OPA171-Q1_15 Datasheet, PDF (18/36 Pages) Texas Instruments – OPAx171-Q1 36-V, Single-Supply, General-Purpose Operational Amplifier

OPA171-Q1, OPA2171-Q1, OPA4171-Q1
SBOS556B – JUNE 2011 – REVISED DECEMBER 2014
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8.2 Typical Application
8.2.1 Capacitive Load Drive Solution Using an Isolation Resistor
The OPA171-Q1 device can be used capacitive loads such as cable shields, reference buffers, MOSFET gates,
and diodes. The circuit uses an isolation resistor (R_ISO) to stabilize the output of an op amp. R_ISO modifies the
open loop gain of the system to ensure the circuit has sufficient phase margin.
+V_S
R_ISO
V_OUT
+
V_{IN ±}
-V_S
C_LOAD
Figure 41. Unity-Gain Buffer with R_ISO Stability Compensation
8.2.1.1 Design Requirements
The design requirements are:
• Supply voltage: 30 V (±15 V)
• Capacitive loads: 100 pF, 1000 pF, 0.01 μF, 0.1 μF, and 1 μF
• Phase margin: 45° and 60°
8.2.1.2 Detailed Design Procedure
shows a unity-gain buffer driving a capacitive load. Equation 1 shows the transfer function for the circuit in . Not
shown in 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/decade. Figure 42 depicts the concept. The 1/β curve for a unity-gain buffer is 0 dB.
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