9

Applications Information

Product Description

The EL2176/EL2276 are current-feedback operational

amplifiers that offer a wide -3dB bandwidth of 70MHz, a low

supply current of 1mA per amplifier and the ability to disable

to 0mA. Both products also feature high output current drive.

The EL2176 can output 100mA, while the EL2276 can

output 55mA per amplifier. The EL2176/EL2276 work with

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

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

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

topology, the EL2176/EL2276 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 EL2176/EL2276 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 EL2170 (8-Pin Single), EL2270 (8-Pin Dual)

and EL2470 (14-Pin Quad). If more AC performance is

required, refer to the EL2180/EL2186 family which provides

Singles, Duals, and Quads with 250MHz of bandwidth while

consuming 3mA 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 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 EL2176/EL2276 amplifiers can be disabled, placing

their output in a high-impedance state. When disabled, each

amplifier's supply current is reduced to 0mA. Each

EL2176/EL2276 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 EL2176/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 EL2176/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 EL2176 at ±5V, if ENABLE is tied to -5V

(rather than ground) the supply current will increase about

15% to 1.15mA.

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 EL2176/EL2276 have 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 these amplifiers, 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

EL2176/EL2276 remain very stable. For less experienced

users, this feature makes the EL2176/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

EL2176/EL2276 have less bandwidth than expected. In this

case, the inverting input may have less parasitic capacitance

than expected by the internal compensation circuitry of the

EL2176/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 see the curves for Frequency

EL2176, EL2276