VGAX49

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VGAX49 P901-cropped-720px.jpg

Features[edit]

  • 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[edit]

VGAX49.PNG

J1 - Digital Connections[edit]

P1 - VGA connector[edit]

Simulation[edit]

  • 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 2:2:2 Case[edit]

  • 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
  • Voltage steps are:

VGA Sim 2R Voltage.PNG

  • Current steps are:

VGA Sim 2R Current.PNG

5:6:5 Resistor Simulation[edit]

  • 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

VideoDAC 6bit Voltage.PNG

  • Current steps

VideoDAC 6bit Current.PNG

Schematic[edit]

Assembly Sheet[edit]