EL2186CS datasheet(10/19 Pages) ELANTEC

10 / 19 page

EL2186C, EL2286C

250MHz/3mA Current Mode Feedback Amp w/Disable

Applications Information

Product Description

The EL2186C/EL2286C are current-feedback opera-

tional amplifiers that offer a wide -3dB bandwidth of

250MHz, a low supply current of 3mA per amplifier and

the ability to disable to 0mA. Both products also feature

high output current drive. The EL2186C can output

100mA, while the EL2286C can output 55mA per

amplifier. The EL2186C/EL2286C work with supply

voltages ranging from a single 3V to ±6V, and they are

also capable of swinging to within 1V of either supply

on the input and the output. Because of their current-

feedback topology, the EL2186C/EL2286C do not have

the normal gain- bandwidth product associated with

voltage-feedback operational amplifiers. This allows

their -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 EL2186C/EL2286C the ideal choice

for many low-power/high-bandwidth applications such

as portable computing, HDSL, and video processing.

For Single, Dual and Quad applications without disable,

consider the EL2180C (8-Pin Single), EL2280C (8-Pin

Dual) and EL2480C (14-Pin Quad). If lower power is

required, refer to the EL2170C/EL2176C family which

provides Singles, Duals, and Quads 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.

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 capac-

itor in parallel with a 0.1µF capacitor has been shown to

work well when placed at each supply pin.

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 peak-

ing and overshoot.

Disable/Power-Down

The EL2186C/EL2286C amplifiers can be disabled,

placing their output in a high-impedance state. When

disabled, each amplifier's supply current is reduced to

0mA. Each EL2186C/EL2286C amplifier is disabled

when its ENABLE pin is floating or pulled up to within

0.5V of the positive supply. Similarly, each amplifier is

enabled by pulling its ENABLE pin at least 3V below

the positive supply. For ±5V supplies, this means that an

EL2186C/EL2286C 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

EL2186C/EL2286C 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 EL2186C 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-feed-

back 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 capaci-

tance (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

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