Datasheet

R
2
OPA1611
R
1
Signal Gain = 1+
Distortion Gain = 1+
R
3 V = 3V
O RMS
Generator
Output
Analyzer
Input
Audio Precision
System Two
(1)
with PC Controller
Load
SIG.
GAIN
DIST.
GAIN
R
1
R
2
R
3
¥
4.99kW
1kW
4.99kW
10W
49.9W
1
-1
101
101
R
2
R
1
R
2
R II R
1 3
[phone]W
4.99kW 49.9W
OPA1611
OPA1612
www.ti.com SBOS450B –JULY 2009–REVISED JULY 2011
TOTAL HARMONIC DISTORTION Validity of this technique can be verified by
MEASUREMENTS duplicating measurements at high gain and/or high
frequency where the distortion is within the
The OPA161x series op amps have excellent
measurement capability of the test equipment.
distortion characteristics. THD + Noise is below
Measurements for this data sheet were made with an
0.00008% (G = +1, V
O
= 3V
RMS
, BW = 80kHz)
Audio Precision System Two distortion/noise
throughout the audio frequency range, 20Hz to
analyzer, which greatly simplifies such repetitive
20kHz, with a 2kΩ load (see Figure 7 for
measurements. The measurement technique can,
characteristic performance).
however, be performed with manual distortion
measurement instruments.
The distortion produced by OPA1611 series op amps
is below the measurement limit of many commercially
available distortion analyzers. However, a special test CAPACITIVE LOADS
circuit (such as Figure 33 shows) can be used to
The dynamic characteristics of the OPA1611 and
extend the measurement capabilities.
OPA1612 have been optimized for commonly
Op amp distortion can be considered an internal error encountered gains, loads, and operating conditions.
source that can be referred to the input. Figure 33 The combination of low closed-loop gain and high
shows a circuit that causes the op amp distortion to capacitive loads decreases the phase margin of the
be 101 times (or approximately 40dB) greater than amplifier and can lead to gain peaking or oscillations.
that normally produced by the op amp. The addition As a result, heavier capacitive loads must be isolated
of R
3
to the otherwise standard noninverting amplifier from the output. The simplest way to achieve this
configuration alters the feedback factor or noise gain isolation is to add a small resistor (R
S
equal to 50Ω,
of the circuit. The closed-loop gain is unchanged, but for example) in series with the output.
the feedback available for error correction is reduced
This small series resistor also prevents excess power
by a factor of 101, thus extending the resolution by
dissipation if the output of the device becomes
101. Note that the input signal and load applied to the
shorted. Figure 19 and Figure 20 illustrate graphs of
op amp are the same as with conventional feedback
Small-Signal Overshoot vs Capacitive Load for
without R
3
. The value of R
3
should be kept small to
several values of R
S
. Also, refer to Applications
minimize its effect on the distortion measurements.
Bulletin AB-028 (literature number SBOA015,
available for download from the TI web site) for
details of analysis techniques and application circuits.
(1) For measurement bandwidth, see Figure 7 through Figure 12.
Figure 33. Distortion Test Circuit
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