OPA2172 Datasheet, PDF (25/42 Pages) Texas Instruments – OPAx172 36-V, Single-Supply, 10-MHz, Rail-to-Rail Output Operational Amplifiers

www.ti.com
OPA172, OPA2172, OPA4172
SBOS618H – DECEMBER 2013 – REVISED SEPTEMBER 2015
Typical Applications (continued)
9.2.1.2 Detailed Design Procedure
Figure 47 depicts a unity-gain buffer driving a capacitive load. Equation 1 shows the transfer function for the
circuit in Figure 47. Not depicted in Figure 47 is the open-loop output resistance of the op amp, R_o.
T(s)
=
1
1 + C_LOAD ×
+ R_o + R_ISO 
R_ISO × s
× C_LOAD
×
s
(1)
The transfer function in Equation 1 has a pole and a zero. The frequency of the pole (f_p) is determined by (R_o +
R_ISO) and C_LOAD. Components R_ISO and C_LOAD determine the frequency of the zero (f_z). A stable system is
obtained by selecting R_ISO such that the rate of closure (ROC) between the open-loop gain (A_OL) and 1 / β is
20 dB per decade. Figure 48 shows the concept. Note that the 1 / β curve for a unity-gain buffer is 0 dB.
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 48. 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. Table 4
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 OPA172, refer to the
precision design, Capacitive Load Drive Solution using an Isolation Resistor (TIPD128).
PHASE MARGIN
45°
60°
Table 4. Phase Margin versus Overshoot and AC Gain Peaking
OVERSHOOT
23.3%
8.8%
AC GAIN PEAKING
2.35 dB
0.28 dB
Copyright © 2013–2015, Texas Instruments Incorporated
Submit Documentation Feedback
25
Product Folder Links: OPA172 OPA2172 OPA4172