OPA1611
OPA1612
www.ti.com........................................................................................................................................................ SBOS450A –JULY 2009–REVISED AUGUST 2009
TOTAL HARMONIC 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.
The OPA161x series op amps have excellent
distortion characteristics. THD + Noise is below
0.00008% (G = +1, VO = 3VRMS, BW = 80kHz)
throughout the audio frequency range, 20Hz to
20kHz, with
a
2kΩ 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 33 shows) can be used to
extend the measurement capabilities.
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 (RS equal to 50Ω,
for example) in series with the output.
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
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.
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 RS. 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
SIG. DIST.
R1
¥
R2
R3
GAIN GAIN
1
101
101
1kW
5kW
10W
50W
R3
OPA1611
VO = 3VRMS
-1
5kW
R2
R1
Signal Gain = 1+
R2
Distortion Gain = 1+
R1 II R3
Generator
Output
Analyzer
Input
Audio Precision
System Two(1)
Load
with PC Controller
(1) For measurement bandwidth, see Figure 7 through Figure 12.
Figure 33. Distortion Test Circuit
Copyright © 2009, Texas Instruments Incorporated
13
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