LM7171AMJ-QML National Semiconductor, LM7171AMJ-QML Datasheet - Page 13

• Manufacturer Part Number: LM7171AMJ-QML
• Description: Very High Speed/ High Output Current/ Voltage Feedback Amplifier
• Manufacturer: National Semiconductor

LM7171 Circuit Operation
ers the inverting input. The triple-buffered output stage iso-
lates the gain stage from the load to provide low output im-
pedance.
LM7171 Slew Rate Characteristic
The slew rate of LM7171 is determined by the current avail-
able to charge and discharge an internal high impedance
node capacitor. This current is the differential input voltage
divided by the total degeneration resistor R
slew rate is proportional to the input voltage level, and the
higher slew rates are achievable in the lower gain configura-
tions. A curve of slew rate versus input voltage level is pro-
vided in the “Typical Performance Characteristics”.
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 resistor such as 1 k
of LM7171, the bandwidth is reduced to help lower the over-
shoot.
Slew Rate Limitation
If the amplifier’s input signal has too large of an amplitude at
too high of a frequency, the amplifier is said to be slew rate
limited; this can cause ringing in time domain and peaking in
frequency domain at the output of the amplifier.
In the “Typical Performance Characteristics” section, there
are several curves of A
nal levels. For the A
the LM7171 responds identically to the different input signal
levels of 30 mV, 100 mV and 300 mV.
For the A
peaking at high frequency (
input signal at high enough frequency that exceeds the am-
plifier’s slew rate. The peaking in frequency response does
not limit the pulse response in time domain, and the LM7171
is stable with noise gain of +2.
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
board and can affect high frequency performance. It is better
to solder the amplifier directly into the PC board without us-
ing 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.
V
= +2 curves, with slight peaking occurs. This
V
= +4 curves, no peaking is present and
V
= +2 and A
>
100 MHz) is caused by a large
V
in series with the input
= +4 versus input sig-
E
(Continued)
. Therefore, the
13
COMPONENT SELECTION AND FEEDBACK RESISTOR
It is important in high speed applications to keep all compo-
nent leads short. For discrete components, choose carbon
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 LM7171, 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 LM7171, a feedback ca-
pacitor of 2 pF is recommended. Figure 1 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.
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 2. Power Supply Bypassing
C
F
gives optimal performance.
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