OPA2171-EP_16 Datasheet, PDF (18/28 Pages) Texas Instruments – 36-V, Single-Supply, SOT553, General-Purpose Operational Amplifiers

OPA2171-EP
SBOS735 – SEPTEMBER 2015
www.ti.com
Typical Application (continued)
ROC stability analysis is typically simulated. The validity of the analysis depends on multiple factors, especially
the accurate modeling of R_o. In addition to simulating the ROC, a robust stability analysis includes a
measurement of overshoot percentage and AC gain peaking of the circuit using a function generator,
oscilloscope, and gain and phase analyzer. Phase margin is then calculated from these measurements. Table 3
shows the overshoot percentage and AC gain peaking that correspond to phase margins of 45° and 60°. For
more details on this design and other alternative devices that can be used in place of the OPA171, refer to the
Precision Design, Capacitive Load Drive Solution using an Isolation Resistor (TIPD128).
Table 3. Phase Margin versus Overshoot and AC Gain
Peaking
PHASE MARGIN
45°
60°
OVERSHOOT
23.3%
8.8%
AC GAIN PEAKING
2.35 dB
0.28 dB
8.2.2.1 Capacitive Load and Stability
The dynamic characteristics of the OPA2171-EP have been optimized for commonly encountered 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 (for example, R_OUT equal to
50 Ω) in series with the output. Figure 41 and Figure 42 illustrate graphs of small-signal overshoot versus
capacitive load for several values of R_OUT. Also, refer to Applications Bulletin AB-028 (SBOA015), available for
download from www.ti.com for details of analysis techniques and application circuits.
50
R_L = 10kW
45
40
35
30
25
20
G = +1
15
R_OUT = 0W
+18V
ROUT
10
R_OUT = 25W
OPA171
-18V
RL
CL
5
R_OUT = 50W
0
0 100 200 300 400 500 600 700 800 900 1000
Capacitive Load (pF)
Figure 41. Small-Signal Overshoot vs Capacitive Load
(100-mV Output Step)
50
45
R_OUT = 0W
40
R_OUT = 25W
35
R_OUT = 50W
30
25
20
15
10
5
0
0
RI = 10kW
RF = 10kW
+18V
OPA171
-18V
G = -1
ROUT
CL
100 200 300 400 500 600 700 800 900 1000
Capacitive Load (pF)
Figure 42. Small-Signal Overshoot vs Capacitive Load
(100-mV Output Step)
18
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