Application Notes

Using the LM6317

LIMITS AND PRECAUTIONS

The absolute maximum supply voltage which may be ap-

plied to the LM6317 is 12V Designers should not design for

more than 10V nominal and carefully check supply toler-

ances under all conditions so that the voltages do not ex-

ceed the maximum

DIFFERENTIAL INPUT VOLTAGE

Differential input voltage is the difference in voltage be-

put of the op amp The absolute maximum differential input

for the LM6317 is 10V across the inputs This limit also ap-

plies when there is no power supplied to the op amp This

may not be a problem in most conventional op amp designs

however designers should avoid using the LM6317 as com-

parators or forcing the inputs to different voltages In some

designs diodes protection may be needed between the in-

puts as shown in

Figure 1

TLH12542 – 20

FIGURE 1 Input Protection for LM6317

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

unused board space needs 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 perform-

ance 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 in-

put impedance and low input capacitance However the

probe ground leads provide a long ground loop that will pro-

duce 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 compo-

nent leads short because wires are inductive at high fre-

quency For discrete components choose carbon composi-

tion-type resistors and mica-type capacitors Surface mount

components are preferred over decrete components for

minimum inductive effect

Large values of feedback resistors can couple with parasitic

capacitance and cause undersirable effects such as ringing

or oscillation in high speed amplifiers Feedback resistor

value around 1 kX is recommended

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

found in the DC Electrical Characteristics Table of the data-

sheet

Figure 2 illustrates the compensation circuit

TLH12542 – 21

FIGURE 2 Compensating for Input Capacitance

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 individual-

ly by placing 001 mF creramic capacitors directly to power

supply pins and 22 mF tantalum capacitors close to the

power supply pins

TLH12542 – 22

FIGURE 3 Power Supply Bypassing

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