EL2245CS datasheet(10/14 Pages) ELANTEC

10 / 14 page

EL2245C, EL2445C

Dual/Quad Low-Power 100MHz Gain-of-2 Stable Op Amp

bandwidth product divided by the noise gain of the cir-

cuit. For gains less than 4, higher-order poles in the

amplifiers' transfer function contribute to even higher

closed loop bandwidths. For example, the

EL2245C/EL2445C have a -3dB bandwidth of 100MHz

at a gain of +2, dropping to 20MHz at a gain of +5. It is

important to note that the EL2245C/EL2445C have been

designed so that this “extra” bandwidth in low-gain

applications does not come at the expense of stability.

As seen in the typical performance curves, the

EL2245C/EL2445C in a gain of +2 only exhibit 1.0dB

of peaking with a 1000

Ω load.

Video Performance

An industry-standard method of measuring the video

distortion of components such as the EL2245C/

EL2445C is to measure the amount of differential gain

(dG) and differential phase (dP) that they introduce. To

make these measurements, a 0.286VPP (40 IRE) signal is

applied to the device with 0V DC offset (0 IRE) at either

3.58MHz for NTSC or 4.43MHz for PAL. A second

measurement is then made at 0.714V DC offset (100

IRE). Differential gain is a measure of the change in

amplitude of the sine wave, and is measured in percent.

Differential phase is a measure of the change in phase,

and is measured in degrees.

For signal transmission and distribution, a back-termi-

nated cable (75

Ω in series at the drive end, and 75Ω to

ground at the receiving end) is preferred since the

impedance match at both ends will absorb any reflec-

tions. However, when double termination is used, the

received signal is halved; therefore a gain of 2 configu-

ration is typically used to compensate for the

attenuation.

The EL2245C/EL2445C have been designed as an eco-

nomical solution for applications requiring low video

distortion. They have been thoroughly characterized for

video performance in the topology described above, and

the results have been included as typical dG and dP

specifications and as typical performance curves. In a

gain of +2, driving 150

Ω, with standard video test levels

at the input, the EL2245C/EL2445C exhibit dG and dP

of only 0.02% and 0.07° at NTSC and PAL. Because dG

and dP can vary with different DC offsets, the video per-

formance of the EL2245C/EL2445C has been

characterized over the entire DC offset range from -

0.714V to +0.714V. For more information, refer to the

curves of dG and dP vs DC Input Offset.

Output Drive Capability

The EL2245C/EL2445C have been designed to drive

low impedance loads. They can easily drive 6VPP into a

150

Ω load. This high output drive capability makes the

EL2245C/EL2445C an ideal choice for RF, IF and video

applications. Furthermore, the current drive of the

EL2245C/EL2445C remains a minimum of 35mA at

low temperatures. The EL2245C/EL2445C are current-

limited at the output, allowing it to withstand shorts to

ground. However, power dissipation with the output

shorted can be in excess of the power-dissipation capa-

bilities of the package.

Capacitive Loads

For ease of use, the EL2245C/EL2445C have been

designed to drive any capacitive load. However, the

EL2245C/EL2445C remain stable by automatically

reducing their gain-bandwidth product as capacitive

load increases. Therefore, for maximum bandwidth,

capacitive loads should be reduced as much as possible

or isolated via a series output resistor (Rs). Similarly,

coax lines can be driven, but best AC performance is

obtained when they are terminated with their character-

istic impedance so that the capacitance of the coaxial

cable will not add to the capacitive load seen by the

amplifier. Although stable with all capacitive loads,

some peaking still occurs as load capacitance increases.

A series resistor at the output of the EL2245C/EL2445C

can be used to reduce this peaking and further improve

stability.

Printed-Circuit Layout

The EL2245C/EL2445C are well behaved, and easy to

apply in most applications. However, a few simple tech-

niques will help assure rapid, high quality results. As

with any high-frequency device, good PCB layout is

necessary for optimum performance. Ground-plane con-

struction is highly recommended, as is good power

supply bypassing. A 0.1µF ceramic capacitor is recom-

mended for bypassing both supplies. Lead lengths

should be as short as possible, and bypass capacitors

should be as close to the device pins as possible. For

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