EL5293, EL5293A

FN7193 Rev 3.00

Page 13 of 16

July 26, 2007

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 SOIC

package, should be avoided if possible. Sockets add

parasitic inductance and capacitance which will result in

additional peaking and overshoot.

Disable/Power-Down

The EL5293A amplifier can be disabled placing its output in

a high impedance state. When disabled, the amplifier supply

current is reduced to < 300µA. The EL5293A 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 EL5293A 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 EL5293A 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 EL5293 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 EL5293 has been designed and specified at a gain of +2

175MHz of bandwidth with 0.2dB of peaking. Since the

EL5293 is a current-feedback amplifier, it is also possible to

bandwidth and peaking can be easily modified by varying the

value of the feedback resistor.

Because the EL5293 is a current-feedback amplifier, its

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

loop gains. This feature actually allows the EL5293 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

resulting in only a slight loss of bandwidth with increased

closed-loop gain.

Supply Voltage Range and Single-Supply

Operation

The EL5293 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 EL5293 will operate

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

supply, the EL5293 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

EL5293 has an input range which extends to within 2V of

either supply. So, for example, on +5V supplies, the EL5293

has an input range which spans ±3V. The output range of the

EL5293 is also quite large, extending to within 1V of the

supply rail. On a ±5V supply, the output is therefore capable

larger because of the increased negative swing due to the

external pull-down resistor to ground.

Video Performance

For good video performance, an amplifier is required to

maintain the same output impedance and the same

frequency response as DC levels are changed at the output.

This is especially difficult when driving a standard video load

of 150

, because of the change in output current with DC

level. Previously, good differential gain could only be

achieved by running high idle currents through the output

transistors (to reduce variations in output impedance.)

These currents were typically comparable to the entire 4mA

supply current of each EL5293 amplifier. Special circuitry

has been incorporated in the EL5293 to reduce the variation

of output impedance with current output. This results in dG

and dP specifications of 0.03% and 0.04°, while driving

150

 at a gain of 2.

Video performance has also been measured with a 500



load at a gain of +1. Under these conditions, the EL5293 has

dG and dP specifications of 0.03% and 0.04°.

Output Drive Capability

In spite of its low 4mA of supply current, the EL5293 is

capable of providing a minimum of ±95mA of output current.

With a minimum of ±95mA of output drive, the EL5293 is