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

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

2:2:2 Resistor Simulation

• 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
• Ideal resistor values (1%) are 348 ohms and 698 ohms for 8 mA drive

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
• 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)

ESP32 Drive

• The ESP32 drive current is significantly more than an FPGA
  • 40 mA
  • More than enough to drive to 1V (saturation level on VGA).
• 1V at 75 Ohms is 13.3 mA
• 2:2:2 270/470 Ohm simulation
• -10 mA (max)
• 0.96V (max)

Ideal Drive 2:2:2 Case

• Ideal case drive current
  • 1V into 75 Ohms = 13.33 mA
  • 3.3V driver which can drive more than 10 mA
  • 1% resistors
    • Get closest values
    • R-2R values
  • 2/3 of the drive comes from the smaller resistor
  • Ideal resistor values are 261, 523 ohms
    • 0V, 0.331V, 0.661V, 0.992 V

Schematic

• VGAX49 schematic

Assembly Sheet

• VGAX49 Assembly Sheet