Datasheet
LM6172
SNOS792D –MAY 1999–REVISED MARCH 2013
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Figure 45. The LM6172 Driving a 220 pF Load with a 50Ω Isolation Resistor
LAYOUT CONSIDERATION
PRINTED CIRCUIT BOARDS AND HIGH SPEED OP AMPS
There are many things to consider when designing PC boards for high speed op amps. Without proper caution, it
is very easy to have excessive ringing, oscillation and other degraded AC performance in high speed circuits. As
a rule, the signal traces should be short and wide to provide low inductance and low impedance paths. Any
unused board space needs to be grounded to reduce stray signal pickup. Critical components should also be
grounded at a common point to eliminate voltage drop. Sockets add capacitance to the board and can affect
frequency performance. It is better to solder the amplifier directly into the PC board without using any socket.
USING PROBES
Active (FET) probes are ideal for taking high frequency measurements because they have wide bandwidth, high
input impedance and low input capacitance. However, the probe ground leads provide a long ground loop that
will produce errors in measurement. Instead, the probes can be grounded directly by removing the ground leads
and probe jackets and using scope probe jacks.
COMPONENTS SELECTION AND FEEDBACK RESISTOR
It is important in high speed applications to keep all component leads short because wires are inductive at high
frequency. For discrete components, choose carbon composition-type resistors and mica-type capacitors.
Surface mount components are preferred over discrete components for minimum inductive effect.
Large values of feedback resistors can couple with parasitic capacitance and cause undesirable effects such as
ringing or oscillation in high speed amplifiers. For LM6172, a feedback resistor less than 1kΩ gives optimal
performance.
COMPENSATION FOR INPUT CAPACITANCE
The combination of an amplifier's input capacitance with the gain setting resistors adds a pole that can cause
peaking or oscillation. To solve this problem, a feedback capacitor with a value
C
F
> (R
G
× C
IN
)/R
F
(1)
can be used to cancel that pole. For LM6172, a feedback capacitor of 2pF is recommended. Figure 46 illustrates
the compensation circuit.
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