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

Connectors

P1 - VGA connector

• Ex: R0 is the least significant Red bit (connects to 10K resistor)

Simulation

• 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

VGA - Ideal Drive 2 bit Case

• 3.3V driver requires less than 8 mA
  • Configure FPGAs output as 3.3V LVTTL and 8mA drive current
• Ideal case drive current
• Full scale video = 0.7V
• VGA Termination resistor = 75 Ohms
  • 0.7V into 75 Ohms = 9.33 mA
• R-2R values
  • Ideal resistor values are 417.9 ohms and 835.7 ohms
  • Standard value 1% resistors
  • 1% standard values are:
    • R1 = 422 (Mouser) Ohms
    • R2 = 845 (Mouser) Ohms
    • Get closest values
    • 0V, 0.228V, 0.460V, 0.693V
• Voltage steps are:

• Current steps are:

VGA - Ideal Drive 3 bit Case

• 3.3V driver requires less than 8 mA
  • Configure FPGAs output as 3.3V LVTTL and 8mA drive current
• Ideal case drive current
• Full scale video = 0.7V
• VGA Termination resistor = 75 Ohms
  • 0.7V into 75 Ohms = 9.33 mA
• R-2R values
  • 5% resistor values are 470 ohms, 1K ohms and 2.2K ohms
    • Produces 0.697V

5:6:5 Resistor Simulation

• Assumptions
  • 3.3V Drive out of FPGA
  • 8 mA max drive current (typical FPGA drive current)
• Using common value, 5% resistors
• Model most significant bit resistor as 1/2 the full scale voltage (0.7V) single resistor value
• 75 Ohm terminator in monitor
• 0.7V at 75 Ohms = 9.33mA
• Series resistor - single resistor value is (3.3V-0.7V)/9.33mA is 278.6 Ohms
• First resistor is 2x 278.57 = 557.1 Ohms
• Each resistor is 2x the previous resistor
• Simulation uses the same 4 value steps (real case would have 2^5 or 2^6 steps)
• Voltage steps

• Current steps

Schematic

• VGAX49 schematic

Assembly Sheet

• VGAX49 Assembly Sheet