EL4501 데이터 시트보기 (PDF) - Renesas Electronics

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EL4501

Video Front End

Renesas Electronics 

EL4501

DATA SLICER

The data slicer is a fast comparator with the output of the video

amplifier connected to its inverting input and the

DS REF connected to its non-inverting input. The DS OUT is

logical inverse of the video output sliced at the DS REF

voltage. The propagation delay from the video amplifier output

to the DS OUT is about 18ns. There is about 10mV hysteresis

added internally in the comparator to prevent the oscillation at

the DS OUT when the voltages at the two inputs are very close

or equal. An adjustable DS REF voltage may be used to

extract data embedded within the active video or video

blanking interval regions of a video signal. Logic low at the DS

ENABLE pin enables the comparator and logic low lets the DS

OUT be three-state. The DS MODE pin sets the mode of the

DS comparator. Logic low at the DS MODE pin selects a

standard logic output and a logic high selects an open

drain/collector output.

VIDEO AMPLIFIER

The EL4501 DC-restore block incorporates a wide bandwidth,

single supply, low power, rail-to-rail output, voltage feedback

operational amplifier. The amplifier is internally compensated

for closed loop feedback gains of +1 or greater. Larger gains

are acceptable but bandwidth will be reduced according to the

familiar Gain-Bandwidth product.

Connected in a voltage follower mode and driving a high

impedance load, the amplifier has a -3dB bandwidth of

100MHz. Driving a 150 load, the -3dB bandwidth reduces to

60MHz while maintaining a 200V/µs slew rate.

CHOICE OF FEEDBACK RESISTOR, RF

The video amplifier is optimized for applications that require a

gain of +1. Hence, no feedback resistor is required. However,

for gains greater than +1, the feedback resistor forms a pole

with the hold capacitance. As this pole becomes larger, phase

margin is reduced. This causes ringing in the time domain and

peaking in the frequency domain. Therefore, RF has some

maximum value that should not be exceeded for optimum

performance. 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 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 is optimum

• For AV = +2, RF between 300 and 1k is optimum

FN7327 Rev 3.00

November 12, 2010

VIDEO PERFORMANCE

For good video signal integrity, an amplifier is required to

maintain the same output impedance and frequency response

as DC levels are changed at the output. This can be difficult

when driving a standard video load of 150 because of the

change in output current with DC level. A look at the

Differential Gain and Differential Phase curves will help to

obtain optimal performance. Curves are provided for AV = +1

and +2, and RL = 150 and 10k. As with all video amplifiers,

there is a common mode sweet spot for optimum differential

gain/differential phase. For example, with AV = +1 and RL =

150 and the video level kept between 1V and 3V, the

amplifier will provide dG/dP performance of 0.17%/0.07°. This

condition is representative of using the amplifier as a buffer

driving a DC coupled, double terminated, 75 coaxial cable.

Driving high impedance loads, such as signals on computer

video cards gives much better dG/dP performance. For AV = 1,

RL = 10k, and the video level kept between 1V and 3V, the

dG/dP are 0.03%/0.02°.

SHORT-CIRCUIT CURRENT LIMIT

The EL4501 video amplifier has no internal short circuit

protection circuitry. Short circuit current of 90mA sourcing and

65mA sinking typically will flow if the output is shorted midway

between the rails. If the output is shorted indefinitely, the power

dissipated could easily increase the die temperature such that

the part will be destroyed. Maximum reliability is maintained if

the output current never exceeds ±50mA. This limit is set by

internal metal interconnect limitations. Obviously, short circuit

conditions must not be allowed to persist or internal metal

connections will be damaged or destroyed.

DRIVING CABLES AND CAPACITIVE LOADS

The EL4501 video amplifier can drive 39pF loads in parallel

with 150 with 5dB of peaking. For less peaking in theses

applications a small series resistor of between 5 and 50

can be placed in series with the output. However, this will

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

the gain resistor RG can be adjusted to make up for any lost

gain caused by the additional output resistor. Peaking may

also be reducing by adding a “snubber” circuit at the output. A

snubber is a resistor in series with a capacitor, 150 and

100pF being typical values. The advantage of a snubber is that

it does not draw DC load current.

When used as a cable driver, double termination is always

recommended for reflection-free performance. For those

applications, the back-termination series resistor decouples

the video amplifier from the cable and enables extensive

capacitive drive. However, other applications may have high

capacitive loads without a back-termination resistor. Again, a

small series resistor at the output can reduce peaking.

VIDEO SYNC SEPARATOR

The EL4501 includes an advanced sync separator, which is

used to generate the DC-restore control signal and seven

major sync outputs. The advanced sync separator operates at

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