OPA171-Q1_15 Datasheet, PDF (19/36 Pages) Texas Instruments – OPAx171-Q1 36-V, Single-Supply, General-Purpose Operational Amplifier

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Typical Application (continued)
OPA171-Q1, OPA2171-Q1, OPA4171-Q1
SBOS556B – JUNE 2011 – REVISED DECEMBER 2014
120
AOL
100
1
80
f_p
  2 u Πu R_ISO  R_o u C_LOAD
60
40
20
0
10
40 dB
1/
1 dec
1
f_z
2 u Πu R_ISO u C_LOAD
ROC ^{20 dB}
dec
100
1k
10k 100k 1M 10M 100M
Frequency (Hz)
Figure 42. Unity-Gain Amplifier with R_ISO Compensation
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. 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).
PHASE MARGIN
45°
60°
Table 3. Phase Margin versus Overshoot and AC Gain Peaking
OVERSHOOT
23.3%
8.8%
AC GAIN PEAKING
2.35 dB
0.28 dB
8.2.1.3 Application Curves
The OPA171-Q1 device meets the supply voltage requirements of 30 V. The OPA171-Q1 device was tested for
various capacitive loads and R_ISO was adjusted to achieve an overshoot corresponding to . shows the test
results.
10000
45q Phase Margin
60q Phase Margin
1000
100
10
1
0.01
0.1
1
10
100
Capacitive Load (nF)
1000
D001
Figure 43. R_ISO vs C_LOAD
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