Datasheet

R
2
OPA166x
R
1
Signal Gain = 1+
Distortion Gain = 1+
R
3 V = 3 V
O RMS
Generator
Output
Analyzer
Input
Audio Precision
System Two
(1)
with PC Controller
SIGNAL
GAIN
DISTORTION
GAIN
R
1
R
2
R
3
¥
4.99 kW
1 kW
4.99 kW
10 W
49.9 W
+1
-1
101
549 W 4.99 kW 49.9 W+10 110
101
R
2
R
1
R
2
R II R
1 3
Load
OPA1662
OPA1664
SBOS489 –DECEMBER 2011 www.ti.com
TOTAL HARMONIC DISTORTION The 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 OPA166x 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
0.0006% (G = +1, V
O
= 3 V
RMS
, BW = 80kHz)
Audio Precision System Two distortion/noise
throughout the audio frequency range, 20 Hz to 20
analyzer, which greatly simplifies such repetitive
kHz, with a 2-kΩ load (see Figure 7 for characteristic
measurements. The measurement technique can,
performance).
however, be performed with manual distortion
measurement instruments.
The distortion produced by the OPA166x series op
amps is below the measurement limit of many
commercially available distortion analyzers. However, CAPACITIVE LOADS
a special test circuit (such as Figure 47 shows) can
The dynamic characteristics of the OPA1662 and
be used to extend the measurement capabilities.
OPA1664 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 47 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 gained up (refer to the table in Figure 47 for the amplifier and can lead to gain peaking or oscillations.
distortion gain factor for various signal gains). The As a result, heavier capacitive loads must be isolated
addition of R
3
to the otherwise standard noninverting from the output. The simplest way to achieve this
amplifier configuration alters the feedback factor or isolation is to add a small resistor (R
S
equal to 50 Ω,
noise gain of the circuit. The closed-loop gain is for example) in series with the output.
unchanged, but the feedback available for error
This small series resistor also prevents excess power
correction is reduced by the distortion gain factor,
dissipation if the output of the device becomes
thus extending the resolution by the same amount.
shorted. Figure 25 illustrates a graph of Small-Signal
Note that the input signal and load applied to the op
Overshoot vs Capacitive Load for several values of
amp are the same as with conventional feedback
R
S
. Also, refer to Applications Bulletin AB-028
without R
3
. The value of R
3
should be kept small to
(literature number SBOA015, available for download
minimize its effect on the distortion measurements.
from the TI web site) for details of analysis
techniques and application circuits.
(1) For measurement bandwidth, see Figure 7 through Figure 12.
Figure 47. Distortion Test Circuit
16 Copyright © 2011, Texas Instruments Incorporated
Product Folder Link(s): OPA1662 OPA1664