EL2386

FN7155 Rev 1.00

Page 9 of 13

June 24, 2004

Applications Information

Product Description

The EL2386 is a current-feedback operational amplifier that

offers a wide -3dB bandwidth of 250MHz, a low supply current

of 3mA per amplifier and the ability to power down to 0mA. It

also features high output current drive. The EL2386 can output

55mA per amplifier. The EL2386 works with supply voltages

ranging from a single 3V to ±6V, and it is also capable of

swinging to within 1V of either supply on the input and the

output. Because of its current-feedback topology, the EL2386

does not have the normal gain-bandwidth product associated

with voltage-feedback operational amplifiers. This allows its -

3dB bandwidth to remain relatively constant as closed-loop

gain is increased. This combination of high bandwidth and low

power, together with aggressive pricing make the EL2386 the

ideal choice for many low-power/high-bandwidth applications

such as portable computing, HDSL, and video processing.

For single and dual applications, consider the EL2186/EL2286.

For single, dual and quad applications without disable,

consider the EL2180, EL2280, or EL2480, all in industry

standard pin outs. The EL2180 also is available in the tiny

SOT-23 package, which is 28% the size of an SO8 package.

For lower power applications where speed is still a concern,

consider the EL2170/EL2176 family which also comes in

similar single, dual and quad configurations with 70MHz of

bandwidth while consuming 1mA of supply current per

amplifier.

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 capacitor has

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

ground plane, a single 4.7µF tantalum capacitor in parallel with

a 0.1µF ceramic capacitor across pins 14 and 3 will suffice.

For good AC performance, parasitic capacitance should be

kept to a minimum especially at the inverting input (see the

Capacitance at the Inverting Input section). Ground plane

construction should be used, but it should be removed from the

area near the inverting input to minimize any stray capacitance

at that node. 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.

Disable/Power-Down

The EL2386 amplifier can be disabled, placing its output in a

high-impedance state. When disabled, the supply current is

reduced to 0mA. The EL2386 is disabled when its ENABLE pin

is floating or pulled up to within 0.5V of the positive supply.

Similarly, the amplifier is enabled by pulling its ENABLE pin at

least 3V below the positive supply. For ±5V supplies, this

means that an EL2386 amplifier will be enabled when ENABLE

is at 2V or less, and disabled when ENABLE is above 4.5V.

Although the logic levels are not standard TTL, this choice of

logic voltages allows the EL2386 to be enabled by tying

ENABLE to ground, even in +3V single-supply applications.

The ENABLE pin can be driven from CMOS outputs or open-

collector TTL.

When enabled, supply current does vary somewhat with the

voltage applied at ENABLE. For example, with the supply

voltages of the EL2186 at ±5V, if ENABLE is tied to -5V (rather

than ground) the supply current will increase about 15% to

3.45mA.

Capacitance at the Inverting Input

Any manufacturer’s high-speed voltage- or current-feedback

amplifier can be affected by stray capacitance at the inverting

input. For inverting gains this parasitic capacitance has little

effect because the inverting input is a virtual ground, but for

non-inverting gains this capacitance (in conjunction with the

feedback and gain resistors) creates a pole in the feedback

path of the amplifier. This pole, if low enough in frequency, has

the same destabilizing effect as a zero in the forward open-

loop response. The use of large value feedback and gain

resistors further exacerbates the problem by further lowering

the pole frequency.

The experienced user with a large amount of PC board layout

experience may find in rare cases that the EL2386 has less

bandwidth than expected. In this case, the inverting input may

have less parasitic capacitance than expected. The reduction

of feedback resistor values (or the addition of a very small

amount of external capacitance at the inverting input, e. g.

0.5pF) will increase bandwidth as desired. Please see the

Feedback Resistor Values

The EL2386 has been designed and specified at gains of +1

0.1dB of peaking. Since the EL2386 is a current-feedback

more bandwidth. As seen in the curve of Frequency Response

modified by varying the value of the feedback resistor.

Because the EL2386 is a current-feedback amplifier, its gain-

bandwidth product is not a constant for different closed-loop

gains. This feature actually allows the EL2386 to maintain