8 bit R-2R

[Andrew Cowan]

Any ideas why this doesn't work?

I was expecting SW1 to make it 2.5V (which it does), SW2 to make it 1.25V, SW3 to make it 0.625V etc.

I am sure I am missing something obvious.

Andrew

 

[moxhamj]

Ah, I think I can see a problem. The inputs on your circuit are either High or Open Circuit. In a R/2R network the inputs must be High or Low.

 

[moxhamj]

Also, just note that the most significant bit on your circuit is on the right, so binary numbers read the opposite. In your first example the actual number you are putting in is 00000001 and if all the other inputs are connected to low, the output will be 1/256 of 5V.

I've built these with 10k/20k networks; they have a fairly high impedence which is probably ok for DMMs but would not be ok for 20k/volt analog meters. I've used ca3140s as buffers, and you do need to supply that chip with more than 5V (6-9V) to get the full 0-5V span.

 

[Andrew Cowan]

Thanks for pointing that out.

Andrew