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FN7061.3

May 19, 2006

The output range of the EL2250/EL2450 is also quite large.

It includes the negative rail, and extends to within 1V of the

top supply rail with a 1k

Ω load. On a +5V supply, the output

is therefore capable of swinging from 0V to +4V. On split

supplies, the output will swing ±4V. If the load resistor is tied

to the negative rail and split supplies are used, the output

range is extended to the negative rail.

The feedback resistor forms a pole with the input

capacitance. As this pole becomes larger, phase margin is

reduced. This increases ringing in the time domain and

maximum value which should not be exceeded for optimum

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,

As this combination gets smaller, the bandwidth falls off.

exceeded for optimum performance.

500

between 2k

Ω and 10kΩ.

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 can be difficult when driving a standard video load of

150

Ω, because of the change in output current with DC level.

Differential Gain and Differential Phase for the

EL2250/EL2450 are specified with the black level of the

output video signal set to +1.2V. This allows ample room for

the sync pulse even in a gain of +2 configuration. This

results in dG and dP specifications of 0.05% and 0.05° while

driving 150

Ω at a gain of +2. Setting the black level to other

values, although acceptable, will compromise peak

performance. For example, looking at the single supply dG

is reduced from 1.2V to 0.6V dG/dP will increase from

0.05%/0.05° to 0.08%/0.25° Note that in a gain of +2

configuration, this is the lowest black level allowed such that

the sync tip doesn’t go below 0V.

If your application requires that the output goes to ground,

then the output stage of the EL2250/EL2450, like all other

single supply op amps, requires an external pull down

resistor tied to ground. As mentioned above, the current

flowing through this resistor becomes the DC bias current for

the output stage NPN transistor. As this current approaches

zero, the NPN turns off, and dG and dP will increase. This

becomes more critical as the load resistor is increased in

value. While driving a light load, such as 1k

Ω, if the input

black level is kept above 1.25V, dG and dP are a respectable

0.03% and 0.03°.

For other biasing conditions see the Differential Gain and

Differential Phase vs. Input Voltage curves.

Output Drive Capability

In spite of their moderately low 5mA of supply current, the

EL2250/EL2450 are capable of providing ±100mA of output

current into a 10

Ω load, or ±60mA into 50Ω. With this large

output current capability, a 50

Ω load can be driven to ±3V

isolation transformers in telecommunications applications.

Driving Cables and Capacitive Loads

When used as a cable driver, double termination is always

recommended for reflection-free performance. For those

applications, the back-termination series resistor will de-

couple the EL2250/EL2450 from the cable and allow

extensive capacitive drive. However, other applications may

have high capacitive loads without a back-termination

resistor. In these applications, a small series resistor (usually

between 5

Ω and 50Ω) can be placed in series with the

then be chosen to make up for any gain loss which may be

created by this additional resistor at the output.

Video Sync Pulse Remover Application

All CMOS Analog to Digital Converters (A/Ds) have a

parasitic latch-up problem when subjected to negative input

voltage levels. Since the sync tip contains no useful video

information and it is a negative going pulse, we can chop it

off.

Figure 1 shows a unity gain connected amplifier A of an

EL2250. Figure 2 shows the complete input video signal

applied at the input, as well as the output signal with the

negative going sync pulse removed.

FIGURE 1.

EL2250, EL2450