Minimig Video d/a resistor ladder

Model:

D/A resistor ladder electrical model

Equations:

Defining constants:

U_vga = 0.7 V

R_vga = 75 Ω

U_vcc = 3.3 V

Total resistance = 1/(1/4000+1/2000+1/1000+1/560)+75 = 357.8282.. Ω

Common current through resistor ladder plus vga impedance (75Ω) = 3.3V / 357.8282.. = 0.009222 A

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

Base resistor value = (1/8+1/4+1/2+1/1)/(1/((R_vga*U_vcc)/U_vga-R_vga)) = 522.3214Ω

Deviation summary:

0523/523 = 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. Resistor combinations will add tolerances and use valuable pcb space.

Maximum output voltage if VGA impedance is 75Ω is: (75*3.3)/(1/(1/4220+1/2100+1/1050+1/523)+75) = 0.6981 V

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