EL5493, EL5493A

FN7202 Rev 1.00

Page 13 of 14

April, 2003

Applications Information

Product Description

The EL5493 is a current-feedback operational amplifier that

offers a wide -3dB bandwidth of 300MHz and a low supply

current of 4mA per amplifier. The EL5493 works with supply

voltages ranging from a single 5V to 10V and they are also

capable of swinging to within 1V of either supply on the output.

Because of its current-feedback topology, the EL5493 does not

have the normal gain-bandwidth product associated with

voltage-feedback operational amplifiers. Instead, 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 EL5493 the ideal

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

as portable, handheld, or battery-powered equipment.

For varying bandwidth needs, consider the EL5191 with 1GHz

on a 9mA supply current or the EL5192 with 600MHz on a 6mA

supply current. Versions also include single, dual, triple, and

quad amp packages.

Power Supply Bypassing and Printed Circuit Board

Layout

As with any high frequency device, good printed circuit board

layout is necessary for optimum performance. Low impedance

ground plane construction is essential. Surface mount

components are recommended, but if leaded components are

used, 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 0.01µF 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) Even when ground

plane construction is used, 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 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 additional peaking and

overshoot.

Disable/Power-Down

The EL5493A amplifier can be disabled placing its output in a

high impedance state. When disabled, the amplifier supply

current is reduced to < 600µA. The EL5493A is disabled when

its CE pin is pulled up to within 1V of the positive supply.

Similarly, the amplifier is enabled by floating or pulling its CE

pin to at least 3V below the positive supply. For ±5V supply,

this means that an EL5493A amplifier will be enabled when CE

is 2V or less, and disabled when CE is above 4V. Although the

logic levels are not standard TTL, this choice of logic voltages

allows the EL5493A to be enabled by tying CE to ground, even

in 5V single supply applications. The CE pin can be driven

from CMOS outputs.

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 exacerbates the problem by further lowering the pole

frequency (increasing the possibility of oscillation.)

The EL5493 has been optimized with a 475

 feedback

resistor. With the high bandwidth of these amplifiers, these

resistor values might cause stability problems when combined

with parasitic capacitance, thus ground plane is not

recommended around the inverting input pin of the amplifier.

Feedback Resistor Values

The EL5493 has been designed and specified at a gain of +2

with RF approximately 500. This value of feedback resistor

gives 200MHz of -3dB bandwidth at AV=2 with 2dB of peaking.

With AV=-2, an RF of approximately 500 gives 175MHz of

bandwidth with 0.2dB of peaking. Since the EL5493 is a

current-feedback amplifier, it is also possible to change the

value of RF to get more bandwidth. As seen in the curve of

Frequency Response for Various RF and RG, bandwidth and

peaking can be easily modified by varying the value of the

feedback resistor.

Because the EL5493 is a current-feedback amplifier, its gain-

bandwidth product is not a constant for different closed-loop

gains. This feature actually allows the EL5493 to maintain

about the same -3dB bandwidth. As gain is increased,

bandwidth decreases slightly while stability increases. Since

the loop stability is improving with higher closed-loop gains, it

becomes possible to reduce the value of RF below the

specified 475

 and still retain stability, resulting in only a slight

loss of bandwidth with increased closed-loop gain.

Supply Voltage Range and Single-Supply Operation

The EL5493 has been designed to operate with supply

voltages having a span of greater than 5V and less than 10V.

In practical terms, this means that the EL5493 will operate on

dual supplies ranging from ±2.5V to ±5V. With single-supply,

the EL5493 will operate from 5V to 10V.

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 EL5493

has an input range which extends to within 2V of either supply.

So, for example, on +5V supplies, the EL5493 has an input

range which spans ±3V. The output range of the EL5493 is