OPA27FJ Datasheet, PDF (10/22 Pages) Texas Instruments – Ultra-Low Noise, Precision OPERATIONAL AMPLIFIERS

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.
+V_CC
(1)
7
2
8
1
OPA27/37
3
NOTE: (1) 10kΩ to 1MΩ
Trim Potentiometer
(10kΩ Recommended).
6
4
±4mV Typical Trim Range
–V_CC
FIGURE 3. Offset Voltage Trim.
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.
R_F
≈ 1kΩ
+V_CC
(1)
NOTE: (1) 1kΩ Trim Potentiometer.
7 4.7kΩ 4.7kΩ
2
8
1
OPA27/37
6
3
4 ±280µV Typical Trim Range
–V_CC
FIGURE 4. High Resolution Offset Voltage Trim.
1k
OPA27 + Resistor
E_O
OPA111 + Resistor
100
R_S
OPA111 + Resistor
10
Resistor Noise Only
1
100
Resistor Noise Only
OPA27 + Resistor
1k
10k
100k
1M
10M
Source Resistance, R_S (Ω)
E_O = √e_n2 + (i_nR_S)² + 4kTR_S
F_O = 1kHz
–
+
Input
OPA27
Output
1.9V/µs
FIGURE 6. Pulsed Operation.
G ≈ 40dB at 1kHz.
Metal film resistors.
Film capacitors.
R_L and C_L per cartridge
manufacturer’s
recommendations.
100Ω
Moving
Magnet
R_L
Cartridge
7.87kΩ
0.01µF 0.03µF
97.6kΩ
2
6
3 ^OPA37
C_L
1µF _Output
20kΩ
FIGURE 7. Low-Noise RIAA Preamplifier.
1kΩ
1kΩ
2
Input
6
OPA27
3
Output
FIGURE 5. Voltage Noise Spectral Density Versus Source
Resistance.
FIGURE 8. Unity-Gain Inverting Amplifier.
10
OPA27, OPA37
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