Difference between revisions of "Minimig Video d/a resistor ladder"
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{| class="wikitable" | {| class="wikitable" | ||
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− | ! Value !! Order number | + | ! Value !! Order number (digikey.com) |
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| 523 || RR12P523DCT-ND | | 523 || RR12P523DCT-ND |
Latest revision as of 18:50, 28 August 2007
Contents
Electrical model[edit]
Equations[edit]
- 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 = 0.6917 V (specification says max 0.7V p-p)
- Output voltage = 75 * (3.3/(1/(1/4000+1/2000+1/1000+1/560)+75)) = 0.6917 V
- 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[edit]
- Wikipedia: Resistor
- Wikipedia: Preferred number
- Digikey: Resistors
- Wikipedia: Thin vs Thickfilm resistor
- In essence Thin = More expensive, but without nasty temperature coefficient.
Digikey ordering:
Value | Order number (digikey.com) |
---|---|
523 | RR12P523DCT-ND |
1.05K | RG20P1.05KBCT-ND |
2.1K | RR12P2.1KDCT-ND |
4.22K | RR12P4.22KBCT-ND |
Value-vs-Amplitude Originaly[edit]
The original 4000Ω 2000Ω 1000Ω 560Ω resistors, Gives this curve for the desired vs actual amplitude:
Note the dent in the middle of the curve!
Value | Amplitude [V] |
---|---|
0 | 0.000002 |
1 | 0.060736 |
2 | 0.119277 |
3 | 0.175740 |
4 | 0.230233 |
5 | 0.282857 |
6 | 0.333708 |
7 | 0.382873 |
8 | 0.389764 |
9 | 0.437103 |
10 | 0.482927 |
11 | 0.527307 |
12 | 0.570310 |
13 | 0.612000 |
14 | 0.652436 |
15 | 0.691673 |
Value-vs-Amplitude Modified[edit]
The modification with 4220Ω 2100Ω 1050Ω 523Ω resistors, Gives this curve for the desired vs actual amplitude:
Value | Amplitude [V] |
---|---|
0 | 0.000002 |
1 | 0.057625 |
2 | 0.113793 |
3 | 0.167545 |
4 | 0.220000 |
5 | 0.270256 |
6 | 0.319355 |
7 | 0.366446 |
8 | 0.413880 |
9 | 0.458053 |
10 | 0.501297 |
11 | 0.542855 |
12 | 0.583575 |
13 | 0.622742 |
14 | 0.661153 |
15 | 0.698131 |
Gnuplot[edit]
Gnuplot command:
gnuplot> plot "dac_sim.txt" using ($1):($2) smooth csplines
Simulation script[edit]
Simulation program (perl):
#!/usr/bin/perl # # Created: 2007-08-23 05:25.32 # Purpose: Minimig Video D/A resistor ladder simulation #----------------------------------------------------------------------------- $map[$n++]=0; $map[$n++]=8; $map[$n++]=4; $map[$n++]=12; $map[$n++]=2; $map[$n++]=10; $map[$n++]=6; $map[$n++]=14; $map[$n++]=1; $map[$n++]=9; $map[$n++]=5; $map[$n++]=13; $map[$n++]=3; $map[$n++]=11; $map[$n++]=7; $map[$n++]=15; for($v=0; $v<=15; $v++) { $d=$map[$v]; $d3 = ($d & 8)?1:0; $d2 = ($d & 4)?1:0; $d1 = ($d & 2)?1:0; $d0 = ($d & 1)?1:0; if( !($d3==0 && $d2==0 && $d1==0 && $d0==0) ) { $r_dac=1/( # ${d3}/4000+ ${d2}/2000+ ${d1}/1000+ ${d0}/560 ${d3}/4220+ ${d2}/2100+ ${d1}/1050+ ${d0}/523 ); } else { $r_dac=100*1000*1000; } $u = (75*3.3)/($r_dac+75); printf("%d %f\n",$v,$u); } #-----------------------------------------------------------------------------