OPA197 Datasheet, PDF (23/41 Pages) Texas Instruments – Operational Amplifiers

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45
R_ISO = 0 :
40
R_ISO = 25 :
35
R_ISO = 50 :
30
25
20
15
10
5
0
20 30 40 50 70 100 200 300 500 700 1000
Capacitive Load (pF)
2000
Figure 47. Small-Signal Overshoot vs Capacitive Load
(100-mV Output Step, G = –1 V/V)
OPA197, OPA2197, OPA4197
SBOS737 – JANUARY 2016
50
R_ISO = 0 :
45
R_ISO = 25 :
40
R_ISO = 50 :
35
30
25
20
15
10
5
20 30 40 50 70 100 200 300 500 700 1000
Capacitive Load (pF)
2000
Figure 48. Small-Signal Overshoot vs Capacitive Load
(100-mV Output Step, G = 1 V/V)
For additional drive capability in unity-gain configurations, improve capacitive load drive by inserting a small
(10-Ω to 20-Ω) resistor, R_ISO, in series with the output, as shown in Figure 49. This resistor significantly reduces
ringing while maintaining dc performance for purely capacitive loads. However, if there is a resistive load in
parallel with the capacitive load, a voltage divider is created, introducing a gain error at the output and slightly
reducing the output swing. The error introduced is proportional to the ratio R_ISO / R_L, and is generally negligible at
low output levels. A high capacitive load drive makes the OPA197 well suited for applications such as reference
buffers, MOSFET gate drives, and cable-shield drives. The circuit shown in Figure 49 uses an isolation resistor,
R_ISO, to stabilize the output of an op amp. R_ISO modifies the open-loop gain of the system for increased phase
margin, and results using the OPA197 are summarized in Table 3. For additional information on techniques to
optimize and design using this circuit, TI Precision Design TIDU032 details complete design goals, simulation,
and test results.
+V_s
R_iso
V_out
+
V_{in ±}
-V_s
C_load
Figure 49. Extending Capacitive Load Drive with the OPA197
Table 3. OPA197 Capacitive Load Drive Solution Using Isolation Resistor Comparison of Calculated and
Measured Results
PARAMETER
Capacitive Load
Phase Margin
R_ISO (Ω)
Measured
Overshoot (%)
Calculated PM
100 pF
45°
60°
47.0
360.0
23.2 8.6
10.4
45.1°
58.1°
1000 pF
45°
60°
24.0
100.0
22.5
9.0
45.8°
59.7°
VALUE
0.01 µF
45°
60°
20.0
51.0
22.1
8.7
46.1°
60.1°
0.1 µF
45°
60°
6.2
15.8
23.1
8.6
45.2°
60.2°
1 µF
45°
60°
2.0
4.7
21.0
8.6
47.2°
60.2°
For step-by-step design procedure, circuit schematics, bill of materials, printed circuit board (PCB) files,
simulation results, and test results, refer to TI Precision Design TIDU032, Capacitive Load Drive Solution using
an Isolation Resistor .
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Product Folder Links: OPA197 OPA2197 OPA4197