# Help with a 4 bit R-2R DAC

Discussion in 'General Electronics Chat' started by tom7891, Dec 18, 2013.

1. ### tom7891 Thread Starter New Member

May 16, 2011
26
0
Hi, i've been asked to build a 4 bit R-2R digital to analogue converter and i need some help. I've built up the circuit, i just need some clarification. Am i right in thinking that the output for the DAC, considering each bit has a 5v digital input, should be (5/16)+(5/8)+(5/4)+(5/2)?
The reason i'm asking is because i get these correct values without a vcc connected to the op amp(0.3115, 0.627, 1.252 and 2.502) because of the R-2R network, so how can the circuit be working?
For clarification here is the circuit i have built:
The part on the left is just to get a digital 5v on each input.
http://i215.photobucket.com/albums/cc25/mercury7891/R-2RDAC.png
Thanks for any help

2. ### eeabe Member

Nov 30, 2013
59
9
I'm assuming this is just a simulation, and you haven't actually built this. The volt meters on each OR gate output are indicating 5V even though you have no 5V power source in the circuit, so the simulation is offering power somewhere, maybe hidden. Maybe it powers the op-amp by default as well. You obviously need to power your op-amps and logic gates in real life. As far as the R2R DAC, that seems correct to me.

Last edited: Dec 18, 2013
3. ### tom7891 Thread Starter New Member

May 16, 2011
26
0
Yeah it's just a simulation on multisim. You're probably right, I just found it weird when I put a volt meter just before the op amp and I got the same voltage as the output of it, as if the op amp isn't even working :/

4. ### tshuck Well-Known Member

Oct 18, 2012
3,531
675
That is a good thing for a voltage follower (meaning the op amp is working). Attaching pretty much anything to the resistive network you have will load it down and distort your outputs. The voltage follower is there to provide a high input impedance (to not load the DAC too much) while maintaining the input voltage on the output...

5. ### tom7891 Thread Starter New Member

May 16, 2011
26
0
Thanks for the help