EL5191CSZ datasheet(14/17 Pages) INTERSIL

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Applications Information

Product Description

The EL5191 is a current-feedback operational amplifier that

offers a wide -3dB bandwidth of 1GHz and a low supply

current of 9mA per amplifier. The EL5191 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 their current-feedback topology, the

EL5191 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 EL5191 the ideal choice for many low-power/high-

bandwidth applications such as portable, handheld, or

battery-powered equipment.

For varying bandwidth needs, consider the EL5192 with

600MHz on a 6mA supply current or the EL5193 with

300MHz on a 4mA supply current. Versions include single,

dual, and triple amp packages with 5-pin SOT-23, 16-pin

QSOP, and 8-pin or 16-pin SO outlines.

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 package,

should be avoided if possible. Sockets add parasitic

inductance and capacitance which will result in additional

peaking and overshoot.

Disable/Power-Down

The EL5191A amplifier can be disabled placing its output in

a high impedance state. When disabled, the amplifier supply

current is reduced to < 150µA. The EL5191A 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 EL5191A 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 EL5191A 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 EL5191 has been optimized with a 250

Ω 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 EL5191 has been designed and specified at a gain of +2

with RF approximately 250Ω. This value of feedback resistor

gives 600MHz of -3dB bandwidth at AV = 2 with about 2dB of

peaking. With AV = -2, that same RF gives 450MHz of

bandwidth with 0.6dB of peaking. Since the EL5191 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 EL5191 is a current-feedback amplifier, its

gain-bandwidth product is not a constant for different closed-

loop gains. This feature actually allows the EL5191 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 250Ω 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 EL5191 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 EL5191 will

operate on dual supplies ranging from ±2.5V to ±5V. With

single-supply, the EL5191 will operate from 5V to 10V.

EL5191, EL5191A

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