A/D Converter - NEC MultiSync LCD1920NX User And Service Manual

2.3 A/D converter

The A/D converter (IC01) is used to convert the RGB video signals of about 0.7Vp-p into the digital signals of
odd-numbered and even-numbered pixels of 8 bits each, 48 bits in total. In regard to the clocks for
conversion into digital signals, the built-in PLL is used to generate the clock signals inside the IC01, based on
the horizontal sync signal.
The video signal passes through the built-in pre-amplifier, and is then A/D-converted by the sampling clock
signal generated from the above-mentioned PLL circuit.
The registers intended for the setting of the clamp level and the pre-amplifier gain are controlled through the
IIC bus of Pins 29 to 30. C81, C82, and R50 connected to Pin 45 are used to compose a loop filter intended
to generate the PLL clock signals. If this section becomes out of order, it is impossible to accomplish normal
reproduction of images.
During the entry of the SOG input and the composite sync input, the vertical blanking period and the PLL
circuit are held by the coast signal input fed from Pin 41. If this signal is not available, the clock frequency is
changed during the vertical period and a curve may be produced at the upper part of the screen.
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