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

Peter JakackiPeter Jakacki Posts: 6,239
edited August 2011 in Propeller 1 Vote Up0Vote Down
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


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