LM6310IM datasheet(12/16 Pages) NSC

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Application Information (Continued)

Using the LM6310

LIMITS AND PRECAUTIONS

SUPPLY VOLTAGE

The absolute maximum supply voltage which may be ap-

plied to the LM6310 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-

tween the non-inverting (a) input and the inverting input

(b) of the op amp The absolute maximum differential input

voltage is g2V across the inputs This limit also applies

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 LM6310 as com-

parator or forcing the inputs to different voltages

Very fast input pulses into high gain circuits may cause the

output to saturate leading to an overload recovery time in

the millisecond range This requires inputs which are faster

than those usually used in video systems and gain levels

which will push the output of the amplifier toward the limit of

its output swing

LAYOUT AND POWER SUPPLY BYPASSING

Since the LM6310 is a high speed (over 50 MHz) device

good high speed circuit layout practices should be followed

This should include the use of ground planes adequate

power supply bypassing removing metal from around the

input pins to reduce capacitance and careful routing of the

output signal lines to keep them away from the input pins

The power supply pins should be bypassed on both the neg-

ative and positive supply inputs with capacitors placed close

to the pins Surface mount capacitors should be used for

best performance and should be placed as close to the

pins as possible It is generally advisable to use two capaci-

tors at each supply voltage pin A small surface mount ca-

pacitor with a value of around 001 mF (10 nF) usually a

ceramic type with good RF performance should be placed

closest to the pin A larger capacitor usually in the range of

10 mF – 47 mF should also be placed near the pin The

larger capacitor should be a device with good RF character-

istics and low ESR (equivalent series resistance) for best

results Ceramic and tantalum capacitors generally work

well as the larger capacitor

It is very important to reduce capacitance at the input and

output pins The ground plane and any other planes (power

etc) should be ‘‘opened up’’ or removed near the pins The

opening should extend to the middle of the nearest pins as

a minimum

The LM6310 is built on a high performance bipolar process

The transistors used in this process have bandwidths much

higher that the LM6310 itself These transistors have a po-

tential to oscillate or ring at 400 b1 GHz when used in

layouts where the components are more than

inch

(6 mm) away from the op amp pins These oscillations may

produce apparent shifts in voltage offset or excess current

consumption

To avoid this keep the input and output resistors as close

as possible to their respective pins Spacing within

inch

(3 mm) or less is recommended for best results

For best performance low inductance resistors such as

chip resistors are recommended The use of wirewound re-

sistors is strongly not recommended

DIP devices should use socket pins which are flush with the

board Conventional sockets have additional capacitance

and are not recommended Obviously the use of wire-

wrapped sockets or the ‘‘white plastic’’ push in prototype

boards is strongly not recommended

FEEDBACK RESISTOR VALUES (RF)

Since the LM6310 is a current feedback amplifier the value

of the feedback resistor is important to the performance of

the op amp For circuits other than voltage followers the

fastest pulse response is usually obtained with a resistor

value of 348X To get higher gain decrease the source re-

sistor value (RS) while leaving the feedback resistor at

348X (Schematics for various gains are shown in the typi-

cal performance curves section of this datasheet)

Current feedback amplifiers generally do not tolerate reac-

tive components in the feedback path Therefore do not

bypass the feedback resistor (RF) with a feedback capaci-

tor This will result in instability

Overshoot and ringing of the LM6310 can be reduced by

increasing the value of the feedback resistor above 348X

The value of 348X will normally produce a near critically

damped pulse response with about 3% – 5% overshoot

Overshoot and bandwidth peaking may be undesirable in

some designs Selecting a larger value for the feedback re-

sistor will reduce the overshoot and bandwidth peaking Too

large a value will reduce the circuit bandwidth and degrade

pulse response Do not place a capacitor across feedback

resistor

For voltage followers (AV ea1) there is no source resist-

ance A feedback resistor must be useddirect connect

from the output to the inverting input will usually result in

oscillation Values in the range of 1 kX –2 kX usually give

good results The pulse response photo for AV ea1 was

obtained witha1kX feedback resistor

Since the small stray capacitance from the circuit layout

other components and specific circuit bandwidth require-

ments will vary it is often useful to select final values based

on prototypes which are similar in layout to the production

circuit boards

Reflections

The output slew rate of the LM6310 is fast enough to pro-

duce reflected signals in many cables and long circuit

traces For best pulse performance it may be necessary to

terminate cables and long circuit traces with their character-

istic impedance to reduce reflected signals

Reflections should not be confused with overshoot Reflec-

tions will depend on cable length while overshoot will de-

pend on load and feedback resistance and capacitance

When determining the type of problem often removing or

drastically shortening the cable will reduce or eliminate re-

flections Overshoot can exist without a cable attached to

the op amp output

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