10

FN7061.2

June 7, 2005

Simplified Schematic

Applications Information

Product Description

The EL2250/EL2450 are part of a family of the industries

fastest single supply operational amplifiers. Connected in

voltage follower mode, their -3dB bandwidth is 125MHz

while maintaining a 275V/µs 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 +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 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

EL2250/EL2450 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 4.7µF tantalum capacitor in parallel with a

0.1µF ceramic capacitor has been shown to work well when

placed at each supply pin. For single supply operation,

where the GND pin is connected to the ground plane, a

single 4.7µF tantalum capacitor in parallel with a 0.1µF

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 parasitic inductance and

capacitance which will result in some additional peaking and

overshoot.

Supply Voltage Range and Single-Supply

Operation

The EL2250/EL2450 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

EL2250/EL2450 will operate on dual supplies ranging from

±1.35V to ±6V. With a single-supply, the EL2250/EL2450 will

operate from +2.7V to +12V. Performance has been

optimized for a single +5V supply.

Pins 8 and 4 are the power supply pins on the EL2250. The

positive power supply is connected to pin 8. When used in

single supply mode, pin 4 is connected to ground. When

used in dual supply mode, the negative power supply is

connected to pin 4.

Pins 4 and 11 are the power supply pins on the EL2450. The

positive power supply is connected to pin 4. When used in

single supply mode, pin 11 is connected to ground. When

used in dual supply mode, the negative power supply is

connected to pin 11.

As supply voltages continue to decrease, it becomes

necessary to provide input and output voltage ranges that

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

EL2250/EL2450 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

EL2250/EL2450 have an input range which spans from 0V to

3.8V.

EL2250, EL2450