EL2257CS datasheet(13/21 Pages) ELANTEC

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EL2257C/EL2357C

125 MHz Single Supply, Clamping Op Amps

Applications Information

Product Description

The EL2257C/EL2357C’s, connected in voltage fol-

lower mode, -3 dB bandwidth is 125 MHz while

maintaining a 275 V/µs slew rate. With an input and out-

put 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 +2.7V or up to

+12V. This makes them ideal for +3V applications,

especially portable computers.

While many amplifiers claim to operate on a single sup-

ply, 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, caus-

ing distortion and recovery delays. However, special

circuitry built into the EL2257C/EL2357C allows the

output 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 highly 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 4.7 µF tantalum capac-

itor in parallel with a 0.1 µF ceramic capacitor has been

shown to work well when placed at each supply pin. For

single supply operation, where the GND pin is con-

nected to the ground plane, a single 4.7 µF tantalum

capacitor in parallel with a 0.1 µF ceramic capacitor

from the VS+ pin to the GND pin will suffice.

For good AC performance, parasitic capacitance should

be kept to a minimum. Ground plane construction

should be used. Carbon or Metal-Film resistors are

acceptable with the Metal-Film resistors giving slightly

less peaking and bandwidth because of their additional

series inductance. Use of sockets, particularly for the SO

package should be avoided if possible. Sockets add par-

asitic inductance and capacitance which will result in

some additional peaking and overshoot.

Supply Voltage Range and Single-Supply

Operation

The EL2257C/EL2357C have been designed to operate

with supply voltages having a span of greater than 2.7V,

and less than 12V. In practical terms, this means that the

EL2257C/EL2357C will operate on dual supplies rang-

ing from ±1.35V to ±6V. With a single-supply, the

EL2257C/EL2357C will operate from +2.7V to +12V.

Performance has been optimized for a single +5V

supply.

Pins 11 and 4 (14 and 3) are the power supply pins on

the EL2257C (EL2357C). The positive power supply is

connected to pin 11 (14). When used in single supply

mode, pin 4 (3) is connected to ground. When used in

dual supply mode, the negative power supply is con-

nected to pin 4 (3).

As supply voltages continue to decrease, it becomes nec-

essary to provide input and output voltage ranges that

can get as close as possible to the supply voltages. The

EL2257C/EL2357C have an input voltage range that

includes the negative supply and extends to within 1.2V

of the positive supply. So, for example, on a single +5V

supply, the EL2257C/EL2357C have an input range

which spans from 0V to 3.8V.

The output range of the EL2257C/EL2357C is also quite

large. It includes the negative rail, and extends to within

1V of the top supply rail with a 1 k

Ω load. On a +5V

supply, the output is therefore capable of swinging from

0V to +4V. On split supplies, the output will swing ±4V.

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 capac-

itance. As this pole becomes larger, phase margin is

reduced. This increases ringing in the time domain and

peaking in the frequency domain. Therefore, RF has

some maximum value which should not be exceeded for

optimum performance. 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 bandwidth.

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