LMH6551 Datasheet, PDF (16/32 Pages) National Semiconductor (TI) – Differential, High Speed Op Amp

LMH6551
SNOSAK7D – FEBRUARY 2005 – REVISED JANUARY 2015
Typical Applications (continued)
V⁺
10 2F
0.01 2F
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10 2F
V⁺
0.01 2F 0.01 2F
+
V_CM
-
0.1 2F
0.1 2F
+
V_CM
-
0.1 2F
10 2F
0.01 2F
V^-
Figure 25. Split Supply Bypassing Capacitors
Figure 26. 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 28 and Figure 4 in Typical
Performance Characteristics.
9.2.1.3 Application Curves
Many application circuits will have capacitive loading. As shown in Figure 4, amplifier bandwidth is reduced with
increasing capacitive load, so parasitic capacitance should be strictly limited.
In order to guarantee 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 5. 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.
2
C_L = 5.7 pF, R_OUT = 60:
1
V_S = ±5V
0
-1 ^CL ^{= 10 pF, R}OUT ^{= 34:}
-2 _{CL = 27 pF, ROUT = 20:}
-3
C_L = 57 pF, R_OUT = 15:
-4
-5 _{LOAD = (CL || 1 k:) IN}
-6 SERIES WITH 2 R_OUTS
-7 ^{VOUT = 0.5 VPP}
DIFFERENTIAL
-8
1
10
100
1000
FREQUENCY (MHz)
Figure 27. Frequency Response vs Capacitive Load
70
60
50
40
30
20
10 ^{LOAD = 1 k: || CAP LOAD}
V_S = ±5V
0
1
10
100
CAPACITIVE LOAD (pF)
Figure 28. Suggested R_OUT vs Cap Load
16
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