EL2020CN View Datasheet(PDF) - Elantec -> Intersil

Part Name

Description

Manufacturer

EL2020CN

EL2020C / 50 MHz Current Feedback Amplifier

Elantec -> Intersil 

EL2020C

50 MHz Current Feedback Amplifier

Application Information Contd

Video Distribution Amplifier

The distribution amplifier shown below features

a difference input to reject common mode signals

on the 75X coax cable input Common mode re-

jection is often necessary to help to eliminate

60 Hz noise found in production environments

Video Distribution Amplifier

with Difference Input

2020 – 15

EL2020 Disable Enable Operation

The EL2020 has an enable disable control input

at pin 8 The device is enabled and operates nor-

mally when pin 8 is left open or returned to pin 7

VCC When more than 250 mA is pulled from pin

8 the EL2020 is disabled The output becomes a

high impedance the inverting input is no longer

driven to the positive input voltage and the sup-

ply current is halved To make it easy to use this

feature there is an internal resistor to limit the

current to a safe level (E1 1 mA) if pin 8 is

grounded

To draw current out of pin 8 an ‘‘open collector

output’’ logic gate or a discrete NPN transistor

can be used This logic interface method has the

advantage of level shifting the logic signal from

5V supplies to whatever supply the EL2020 is op-

erating on without any additional components

Using the EL2020 as a Multiplexer

An interesting use of the enable feature is to com-

bine several amplifiers in parallel with their out-

puts common This combination then acts simi-

lar to a MUX in front of an amplifier A typical

circuit is shown

When the EL2020 is disabled the DC output im-

pedance is very high over 10 kX However there

is also an output capacitance that is non-linear

For signals of less than 5V peak to peak the out-

put capacitance looks like a simple 15 pF capaci-

tor However for larger signals the output capac-

itance becomes much larger and non-linear

The example multiplexer will switch between

amplifiers in 5 ms for signals of less than g2V on

the outputs For full output signals of 20V peak

to peak the selection time becomes 25 ms The

disabled outputs also present a capacitive load

and therefore only three amplifiers can have their

outputs shorted together However an unlimited

number can sum together if a small resistor

(25X) is inserted in series with each output to

isolate it from the ‘‘bus’’ There will be a small

gain loss due to the resistors of course

Using the EL2020 as a Multiplexer

2020 – 16

13

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