LMH6550 Datasheet, PDF (16/33 Pages) National Semiconductor (TI) – Differential, High Speed Op Amp

LMH6550
SNOSAK0I – DECEMBER 2004 – REVISED JANUARY 2015
Typical Applications (continued)
V⁺
10 2F
0.01 2F
10 2F
V⁺
0.01 2F 0.01 2F
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+
V_CM
-
0.1 2F
0.1 2F
10 2F
0.01 2F
V^-
Figure 23. Split Supply Bypassing Capacitors
+
V_CM
-
0.1 2F
Figure 24. Single Supply Bypassing Capacitors
9.2.1.2.2 Capacitive Drive
As noted in Driving Analog-to-Digital Converters, capacitive loads should be isolated from the amplifier output
with small valued resistors. This is particularly the case when the load has a resistive component that is 500 Ω or
higher. A typical ADC has capacitive components of around 10 pF and the resistive component could be 1000 Ω
or higher. If driving a transmission line, such as 50-Ω coaxial or 100-Ω twisted pair, using matching resistors will
be sufficient to isolate any subsequent capacitance. For other applications see Figure 6 and Figure 25 in Typical
Characteristics.
9.2.1.2.3 Application Curves
Many application circuits have capacitive loading. As shown in Figure 25, amplifier bandwidth is reduced with
increasing capacitive load, so parasitic capacitance should be strictly limited.
To ensure stability, resistance should be added between the capacitive load and the amplifier output pins. The
value of the resistor is dependent on the amount of capacitive load as shown in Figure 26. This resistive value is
a suggestion. System testing will be required to determine the optimal value. Using a smaller resistor will retain
more system bandwidth at the expense of overshoot and ringing, while larger values of resistance will reduce
overshoot but will also reduce system bandwidth.
70
60
50
40
30
20
10 ^{LOAD = 1 k: || CAP LOAD}
V_S = 5V
0
1
10
100
CAPACITIVE LOAD (pF)
Figure 25. Suggested R_OUT vs Cap Load
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
0
V_S = 5V
R_L = 500:
R_F = 360:
10 20 30 40 50 60 70 80 90 100
TIME (ns)
Figure 26. 1 V_PP Pulse Response Single-Ended Input
16
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