OPA2604-Q1 Datasheet, PDF (8/16 Pages) Texas Instruments – DUAL FET-INPUT, LOW DISTORTION OPERATIONAL AMPLIFIER

OPA2604ĆQ1
DUAL FETĆINPUT, LOW DISTORTION
OPERATIONAL AMPLIFIER
SGLS209 − NOVEMBER 2003
APPLICATION INFORMATION
The OPA2604 is unity-gain stable, making it easy to use in a wide range of circuitry. Applications with noisy or
high impedance power supply lines may require decoupling capacitors close to the device pins. In most cases
1 µF tantalum capacitors are adequate.
distortion measurements
The distortion produced by the OPA2604 is below the measurement limit of virtually all commercially available
equipment. A special test circuit, however, can be used to extend the measurement capabilities.
Op amp distortion can be considered an internal error source which can be referred to the input. Figure 22
shows a circuit which causes the op amp distortion to be 101 times greater than normally produced by the op
amp. The addition of R₃ to the otherwise standard non-inverting amplifier configuration alters the feedback
factor or noise gain of the circuit. The closed-loop gain is unchanged, but the feedback available for error
correction is reduced by a factor of 101. This extends the measurement limit, including the effects of the
signal-source purity, by a factor of 101. Note that the input signal and load applied to the op amp are the same
as with conventional feedback without R₃.
Validity of this technique can be verified by duplicating measurements at high gain and/or high frequency where
the distortion is within the measurement capability of the test equipment. Measurements for this data sheet were
made with the Audio Precision System One which greatly simplifies such repetitive measurements. The
measurement technique can, however, be performed with manual distortion measurement instruments.
R1
R2
12
R3
OPA2604
VO = 10Vp−p
(3.5Vrms)
SIG. DIST.
GAIN GAIN ^R1
R2
R3
1 101 ∞ 5kΩ 50Ω
10 101 500Ω 5kΩ 500Ω
100 101 50Ω 5kΩ ∞
Generator
Output
Analyzer
Input
Audio Precision
RL
System One
1kΩ
Analyzer*
IBM PC
or
Compatible
* Measurement BW = 80kHz
Figure 22. Distortion Test Circuit
capacitive loads
The dynamic characteristics of the OPA2604 have been optimized for commonly encountered gains, loads and
operating conditions. The combination of low closed-loop gain and capacitive load will decrease the phase
margin and may lead to gain peaking or oscillations. Load capacitance reacts with the op amp’s open-loop
output resistance to form an additional pole in the feedback loop. Figure 23 shows various circuits which
preserve phase margin with capacitive load. Request Application Bulletin AB-028 for details of analysis
techniques and applications circuits.
For the unity-gain buffer, Figure a, stability is preserved by adding a phase-lead network, R_C and C_C. Voltage
drop across R_C will reduce output voltage swing with heavy loads. An alternate circuit, Figure b, does not limit
the output with low load impedance. It provides a small amount of positive feed-back to reduce the net feedback
factor. Input impedance of this circuit falls at high frequency as op amp gain rolloff reduces the bootstrap action
on the compensation network.
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