TLE2144MDWREP datasheet(14/22 Pages) TI

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34830

FUNCTIONAL DEVICE OPERATION

FUNCTIONAL INTERNAL BLOCK DESCRIPTION

INPUT CONSIDERATIONS

As explained in the Input Clamp section, the 34830

features an internal clamp that allows the device to work with

both AC as well as DC-coupled input signals. To AC-couple

the input signal, use a 0.1μF capacitor following the video

signal source. If the signal being AC-coupled has sync, then

the 34830’s clamp circuit ensures that the sync tip is detected

and positioned near ground (Sync Tip Clamp). If the signal

that is being AC-coupled does not have sync (Key Clamp),

care must be taken to ensure that its most negative portions

are not confused as being sync tips and clamped, resulting in

signal distortion. In order to prevent this from happening, the

user must set the DC bias at the input correctly. See the

SETTING KEY CLAMP BIAS section.

When the input to the 34830 goes above 50mV, the clamp

circuit becomes transparent and does not have any effect on

the signal being passed. This allows the 34830 to work with

DC-coupled signals. To DC-couple the input signal, simply

connect the video source directly to the input of the 34830.

OUTPUT CONISDERATIONS

The relationship between input and output for the 34830

follows the equation:

Where the 250mV term is the offset provided by the

internal level shifter. The 100mV term that is added to the

equation represents the worst case errors and offsets that

can be expected from the signal path, due to process and

temperature variations.

The 34830 has been optimized to drive a single standard

video load. A standard video load typically consists of a 75Ω

back-termination resistor, followed by a matched video cable

and a 75Ω load resistor. The 34830 can drive up to 5pF of

load capacitance in parallel with the video cable and load

resistor. Two video loads can be supported by the 34830 with

a minimum tradeoff in performance parameters.

The output of the 34830 can be both AC or DC-coupled.

When the output is AC-coupled the AC-coupling capacitor

forms a high-pass filter with the load resistor. Ensure that the

value of the AC-coupling capacitor is such that the lowest

frequencies of the video signal are passed without

attenuation from this filter. A typical value for the output AC-

coupling capacitor is 220μF.

Place the output termination resistor as close to the output

as possible to minimize parasitic inductance and capacitance

effects that tend to deteriorate signal quality.

VOUT 2VIN 250mV

()

100mV

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