lm7171amwgflqv National Semiconductor Corporation, lm7171amwgflqv Datasheet - Page 17

• Manufacturer Part Number: lm7171amwgflqv
• Description: Lm7171qml Very High Speed, High Output Current, Voltage Feedback Amplifier
• Manufacturer: National Semiconductor Corporation

Application Notes
LM7171 Performance Discussion
The LM7171 is a very high speed, voltage feedback amplifier.
It consumes only 6.5 mA supply current while providing a uni-
ty-gain bandwidth of 200 MHz and a slew rate of 4100V/μs. It
also has other great features such as low differential gain and
phase and high output current.
The LM7171 is a true voltage feedback amplifier. Unlike cur-
rent feedback amplifiers (CFAs) with a low inverting input
impedance and a high non-inverting input impedance, both
inputs of voltage feedback amplifiers (VFAs) have high
impedance nodes. The low impedance inverting input in
CFAs and a feedback capacitor create an additional pole that
will lead to instability. As a result, CFAs cannot be used in
traditional op amp circuits such as photodiode amplifiers, I-to-
V converters and integrators where a feedback capacitor is
required.
LM7171 Circuit Operation
The class AB input stage in the LM7171 is fully symmetrical
and has a similar slewing characteristic to the current feed-
back amplifiers. In the LM7171 Simplified Schematic, Q1
through Q4 form the equivalent of the current feedback input
buffer, R
A buffers the inverting input. The triple-buffered output stage
isolates the gain stage from the load to provide low output
impedance.
LM7171 Slew Rate Characteristic
The slew rate of the LM7171 is determined by the current
available 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 kΩ in series with the input
of the LM7171, the bandwidth is reduced to help lower the
overshoot.
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
levels. For the A
LM7171 responds identically to the different input signal lev-
els of 30 mV, 100 mV and 300 mV.
For the A
at high frequency (>100 MHz) is caused by a large input signal
at high enough frequency that exceeds the amplifier'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
E
V
the equivalent of the feedback resistor, and stage
= +2 curves, slight peaking occurs. This peaking
≥
+2.
V
= +4 curves, no peaking is present and the
V
= +2 and A
V
= +4 versus input signal
E
. Therefore, the
17
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 volt-
age 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 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.
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. 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 the LM7171, a feed-
back resistor of 510Ω 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
can be used to cancel that pole. For the LM7171, a feedback
capacitor of 2 pF is recommended. Figure 1 illustrates the
compensation circuit.
FIGURE 1. Compensating for Input Capacitance
C
F
> (R
G
× C
IN
)/R
F
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