LM6172AMWG-QML National Semiconductor, LM6172AMWG-QML Datasheet - Page 12

• Manufacturer Part Number: LM6172AMWG-QML
• Description: Dual High Speed/ Low Power/ Low Distortion/ Voltage Feedback Amplifiers
• Manufacturer: National Semiconductor

www.national.com
Driving Capacitive Loads
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 de-
graded AC performance in high speed circuits. As a rule, the
signal traces should be short and wide to provide low induc-
tance 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
FIGURE 2. The LM6172 Driving a 510 pF Load
FIGURE 3. The LM6172 Driving a 220 pF Load
FIGURE 1. Isolation Resistor Used
with a 30
with a 50
to Drive Capacitive Load
Isolation Resistor
Isolation Resistor
(Continued)
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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 mea-
surements 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 er-
rors 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 compo-
nent leads short because wires are inductive at high fre-
quency.
composition-type resistors and mica-type capacitors. Sur-
face mount components are preferred over discrete compo-
nents 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 feed-
back resistor less than 1 k
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
can be used to cancel that pole. For LM6172, a feedback ca-
pacitor of 2 pF is recommended. Figure 4 illustrates the com-
pensation circuit.
Power Supply Bypassing
Bypassing the power supply is necessary to maintain low
power supply impedance across frequency. Both positive
and negative power supplies should be bypassed individu-
ally by placing 0.01 µF ceramic capacitors directly to power
supply pins and 2.2 µF tantalum capacitors close to the
power supply pins.
FIGURE 4. Compensating for Input Capacitance
For
discrete
C
F
>
(R
components,
G
gives optimal performance.
x C
IN
)/R
F
choose
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carbon