LM6172

SNOS792D – MAY 1999 – REVISED MARCH 2013

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Figure 45. The LM6172 Driving a 220 pF Load with a 50

Ω Isolation Resistor

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 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 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.

COMPONENTS SELECTION AND FEEDBACK RESISTOR

It is important in high speed applications to keep all component leads short because wires are inductive at high

frequency. 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 LM6172, a feedback resistor less than 1k

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

(1)

can be used to cancel that pole. For LM6172, a feedback capacitor of 2pF is recommended. Figure 46 illustrates

the compensation circuit.

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