Difference between revisions of "Minimig Video d/a resistor ladder"

Revision as of 17:37, 22 August 2007

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

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.

@@ Line 9: / Line 9: @@
 :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.
 :[http://en.wikipedia.org/wiki/Resistor wikipedia: Resistor]
 :[http://en.wikipedia.org/wiki/Preferred_number#Capacitors_and_resistors wikipedia: Preferred number]
 :[http://www.digikey.com/scripts/DkSearch/dksus.dll?Cat=65538;keywords=resistor digikey: Resistors]