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

Ω in series with the input

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

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.

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

Ω 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

C

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.

DS012385-10

FIGURE 1. Compensating for Input Capacitance

DS012385-11

FIGURE 2. Power Supply Bypassing

www.national.com

13