OPA1611_15 Datasheet, PDF (17/31 Pages) Texas Instruments – SoundPlus High-Performance, Bipolar-Input Audio Operational Amplifiers

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OPA1611, OPA1612
SBOS450C – JULY 2009 – REVISED AUGUST 2014
8.3 Total Harmonic Distortion Measurements
The OPA161x series op amps have excellent distortion characteristics. THD + noise is below 0.00008% (G = +1,
V_O = 3 V_RMS, BW = 80 kHz) throughout the audio frequency range, 20 Hz to 20 kHz, with a 2-kΩ load (see
Figure 7 for characteristic performance).
The distortion produced by OPA1611 series op amps is below the measurement limit of many commercially
available distortion analyzers. However, a special test circuit (such as Figure 34 shows) can be used to extend
the measurement capabilities.
Op amp distortion can be considered an internal error source that can be referred to the input. Figure 34 shows a
circuit that causes the op amp distortion to be 101 times (or approximately 40 dB) greater than that normally
produced by the op amp. The addition of R₃ to the otherwise standard noninverting 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, thus extending the resolution by 101. Note that the input signal and
load applied to the op amp are the same as with conventional feedback without R₃. Keep the value of R₃ small to
minimize its effect on the distortion measurements.
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 an audio precision system two distortion and noise analyzer, which greatly simplifies such repetitive
measurements. The measurement technique can, however, be performed with manual distortion measurement
instruments.
R₁
R₂
Signal Gain = 1+ ^R2
R₁
Distortion Gain = 1+ ^R2
R₁ II R₃
R₃
OPA1611
V_O = 3V_RMS
Generator
Output
Analyzer
Input
SIG. DIST.
GAIN GAIN R1
R₂
R₃
1 101 ¥ 1kW 10W
-1 101 4.99kW 4.99kW 49.9W
[phone]W 4.99kW 49.9W
Audio Precision
System Two⁽¹⁾
with PC Controller
Load
(1) For measurement bandwidth, see Figure 7 through Figure 12.
Figure 34. Distortion Test Circuit
8.4 Capacitive Loads
The dynamic characteristics of the OPA1611 and OPA1612 have been optimized for commonly encountered
gains, loads, and operating conditions. The combination of low closed-loop gain and high capacitive loads
decreases the phase margin of the amplifier and can lead to gain peaking or oscillations. As a result, heavier
capacitive loads must be isolated from the output. The simplest way to achieve this isolation is to add a small
resistor (R_S equal to 50 Ω, for example) in series with the output.
This small series resistor also prevents excess power dissipation if the output of the device becomes shorted.
Figure 19 and Figure 20 illustrate graphs of Small-Signal Overshoot vs Capacitive Load for several values of R_S.
Also, refer to Applications Bulletin AB-028, Feedback Plots Define Op Amp AC Performance (SBOA015),
available for download from the TI web site, for details of analysis techniques and application circuits.
Copyright © 2009–2014, Texas Instruments Incorporated
Product Folder Links: OPA1611 OPA1612
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