el2257 Intersil Corporation, el2257 Datasheet - Page 11

• Manufacturer Part Number: el2257
• Description: 125mhz Single Supply, Clamping Op Amp
• Manufacturer: Intersil Corporation

Simplified Schematic
Applications Information
Product Description
The EL2257, connected in voltage follower mode, -3dB
bandwidth is 125MHz while maintaining a 275V/µs slew rate.
With an input and output common mode range that includes
ground, this amplifier was optimized for single supply
operation, but will also accept dual supplies. It operates 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
EL2257 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. Pin 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 ceramic capacitor
from the V
S
+ pin to the GND pin will suffice.
11
(One Channel)
EL2257
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 (0.150") 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 EL2257 has 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 EL2257 will
operate on dual supplies ranging from ±1.35V to ±6V. With a
single-supply, the EL2257 will operate from +2.7V to +12V.
Performance has been optimized for a single +5V supply.
Pins 11 and 4 are the power supply pins on the EL2257. The
positive power supply is connected to pin 11. 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.
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
EL2257 has 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 EL2257
has an input range which spans from 0V to 3.8V.
The output range of the EL2257 is also quite large. It
includes the negative rail, and extends to within 1V of the top