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 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.

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 LM6172, a feed-

back resistor less than 1 k

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 LM6172, a feedback ca-

pacitor of 2 pF is recommended.

Figure 4 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.

DS012581-45

FIGURE 1. Isolation Resistor Used

to Drive Capacitive Load

DS012581-51

FIGURE 2. The LM6172 Driving a 510 pF Load

with a 30

Ω Isolation Resistor

DS012581-52

FIGURE 3. The LM6172 Driving a 220 pF Load

with a 50

Ω Isolation Resistor

DS012581-46

FIGURE 4. Compensating for Input Capacitance

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