LM6171 datasheet(13/17 Pages) NSC

13 / 17 page

Application Information (Continued)

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

Ω to the input of

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.

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 LM6171, 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

F > (RG xCIN)/RF

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.

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.

DS012336-11

FIGURE 1. Compensating for Input Capacitance

DS012336-12

FIGURE 2. Power Supply Bypassing

DS012336-14

FIGURE 3. Properly Terminated Signal

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