz (T2 = 50 µs), the amplifier need
1 3 0 0
not be stabilized for unity gain. The decompensated OP37 Figure 43. Fixed Gain Transformerless Microphone Preamplifier provides a greater bandwidth and slew rate. For many applica- For applications demanding appreciably lower noise, a high tions, the idealized time constants shown can require trimming quality microphone transformer coupled preamplifier (Figure 44) of R1 and R2 to optimize frequency response for nonideal tape incorporates the internally compensated OP27. T1 is a JE- head performance and other factors (see the References section). 115K-E 150 Ω/15 kΩ transformer that provides an optimum The network values of the configuration yield a 50 dB gain at source resistance for the OP27 device. The circuit has an overall 1 kHz, and the dc gain is greater than 70 dB. Thus, the worst- gain of 40 dB, the product of the transformer’s voltage setup and case output offset is just over 500 mV. A single 0.47 µF output the op amp’s voltage gain. capacitor can block this level without affecting the dynamic
C2
range.
1800pF
The tape head can be coupled directly to the amplifier input,
R1 R2 1
because the worst-case bias current of 80 nA with a 400 mH, 100 µ inch head (such as the PRB2H7K) is not troublesome.
2
Amplifier bias-current transients that can magnetize a head
A1 T11 OP27 6 OUTPUT
present one potential tape head problem. The OP27 and OP37
3
are free of bias current transients upon power-up or power-
R3 SOURCE
down. It is always advantageous to control the speed of power
1 T1 – JENSEN JE – 115K – E
4 4 supply rise and fall to eliminate transients.
JENSEN TRANSFORMERS
-0 7 1 3 0 0 In addition, the dc resistance of the head should be carefully Figure 44. High Quality Microphone Transformer Coupled Preamplifier controlled and preferably below 1 kΩ. For this configuration, Gain can be trimmed to other levels, if desired, by adjusting R2 the bias current induced offset voltage can be greater than the or R1. Because of the low offset voltage of the OP27, the output 100 pV maximum offset if the head resistance is not sufficiently offset of this circuit is very low, 1.7 mV or less, for a 40 dB gain. controlled. The typical output blocking capacitor can be eliminated in such A simple, but effective, fixed gain transformerless microphone cases, but it is desirable for higher gains to eliminate switching preamp (Figure 43) amplifies differential signals from low transients. impedance microphones by 50 dB and has an input impedance
+18V
of 2 kΩ. Because of the high working gain of the circuit, an OP37 helps to preserve bandwidth, which is 110 kHz. As the OP37 is a decompensated device (minimum stable gain of 5), a
8 2
dummy resistor, R
7
p, may be necessary if the microphone is to be
OP27 6
unplugged. Otherwise, the 100% feedback from the open input
3 4
can cause the amplifier to oscillate. 5 4 -0 7 Common-mode input noise rejection will depend upon the 1
–18V
3 0 0 match of the bridge-resistor ratios. Either close tolerance (0.1%) Figure 45. Burn-In Circuit types should be used, or R4 should be trimmed for best CMRR. All resistors should be metal film types for best stability and low Capacitor C2 and Resistor R2 form a 2 µs time constant in this noise. circuit, as recommended for optimum transient response by the transformer manufacturer. With C2 in use, A1 must have unity- Noise performance of this circuit is limited more by the Input gain stability. For situations where the 2 µs time constant is not Resistors R1 and R2 than by the op amp, as R1 and R2 each necessary, C2 can be deleted, allowing the faster OP37 to be generate a 4 nV/√Hz noise, while the op amp generates a employed. 3.2 nV/√Hz noise. The rms sum of these predominant noise Rev. H | Page 17 of 21
+
sources is about 6 nV/√Hz, equivalent to 0.9 µV in a 20 kHz
0.47µF OP27
noise bandwidth, or nearly 61 dB below a 1 mV input signal.
TAPE R C A A HEAD – N
Measurements confirm this predicted performance.
R1 N C1 R1 R3 R6 5mF N N R2 N 0.01µF –
2
4
T1 = 3180µs LOW IMPEDANCE
-0
R OP27/ R7
7
P T2 = 50µs
1
MICROPHONE INPUT
3
N
0
N OUTPUT OP37
0
= T2 +
Figure 42. Tape Head Preamplifier
R2 R4 R3 R4
While the tape equalization requirement has a flat high
= N N R1 R2 3 4 -0 7
frequency gain above 3 kHz (T2 = 50 µs), the amplifier need
1 3 0 0
not be stabilized for unity gain. The decompensated OP37 Figure 43. Fixed Gain Transformerless Microphone Preamplifier provides a greater bandwidth and slew rate. For many applica- For applications demanding appreciably lower noise, a high tions, the idealized time constants shown can require trimming quality microphone transformer coupled preamplifier (Figure 44) of R1 and R2 to optimize frequency response for nonideal tape incorporates the internally compensated OP27. T1 is a JE- head performance and other factors (see the References section). 115K-E 150 Ω/15 kΩ transformer that provides an optimum The network values of the configuration yield a 50 dB gain at source resistance for the OP27 device. The circuit has an overall 1 kHz, and the dc gain is greater than 70 dB. Thus, the worst- gain of 40 dB, the product of the transformer’s voltage setup and case output offset is just over 500 mV. A single 0.47 µF output the op amp’s voltage gain. capacitor can block this level without affecting the dynamic
C2
range.
1800pF
The tape head can be coupled directly to the amplifier input,
R1 R2 1
because the worst-case bias current of 80 nA with a 400 mH, 100 µ inch head (such as the PRB2H7K) is not troublesome.
2
Amplifier bias-current transients that can magnetize a head
A1 T11 OP27 6 OUTPUT
present one potential tape head problem. The OP27 and OP37
3
are free of bias current transients upon power-up or power-
R3 SOURCE
down. It is always advantageous to control the speed of power
1 T1 – JENSEN JE – 115K – E
4 4 supply rise and fall to eliminate transients.
JENSEN TRANSFORMERS
-0 7 1 3 0 0 In addition, the dc resistance of the head should be carefully Figure 44. High Quality Microphone Transformer Coupled Preamplifier controlled and preferably below 1 kΩ. For this configuration, Gain can be trimmed to other levels, if desired, by adjusting R2 the bias current induced offset voltage can be greater than the or R1. Because of the low offset voltage of the OP27, the output 100 pV maximum offset if the head resistance is not sufficiently offset of this circuit is very low, 1.7 mV or less, for a 40 dB gain. controlled. The typical output blocking capacitor can be eliminated in such A simple, but effective, fixed gain transformerless microphone cases, but it is desirable for higher gains to eliminate switching preamp (Figure 43) amplifies differential signals from low transients. impedance microphones by 50 dB and has an input impedance
+18V
of 2 kΩ. Because of the high working gain of the circuit, an OP37 helps to preserve bandwidth, which is 110 kHz. As the OP37 is a decompensated device (minimum stable gain of 5), a
8 2
dummy resistor, R
7
p, may be necessary if the microphone is to be
OP27 6
unplugged. Otherwise, the 100% feedback from the open input
3 4
can cause the amplifier to oscillate. 5 4 -0 7 Common-mode input noise rejection will depend upon the 1
–18V
3 0 0 match of the bridge-resistor ratios. Either close tolerance (0.1%) Figure 45. Burn-In Circuit types should be used, or R4 should be trimmed for best CMRR. All resistors should be metal film types for best stability and low Capacitor C2 and Resistor R2 form a 2 µs time constant in this noise. circuit, as recommended for optimum transient response by the transformer manufacturer. With C2 in use, A1 must have unity- Noise performance of this circuit is limited more by the Input gain stability. For situations where the 2 µs time constant is not Resistors R1 and R2 than by the op amp, as R1 and R2 each necessary, C2 can be deleted, allowing the faster OP37 to be generate a 4 nV/√Hz noise, while the op amp generates a employed. 3.2 nV/√Hz noise. The rms sum of these predominant noise Rev. H | Page 17 of 21
