LM6171AIMX National Semiconductor, LM6171AIMX Datasheet - Page 13

• Manufacturer Part Number: LM6171AIMX
• Description: High Speed Low Power Low Distortion Voltage Feedback Amplifier
• Manufacturer: National Semiconductor

Application Information
slew rate is proportional to the input voltage level, and the
higher slew rates are achievable in the lower gain configura-
tions.
When a very fast large signal pulse is applied to the input of
an amplifier, some overshoot or undershoot occurs. By plac-
ing an external series resistor such as 1 k
LM6171, the bandwidth is reduced to help lower the over-
shoot.
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 and frustrating to have excessive ringing, oscilla-
tion and other degraded AC performance in high speed cir-
cuits. As a rule, the signal traces should be short and wide to
provide low inductance and low impedance paths. Any un-
used board space needs to be grounded to reduce stray sig-
nal pickup. Critical components should also be grounded at
a common point to eliminate voltage drop. Sockets add ca-
pacitance to the board and can affect frequency perfor-
mance. 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 LM6171, a feed-
back resistor of 510
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 LM6171, a feedback ca-
pacitor of 2 pF is recommended. Figure 1 illustrates the com-
pensation circuit.
For
discrete
C
F
gives optimal performance.
>
(R
components,
G
x C
IN
)/R
F
(Continued)
choose
to the input of
carbon
13
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.
Termination
In high frequency applications, reflections occur if signals
are not properly terminated. Figure 3 shows a properly termi-
nated signal while Figure 4 shows an improperly terminated
signal.
FIGURE 1. Compensating for Input Capacitance
FIGURE 3. Properly Terminated Signal
FIGURE 2. Power Supply Bypassing
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