EL5244CN View Datasheet(PDF) - Elantec -> Intersil

Part Name

Description

Manufacturer

EL5244CN

100 MHz Single Supply Rail to Rail Amplifier

Elantec -> Intersil 

EL5144C, EL5146C, EL5244C, EL5246C,

EL5444C

100 MHz Single Supply Rail to Rail Amplifier

Figure 1 shows the output of the EL5144C series ampli-

fier swinging rail to rail with RF = 1KΩ, AV = +2 and RL

= 1MΩ. Figure 2 is with RL = 150 Ω.

5V

0V

Figure 1

5V

0V

Figure 2

Choice of Feedback Resistor, RF

These amplifiers are optimized for applications that

require a gain of +1. Hence, no feedback resistor is

required. However, for gains greater than +1, the feed-

back resistor forms a pole with the input capacitance. As

this pole becomes larger, phase margin is reduced. This

causes ringing in the time domain and peaking in the fre-

quency domain. Therefore, RF has some maximum

value that should not be exceeded for optimum perfor-

mance. If a large value of RF must be used, a small

capacitor in the few picofarad range in parallel with RF

can help to reduce this ringing and peaking at the

expense of reducing the bandwidth.

As far as the output stage of the amplifier is concerned,

RF + RG appear in parallel with RL for gains other than

+1. As this combination gets smaller, the bandwidth

falls off. Consequently, RF also has a minimum value

that should not be exceeded for optimum performance.

For AV = +1, RF = 0 Ω is optimum. For AV = -1 or +2

(noise gain of 2), optimum response is obtained with RF

between 300 Ω and 1K Ω. For AV = -4 or +5 (noise gain

of 5), keep RF between 300 Ω and 15K Ω.

Video Performance

For good video signal integrity, an amplifier is required

to maintain the same output impedance and the same fre-

quency response as DC levels are changed at the output.

This can be difficult when driving a standard video load

of 150Ω, because of the change in output current with

DC level. A look at the Differential Gain and Differen-

tial Phase curves for various supply and loading

conditions will help you obtain optimal performance.

Curves are provided for AV = +1 and +2, and RL = 150Ω

and 10 KΩ tied both to ground as well as 2.5V. As with

all video amplifiers, there is a common mode sweet spot

for optimum differential gain / differential phase. For

example, with AV = +2 and RL = 150Ω tied to 2.5V, and

the output common mode voltage kept between 0.8V

and 3.2V, dG/dP is a very low 0.1% / 0.1°. This condi-

tion corresponds to driving an AC-coupled, double

terminated 75Ω coaxial cable. With AV = +1, RL =

150Ω tied to ground, and the video level kept between

0.85V and 2.95V, these amplifiers provide dG/dP per-

formance of 0.05% / 0.20°. This condition is

representative of using the EL5144C series amplifier as

a buffer driving a DC coupled, double terminated, 75Ω

coaxial cable. Driving high impedance loads, such as

signals on computer video cards, gives similar or better

dG/dP performance as driving cables.

Driving Cables and Capacitive Loads

The EL5144C series amplifiers can drive 50pF loads in

parallel with 150 Ω with 4dB of peaking and 100pF with

7dB of peaking. If less peaking is desired in these appli-

cations, a small series resistor (usually between 5 Ω and

50 Ω) can be placed in series with the output to eliminate

most peaking. However, this will obviously reduce the

gain slightly. If your gain is greater than 1, the gain

resistor (RG) can then be chosen to make up for any gain

16

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