Difference between revisions of "VGAX49"
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** 8 mA max drive current (typical FPGA drive current) | ** 8 mA max drive current (typical FPGA drive current) | ||
* Using common value, 5% resistors | * 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) | * Simulation uses the same 4 value steps (real case would have 2^5 or 2^6 steps) | ||
| + | * Voltage steps | ||
| − | [[File: | + | [[File:VideoDAC_6bit_Voltage.PNG]] |
| − | [[File: | + | * Current steps |
| + | |||
| + | [[File:VideoDAC_6bit_Current.PNG]] | ||
== Schematic == | == Schematic == | ||
Revision as of 13:28, 19 February 2020
Contents
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
J1 - Digital Connections
P1 - VGA connector
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 2:2:2 Case
- ESP32 has a 3.3V driver which can drive more than 10 mA
- Ideal case drive current
- 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 422 (Mouser), 845 (Mouser) Ohms
- Get closest values
- 0V, 0.228V, 0.460V, 0.693V
- Voltage steps are:
- Current steps are:
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
