VGA/TV DAC using resistor arrays (and other uses)

A long long time ago, somewhere in the deep recesses of the Propeller forum I posted a diagram of the arrangement that I use to create the video DACs using resistor arrays. Theres are normally made up of discrete resistors of various values that "should" be binary multiples of each other. I find that smd resistor arrays are costing me about half a cent each and they pack 4 resistors of the same value in the same "space" that a single 0805 resistor takes. At the time I implemented my DACs for the TV or VGA using the same pins and just used 470R resnets which seems to work quite well.

Here's what they look like (look to the bottom of the RJ45 socket):

resnet dac.jpg

For the purists however I present a similar implementation using three 240R resnets plus a single (or two) 560R resnet(s). Here's a schematic:


So why do it this way then? Well, the advantage is that you can just use the one or two values of resnets rather than all the individual values of resistors. It's also very compact as you can see from the photo. Plus this is a true binary DAC as the values increase in powers of 2.

There is a slight mismatch on the TV DAC which can be easily corrected if you want by just using another 560R resnet to get 280R or just put in a 33R resistor in series.

Is there any other advantage? Yes, using resnets in place of discrete resistors means that you can stock less values of resistors and instead make up values from combinations of the resistors in the resnet in the same space. Here's an example where we use a 240R resnet to get these odd values:

1) 60R using 4 in parallel
2) 80R using 3 in parallel with 1 spare
3) 120R using 2 in parallel with 2 spare
4) 240R using single elements with 3 spare
5) 320R using 1 in series with 3 in parallel
6) 360R using 1 in series with 2 in parallel with 1 spare
7) 480R using 2 in series with 2 spare
8) 600R using 2 in series with 2 in parallel
9) 720R using 3 in series with 1 spare
10) 960R using all 4 in series

So you can imagine that with as little as 3 values of resnets that you can span a wide range of values. Digikey sells reels of 5,000 for $25, that's 20,000 resistors. So that's my little design tip for the day.
1024 x 700 - 101K
800 x 600 - 245K


  • 10 Comments sorted by Votes Date Added
  • edited August 2011 Posts: 4,426Vote Up0Vote Down
    Seeing nobody made any comments on this one I thought I would throw in a little bit more about using resnets in place of discrete resistors. I wrote a little program that spits out the values from all practical combinations not counting any spare elements left over. I chose values in the hundreds just to give an idea of the range of values obtainable. Did I leave any combinations out? Did I make any mistakes?

    Here are a few common values with uncommon results:
    (2S+2P means 2 in series plus in series with 2 in parallel)
    RESNET  100
     4P           25
     3P           33
     2P           50
     1S           100
     2S           200
     3S           300
     4S           400
     1S+2P        150
     1S+3P        133
     2S+2P        250
    RESNET  220
     4P           55
     3P           73
     2P           110
     1S           220
     2S           440
     3S           660
     4S           880
     1S+2P        330
     1S+3P        293
     2S+2P        550
    RESNET  240
     4P           60
     3P           80
     2P           120
     1S           240
     2S           480
     3S           720
     4S           960
     1S+2P        360
     1S+3P        320
     2S+2P        600
    RESNET  330
     4P           82
     3P           110
     2P           165
     1S           330
     2S           660
     3S           990
     4S           1320
     1S+2P        495
     1S+3P        440
     2S+2P        825
    RESNET  470
     4P           117
     3P           156
     2P           235
     1S           470
     2S           940
     3S           1410
     4S           1880
     1S+2P        705
     1S+3P        626
     2S+2P        1175
    RESNET  560
     4P           140
     3P           186
     2P           280
     1S           560
     2S           1120
     3S           1680
     4S           2240
     1S+2P        840
     1S+3P        746
     2S+2P        1400
    RESNET  680
     4P           170
     3P           226
     2P           340
     1S           680
     2S           1360
     3S           2040
     4S           2720
     1S+2P        1020
     1S+3P        906
     2S+2P        1700
  • edited August 2011 Posts: 0Vote Up0Vote Down
    Hi Peter

    Good work. Nice to have someone pre-think these kind of things through. I think Phil Pilgrim once posted some info about the 430R resnets used on the propeller backpack.

    I've been doing some work with 64 greyscale VGA, and one thing I noticed is the output resistance of the prop pin should be taken into account for small values of external R. I did some brief calcs and estimated the resistance at 27 ohms at 3v3, but haven't had a chance to characterise it properly (high/low logic level, Vdd supply voltage, temperature, shunt reactance, variation from prop to prop).

    The good news about the 27 ohms is some white and blue leds can be directly driven, albeit with not great intensity matching, temperature independence, etc
  • edited August 2011 Posts: 4,426Vote Up0Vote Down
    Thanks Tubular, I think my earlier posts predate even Phil's :)

    The 27ohms source resistance you are seeing is probably close to the mark but since this is the MOSFET channel resistance when on I don't think you can quite count on any particular value like we can with fixed 1% resistors!

    There is a trick to driving LEDs directly from a higher voltage that I have used in designs. Seeing that the LED has a forward voltage drop means you can drive your normal LEDs directly from the Prop I/O lines (through resistors) even if the LEDs are tied to +5V. So much more is the case for LEDs with higher voltage drops such as white and blue. If you have an LED with a 3V drop then you can certainly tie this to +5V and calculate your current limit based on 2V across the resistor.
  • edited August 2011 Posts: 0Vote Up0Vote Down
    Another advantage of using resnets in place of discrete resistors is the reduction of the thermal coefficient ratio. Being packaged together the resistors see the same temp changes. When using the resnets as voltage dividers, the output is much more stable.
  • edited August 2011 Posts: 0Vote Up0Vote Down
    Probably a silly question, but with the circuit in your schematic aren't you going to get crosstalk between HSync, Vsync and B via the TV output?
    Pay for your free software - let the developers know how much you appreciate their work!

    Links to Propeller stuff I've done (mostly composite video)
  • edited August 2011 Posts: 0Vote Up0Vote Down
    I was going to ask the same question as eric. I see it forms a voltage divider of 1120R : 240R which is probably ok.

    BTW from what I have asked, seems the 4th pin for TV is not used often, if at all. I have commandeered this pin for other uses.

    I have pondered the connection of VGA/TV using sommon resistors. I was wondering if the VGA would suffer if 270R & 540R were used instead of 240R & 470R, or alternately would TV suffer if 240R, 480R & 960R were used instead??? This would make a simpler network.

    The only issue with resnets is they are harder to hand solder due to the fine pitch. They are the most difficult part on my RamBlade.
    My Prop boards: CpuBlade, TriBlade, RamBlade,
    Prop Tools (Index)
    Emulators (Index) ZiCog (Z80)
    Prop OS (also see Sphinx, PropDos, PropCmd)
  • edited August 2011 Posts: 4,426Vote Up0Vote Down
    @ericball: Crosstalk? This circuit is used for either VGA or TV but not both at the same time. Any signals that appear to be cross-coupled are terminating back into low-impedance source pins (the Prop's I/O) and so are "swamped" from causing any interference. Sketch the paths (include the source/sink resistance of the Prop pins as well) and do the maths, you can see there is no problem.

    @Cluso99: The VGA resistors will only affect the luminance of each of the primary colors and there is enough difference from monitor to monitor anyway so it shouldn't matter. The series resistors in the sync lines aren't really necessary but does protect the Prop. I'm using 470R resnets exclusively for both the VGA and TV and I can tell you that it just works.
  • edited August 2011 Posts: 0Vote Up0Vote Down
    Thanks Peter.
    My Prop boards: CpuBlade, TriBlade, RamBlade,
    Prop Tools (Index)
    Emulators (Index) ZiCog (Z80)
    Prop OS (also see Sphinx, PropDos, PropCmd)
  • edited August 2011 Posts: 0Vote Up0Vote Down
    Pondering a little further...

    I can get this to 3 resnets, 2x 240R and 1x 470R. I think this would work. You will note I include an optional terminating resistor for TV of 191R-200R.


    I have marked 3 links, but I think they could be permanently connected without problems. Just select the correct output in software.
    702 x 376 - 33K
    My Prop boards: CpuBlade, TriBlade, RamBlade,
    Prop Tools (Index)
    Emulators (Index) ZiCog (Z80)
    Prop OS (also see Sphinx, PropDos, PropCmd)
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