Minimig Video d/a resistor ladder

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.

wikipedia: Resistor

wikipedia: Preferred number

digikey: Resistors