OPA1641_15 Datasheet, PDF (12/29 Pages) Texas Instruments – High-Performance, JFET-Input AUDIO OPERATIONAL AMPLIFIERS

OPA1641
OPA1642
OPA1644
SBOS484B – DECEMBER 2009 – REVISED AUGUST 2010
TOTAL HARMONIC DISTORTION
MEASUREMENTS
The OPA164x series op amps have excellent
distortion characteristics. THD + Noise is below
0.00005% (G = +1, V_O = 3V_RMS, BW = 80kHz)
throughout the audio frequency range, 20Hz to
20kHz, with a 2kΩ load (see Figure 7 for
characteristic performance).
The distortion produced by the OPA164x series op
amps is below the measurement limit of many
commercially available distortion analyzers. However,
a special test circuit (such as Figure 33 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 33
shows a circuit that causes the op amp distortion to
be 101 times (or approximately 40dB) greater than
that normally produced by the op amp. The addition
of R₃ to the otherwise standard noninverting amplifier
R₁
R₂
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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. The value of R₃ should be kept 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/noise
analyzer, which greatly simplifies such repetitive
measurements. The measurement technique can,
however, be performed with manual distortion
measurement instruments.
space
Signal Gain = 1+ ^R2
R₁
Distortion Gain = 1+ ^R2
R₁ II R₃
R₃
OPA1641
V_O = 3V_RMS
SIGNAL DISTORTION
GAIN
GAIN
R₁
R₂
R₃
1
101
Â¥ 1kW 10W
11
101
100W 1kW 11W
Generator
Output
Analyzer
Input
Audio Precision
System Two⁽¹⁾
with PC Controller
Load
(1) For measurement bandwidth, see Figure 7 through Figure 12.
Figure 33. Distortion Test Circuit
12
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