10

FN7365.6

September 14, 2010

Applications Information

Product Description

The EL5166 and EL5167 are current-feedback operational

amplifiers that offers a wide -3dB bandwidth of 1.4GHz and a

low supply current of 8.5mA per amplifier. The EL5166 and

EL5167 work 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 EL5166 and EL5167 do not have the normal

gain-bandwidth product associated with voltage-feedback

operational amplifiers. Instead, their -3dB bandwidth remains

relatively constant as closed-loop gain is increased. This

combination of high bandwidth and low power, together with

aggressive pricing make the EL5166 and EL5167 ideal

choices for many low-power/high-bandwidth applications,

such as portable, handheld, or battery-powered equipment.

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

“Capacitance at the Inverting Input” on page 10). 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 EL5166 amplifier can be disabled, placing its output in a

high impedance state. When disabled, the amplifier supply

current is reduced to 13µA. The EL5166 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 EL5166 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 EL5166 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 EL5166 and EL5167 frequency responses are

optimized with the resistor values in Figure 3. 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 EL5166 and EL5167 have been designed and specified

with about 0.5dB of peaking. Since the EL5166 and EL5167

are current-feedback amplifiers, it is also possible to change

“Typical Performance Curves” on page 4, bandwidth and

peaking can be easily modified by varying the value of the

feedback resistor.

Because the EL5166 and EL5167 are current-feedback

amplifiers, their gain-bandwidth product is not a constant for

different closed-loop gains. This feature actually allows the

EL5166 and EL5167 to maintain a reasonably constant -3dB

bandwidth for different gains. As gain is increased,

bandwidth decreases slightly while stability increases. Since

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

specified 250

Ω 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 EL5166 and EL5167 have 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 EL5166

and EL5167 will operate on dual supplies ranging from

±2.5V to ±5V. With single-supply, they 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

EL5166 and EL5167 have an input range that extends to

within 1.8V of either supply. So, for example, on ±5V

EL5166, EL5167