OPA1602_15 Datasheet, PDF (13/29 Pages) Texas Instruments – High-Performance, Bipolar-Input AUDIO OPERATIONAL AMPLIFIERS

OPA1602
OPA1604
www.ti.com
TOTAL HARMONIC DISTORTION
MEASUREMENTS
The OPA160x series op amps have excellent
distortion characteristics. THD + noise is below
0.00008% (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 OPA160x series op
amps is below the measurement limit of many
commercially available distortion analyzers. However,
a special test circuit (such as Figure 35 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 35
shows a circuit that causes the op amp distortion to
be gained up (refer to the table in Figure 35 for the
distortion gain factor for various signal gains). 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 the distortion gain factor,
thus extending the resolution by the same amount.
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.
SBOS474B – APRIL 2011 – REVISED NOVEMBER 2011
The 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.
CAPACITIVE LOADS
The dynamic characteristics of the OPA1602 and
OPA1604 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 50Ω,
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 illustrates a graph of Small-Signal
Overshoot vs Capacitive Load for several values of
R_S. Also, refer to Applications Bulletin AB-028
(literature number SBOA015, available for download
from the TI web site) for details of analysis
techniques and application circuits.
R₁
R₂
Signal Gain = 1+ ^R2
R₁
Distortion Gain = 1+ ^R2
R₁ II R₃
R₃
OPA160x
V_O = 3V_RMS
Generator
Output
Analyzer
Input
SIGNAL DISTORTION
GAIN
GAIN
R₁
R₂
R₃
+1
101
Â¥ 1kW 10W
-1
101
4.99kW 4.99kW 49.9W
+10
110
549W 4.99kW 49.9W
Audio Precision
System Two⁽¹⁾
with PC Controller
Load
(1) For measurement bandwidth, see Figure 7 through Figure 12.
Figure 35. Distortion Test Circuit
Copyright © 2011, Texas Instruments Incorporated
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