Difference between revisions of "VGAX49"

Revision as of 13:22, 14 July 2022

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

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:

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

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

@@ Line 75: / Line 75: @@
 ** 0.7V into 75 Ohms = 9.33 mA
 * R-2R values
-** 5% resistor values are 510 ohms, 1K ohms and 2K ohms
+** 5% resistor values are 470 ohms, 1K ohms and 2.2K ohms
-*** Produce 0.679V
+*** Produces 0.697V
 [[file:VideoDAC_3bit_Voltage.PNG]]