LM7171AIN National Semiconductor, LM7171AIN Datasheet - Page 3

• Manufacturer Part Number: LM7171AIN
• Manufacturer: National Semiconductor

MICROCIRCUIT DATA SHEET
MNLM7171AM-X-RH REV 0C0
Applications
(Continued)
CIRCUIT OPERATION: The class AB input stage in the LM7171 is fully symmetrical and has a
similar slewing characteristic to the current feedback amplifiers. In the LM7171
Simplified Schematic, (see AN00006) Q1 through Q4 form the equivalent of the current
feedback input buffer, RE the equivalent of the feedback resistor, and stage A buffers the
inverting input. The triple-buffered output stage isolates the gain stage from the load to
provide low output impedance.
SLEW RATE CHARACTERISTIC: The slew rate of 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 RE. Therefore, the
slew rate is proportional to the input voltage level, and the higher slew rates are
achievable in the lower gain configurations. See the LM7171 Commercial Data Book for slew
rate Vs input voltage level curve.
When a very fast, large signal, pulse is applied to the input of an amplifier, some
overshoot or undershoot occurs. By placing an external resistor such as 1K Ohm 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 Commercial Data Book "Typical Performance Characteristics" section, there are
several curves of Av = +2 and Av = +4 versus input power levels. For the Av = +4 curves,
no peaking is present and the LM7171 responds identically to the different input power
levels of 30 mV, 100 mV and 300mV.
For the Av = +2 curves, slight peaking occurs. This peaking at high frequency (>100MHz) is
caused by a large input signal at high enough frequency, that it exceeds the amplifier's
slew rate. The peaking in frequency response does not limit the pulse response in time
domain.
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 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 must 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 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.
COMPONENT SELECTION & FEEDBACK RESISTOR: It is important in high speed applications to
keep all component 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 LM7171, a
feedback resistor of 510 Ohms gives optimal performance.
COMPENSATION FOR INPUT CAPACITANCE: The combinations of an amplfier'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 Cf>(Rg X Cin)/Rf can be used to
cancel that pole. For LM7171, a feedback capacitor of 2pF is recommended. AN00003
illustrates the compensation circuit.
POWER SUPPLY BYPASSING: Bypassing the power supply is necessary to maintain low power
supply impedance across the frequency spectrum. Both positive and negative power supplies
should be bypassed individually by placing 0.01uF ceramic capacitors directly to the power
supply pins and 2.2uF tantalum capacitors close to the power supply pins. See AN00004.
TERMINATION: In high frequency applications, reflection occur if signals are not properly
terminated. Figure 3, in the Commercial Data Book, shows a properly terminated signal,
while Figure 4, in the Commercial Data Book, shows an improperly terminated signal.
To minimize reflection, coaxial cable with matching characteristic impedance to the signal
source should be used. The other end of the cable should be terminated with the same value
terminator or resistor. For the commonly used cables, RG59 has 75 Ohm characteristic
impedance, and RG58 has 50 Ohm characteristic impedance.
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