VGAX49

Features

Digital VGA adapter
16-bit digital video
- 5:6:5 (R:G:B) mapping (maximum)
- 2:2:2 (R:G:B) mapping (option)
Uses summing resistors
DB-15F connector
49x49mm ODAS form factor
Mounting holes

Each color is independently driven and can be considered individually
Critical design criteria is output voltage and current capability of the driving part
Use common value, 5% resistors
Each resistor is about 2X the value of the previous resistor
1V is drive level for VGA specification
- VGA presents a 75 Ohm load
- 1V at 75 Ohms is 13.3 mA
Series resistors present a voltage divider between the resistors on the card and the VGA load
The resistor with the smallest resistance value has larger current from the source device pin
Current switches from source to sink depending on the voltages on the other resistors
Each resistor is 2X the value of the previous resistor
Too much effort is put into using precision resistors - the human eye is not that picky

Assumptions
- 3.3V Drive out of FPGA
- 8 mA max drive current (typical FPGA drive current)
Using common value, 5% resistors
330 Ohm series resistor into a 75 Ohm load is 8.15 mA (close enough to max of FPGA)
- Voltage with 330/75 ohm voltage divider is 0.61V - not full brightness
Next standard value from 330 ohms is 680 ohms
- Voltage is 0.83V - better brightness
Voltage with all 6 resistors (Green case) is 1.07V (full brightness)
Simulation uses the same 4 value steps (real case would have 2^5 or 2^6 steps)