Minimig Video d/a resistor ladder
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- Equations:
- Total resistance = 1/(1/4000+1/2000+1/1000+1/560)+75
- Common current through resistor ladder plus vga impedance (75 ohm) = 3.3V / 357.8282..
- Current * vga impedance = 0.6917V (specification says 0.7 p-p)
- Output voltage = 75 * (3.3/(1/(1/4000+1/2000+1/1000+1/560)+75)) = 0.6917V
- Simplification:
- U = (75*3.3)/(1/(1/4000+1/2000+1/1000+1/560)+75) = 0.6917V
- U_vga = 0.7 Volt
- R_vga = 75 ohm
- U_vcc = 3.3 Volt
- Base constant = (1/8+1/4+1/2+1/1)/(1/((R_vga*U_vcc)/U_vga-R_vga)) = 522.3214 ohm
- Deviation
- 523/523 = 1.0000
- 1050/523 = 2.0076
- 2100/523 = 4.0153
- 4220/523 = 8.0688
- As can be seen no more than 1% is useful with digikey resistors.