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I know the problem .. I design a current DAC .18um
but current too small ~uA ...
and someone tell me VGA RAMDAC have large current scale
~ 17mA and driver 50 ohm load (swing < 1v) ..
In RAMDAC design, its spec need a 17~19mA current to drive
an 37 Ohm resistor (double terminaled by 75 Ohm).
In general design, we will use a V to I circuit to generate a current
source and mirror it to a set of current cell controlled by the
opposite digital code.
So if you want to design this VGA DAC, I think you may change your
design meet the output current requirment. Becuase you will find
many issue when you need to operate in high speed.
And also remeber you will need three DACs for R G B channel.
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