EL2157CS datasheet(12/20 Pages) ELANTEC

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EL2150CEL2157C

125 MHz Single Supply Clamping Op Amps

Applications Information

Product Description

The EL2150CEL2157C are the industry’s fastest

single supply operational amplifiers Connected

in voltage follower mode their b3dB bandwidth

is 125 MHz while maintaining a 275 V

ms slew

rate With an input and output common mode

range that includes ground these amplifiers were

optimized for single supply operation but will

also accept dual supplies They operate on a total

supply voltage range as low as a27V or up to

12V This makes them ideal for a3V applica-

tions especially portable computers

While many amplifiers claim to operate on a sin-

gle supply and some can sense ground at their

inputs most fail to truly drive their outputs to

ground If they do succeed in driving to ground

the amplifier often saturates causing distortion

and recovery delays However special circuitry

built into the EL2150CEL2157C allows the out-

put to follow the input signal to ground without

recovery delays

Power Supply Bypassing And Printed

Circuit Board Layout

As with any high-frequency device good printed

circuit board layout is necessary for optimum

performance Ground plane construction is high-

ly recommended Lead lengths should be as short

as possible The power supply pins must be well

bypassed to reduce the risk of oscillation The

combination of a 47

mF tantalum capacitor in

parallel with a 01

mF ceramic capacitor has been

shown to work well when placed at each supply

pin For single supply operation where pin 4

(VSb) is connected to the ground plane a single

mF tantalum capacitor in parallel with a 01

mF ceramic capacitor across pins 7 and 4 will suf-

fice

For good AC performance parasitic capacitance

should be kept to a minimum Ground plane con-

struction should be used Carbon or Metal-Film

resistors are acceptable with the Metal-Film re-

sistors giving slightly less peaking and band-

width because of their additional series induc-

tance Use of sockets particularly for the SO

package should be avoided if possible Sockets

add parasitic inductance and capacitance which

will result in some additional peaking and over-

shoot

Supply Voltage Range and

Single-Supply Operation

The EL2150CEL2157C have been designed to

operate with supply voltages having a span of

greater than 27V and less than 12V In practical

terms this means that the EL2150CEL2157C

will operate on dual supplies ranging from

135V to g6V

With a single-supply

the

EL2150CEL2157C will operate from a27V to

12V Performance has been optimized for a sin-

gle a5V supply

Pins 7 and 4 are the power supply pins The posi-

tive power supply is connected to pin 7 When

used in single supply mode pin 4 is connected to

ground When used in dual supply mode the neg-

ative power supply is connected to pin 4

As supply voltages continue to decrease it be-

comes necessary to provide input and output

voltage ranges that can get as close as possible to

the supply voltages

The EL2150CEL2157C

have an input voltage range that includes the

negative supply and extends to within 12V of the

positive supply So for example on a single a5V

supply the EL2150CEL2157C have an input

range which spans from 0V to 38V

The output range of the EL2150CEL2157C is

also quite large It includes the negative rail and

extends to within 1V of the top supply rail On a

5V supply the output is therefore capable of

swinging from 0V to a4V On split supplies the

output will swing g4V If the load resistor is tied

to the negative rail and split supplies are used

the output range is extended to the negative rail

Choice Of Feedback Resistor RF

The feedback resistor forms a pole with the input

capacitance As this pole becomes larger phase

margin is reduced This increases ringing in the

time domain and peaking in the frequency do-

main Therefore RF has some maximum value

which should not be exceeded for optimum per-

formance If a large value of RF must be used a

small capacitor in the few picofarad range in par-

allel with RF can help to reduce this ringing and

peaking at the expense of reducing the band-

width

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