Datasheet

(2)
(3)
(4)
(1)
OUTPUT
R
6
1:
R
5
1:
R
4
1:
R
8
3.5 k:
R
2
3 k:
R
1
750:
R
3
50:
e
in
R
11
20:
+
-
-
+
+
-
C
1
680 pF
-
+
R
7
1:
R
9
3.5 k:
R
10
3.5 k:
+
-
R
IN
51:
R
F
1.2k:
X1
-
+
R
OUT
51:
RG
1.2k:
CL
10pF
R
L
1k:
LMH6723, LMH6724, LMH6725
SNOSA83H –AUGUST 2003–REVISED APRIL 2013 www.ti.com
DRIVING CAPACITIVE LOADS
Capacitive output loading applications will benefit from the use of a series output resistor as shown in Figure 41.
The charts "Suggested R
OUT
vs. Cap Load" give a recommended value for selecting a series output resistor for
mitigating capacitive loads. The values suggested in the charts are selected for .5 dB or less of peaking in the
frequency response. This gives a good compromise between settling time and bandwidth. For applications where
maximum frequency response is needed and some peaking is tolerable, the value of R
OUT
can be reduced
slightly from the recommended values.
There will be amplitude lost in the series resistor unless the gain is adjusted to compensate; this effect is most
noticeable with heavy loads (R
L
< 150Ω).
An alternative approach is to place R
OUT
inside the feedback loop as shown in Figure 42. This will preserve gain
accuracy, but will still limit maximum output voltage swing.
Figure 42. Series Output Resistor Inside Feedback Loop
INVERTING INPUT PARASITIC CAPACITANCE
Parasitic capacitance is any capacitance in a circuit that was not intentionally added. It is produced through
electrical interaction between conductors and can be reduced but never entirely eliminated. Most parasitic
capacitances that cause problems are related to board layout or lack of termination on transmission lines. Please
see LAYOUT CONSIDERATIONS for hints on reducing problems due to parasitic capacitances on board traces.
Transmission lines should be terminated in their characteristic impedance at both ends.
High speed amplifiers are sensitive to capacitance between the inverting input and ground or power supplies.
This shows up as gain peaking at high frequency. The capacitor raises device gain at high frequencies by
making R
G
appear smaller. Capacitive output loading will exaggerate this effect.
One possible remedy for this effect is to slightly increase the value of the feedback (and gain set) resistor. This
will tend to offset the high frequency gain peaking while leaving other parameters relatively unchanged. If the
device has a capacitive load as well as inverting input capacitance, using a series output resistor as described in
DRIVING CAPACITIVE LOADS will help.
Figure 43. High Output Current Composite Amplifier
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