Datasheet

OPA27, OPA37
10
SBOS135C
www.ti.com
COMPENSATION
Although internally compensated for unity-gain stability, the
OPA27 may require a small capacitor in parallel with a
feedback resistor (R
F
) which is greater than 2kΩ. This ca-
pacitor will compensate the pole generated by R
F
and C
IN
and eliminate peaking or oscillation.
INPUT PROTECTION
Back-to-back diodes are used for input protection on the
OPA27 and OPA37. Exceeding a few hundred millivolts differ-
ential input signal will cause current to flow, and without
external current limiting resistors, the input will be destroyed.
Accidental static discharge, as well as high current, can dam-
age the amplifier’s input circuit. Although the unit may still be
functional, important parameters such as input offset voltage,
drift, and noise may be permanently damaged, as will any
precision operational amplifier subjected to this abuse.
Transient conditions can cause feedthrough due to the amplifier’s
finite slew rate. When using the OPA27 as a unity-gain buffer
(follower) a feedback resistor of 1kΩ is recommended, as
shown in Figure 6.
NOISE: BIPOLAR VERSUS FET
Low-noise circuit design requires careful analysis of all noise
sources. External noise sources can dominate in many
cases, so consider the effect of source resistance on overall
operational amplifier noise performance. At low source im-
pedances, the lower voltage noise of a bipolar operational
amplifier is superior, but at higher impedances the high
current noise of a bipolar amplifier becomes a serious liabil-
ity. Above about 15kΩ, the OPA111 low-noise FET opera-
tional amplifier is recommended for lower total noise than the
OPA27, as shown in Figure 5.
FIGURE 3. Offset Voltage Trim.
FIGURE 5. Voltage Noise Spectral Density Versus Source
Resistance.
FIGURE 6. Pulsed Operation.
FIGURE 8. Unity-Gain Inverting Amplifier.
FIGURE 7. Low-Noise RIAA Preamplifier.
FIGURE 4. High Resolution Offset Voltage Trim.
1
2
3
4
6
±4mV Typical Trim Range
NOTE: (1) 10kΩ to 1MΩ
Trim Potentiometer
(10kΩ Recommended).
+V
CC
–V
CC
OPA27/37
7
8
(1)
1
2
3
4
6
±280µV Typical Trim Range
NOTE: (1) 1kΩ Trim Potentiometer.
+V
CC
–V
CC
OPA27/37
7
8
4.7kΩ 4.7kΩ
(1)
100 1k 10k 100k 1M 10M
1k
100
10
1
Voltage Noise Spectral Density, E
O
Typical at 1kHz (nV/√Hz)
OPA111 + Resistor
OPA27 + Resistor
Source Resistance, R
S
(Ω)
E
O
R
S
E
O
= √e
n
2
+ (i
n
R
S
)
2
+ 4kTR
S
F
O
= 1kHz
Resistor Noise Only
OPA27 + Resistor
OPA111 + Resistor
Resistor Noise Only
OPA27 Output
1.9V/µs
R
F
≈ 1kΩ
Input
–
+
OPA37
Output
97.6kΩ
G ≈ 40dB at 1kHz.
Metal film resistors.
Film capacitors.
R
L
and C
L
per cartridge
manufacturer’s
recommendations.
100Ω
2
3
6
0.03µF0.01µF
7.87kΩ
1µF
20kΩ
R
L
Moving
Magnet
Cartridge
C
L
OPA27
Output
Input
1kΩ
1kΩ
2
3
6