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Applications Information

Product Description

The EL2276 is a current-feedback operational amplifier that

offers a wide -3dB bandwidth of 70MHz, a low supply current

of 1mA per amplifier and the ability to disable to 0mA. This

product also features high output current drive. The EL2276

can output 55mA per amplifier. The EL2276 works with

supply voltages ranging from a single 3V to ±6V, and it is

also capable of swinging to with in 1V of either supply on the

input and the output. Because of its current-feedback

topology, the EL2276 does not have the normal gain-

bandwidth product associated with voltage-feedback

operational amplifier. 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 EL2276 the ideal choice for

many low-power/high-bandwidth applications such as

portable computing, HDSL, and video processing.

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 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 peaking and overshoot.

Disable/Power-Down

The EL2276 amplifier can be disabled, placing its output in a

high-impedance state. When disabled, the amplifier's supply

current is reduced to 0mA. The EL2276 amplifier 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 EL2276 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

EL2276 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 EL2276 at ±5V, if ENABLE is tied to -5V

(rather than ground) the supply current will increase about

15% to 2.3mA.

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 EL2276 has been specially designed to reduce power

dissipation in the feedback network by using large 1.0k

Ω

feedback and gain resistors. With the high bandwidths of this

amplifier, these large resistor values would normally cause

stability problems when combined with parasitic

capacitance, but by internally canceling the effects of a

nominal amount of parasitic capacitance, the EL2276

remains very stable. For less experienced users, this feature

makes the EL2276 much more forgiving, and therefore

easier to use than other products not incorporating this

proprietary circuitry.

The experienced user with a large amount of PC board

layout experience may find in rare cases that the EL2276

has less bandwidth than expected. In this case, the inverting

input may have less parasitic capacitance than expected by

the internal compensation circuitry of the EL2276. 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

EL2276