Difference between revisions of "Minimig Video d/a resistor ladder"
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− | :Equations: | + | Model:<br> |
− | ::Total resistance = 1/(1/4000+1/2000+1/1000+1/560)+75 | + | [[Image:Minimig_dac_resistor_ladder_model.png|thumb| none |150px|D/A resistor ladder electrical model]]<br><!-- http://en.wikipedia.org/wiki/Wikipedia:Extended_image_syntax --> |
− | ::Common current through resistor ladder plus vga impedance ( | + | |
− | ::Current * | + | Equations:<br> |
+ | :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 * [[Video Graphics Array|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 | ::Output voltage = 75 * (3.3/(1/(1/4000+1/2000+1/1000+1/560)+75)) = 0.6917V | ||
+ | |||
:Simplification: | :Simplification: | ||
− | :U = (75*3.3)/(1/(1/4000+1/2000+1/1000+1/560)+75) = 0.6917V | + | ::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 | |
− | |||
− | |||
− | |||
− | :Base | ||
− | :Deviation | + | :Deviation summary: |
− | :: | + | ::0523/523 = 1.0000 |
::1050/523 = 2.0076 | ::1050/523 = 2.0076 | ||
::2100/523 = 4.0153 | ::2100/523 = 4.0153 | ||
::4220/523 = 8.0688 | ::4220/523 = 8.0688 | ||
− | |||
− | |||
− | :[http://en.wikipedia.org/wiki/Resistor | + | :As can be seen no more than 1% is useful with digikey resistors. Resistor combinations will add tolerances and use valuable pcb space. |
− | :[http://en.wikipedia.org/wiki/Preferred_number#Capacitors_and_resistors | + | :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 |
− | :[http://www.digikey.com/scripts/DkSearch/dksus.dll?Cat=65538;keywords=resistor | + | |
+ | Useful links:<br> | ||
+ | :[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] |
Revision as of 18:35, 22 August 2007
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
Useful links: