VGA voltage range on LogicStart Shield

Larry McGovern

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I've been making my way through Mike Field's terrific tutorial using the Papilio DUO and LogicStart Shield.  I have to say, I've been nothing but pleased with the boards, the tutorial, everything!  It has been the perfect way to jump start my knowledge of FPGAs.  What a great education!

I'm currently on the VGA chapter, and am successfully driving my LCD monitor with a VGA signal.  However, there must be something I'm missing.  The analog output is supposed to range from 0 to 0.7 volts, but as far as I can tell from the schematic, if I enable all four bits, then the analog output will go to 3.3V.  With all four bits enabled, the "resistor ladder" does not act as a voltage divider anymore, and the output voltage is the same as the FPGA pin output.  Indeed, I've confirmed this on my multimeter. 

What am I missing? 


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Ok, I feel like a dummy.  I'll answer my own question.   It appears the resistor ladder on the LogicStart board depends on the monitor having a 75 Ohm impedance, providing the other half of the voltage divider.  I had assumed the impedance would be much higher, but now understand 75 is the standard.  VGA test circuit working now on my Papilio Duo.  

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