Attempt at R2R DAC switching

Yeah the results are weird.

Uh... in R2R, aren't you meant to use just 2 resistor values, one for the series chain and the other going to the switches?

Not necessarily, as long as the series resistor and the switch resistor have the ratio R-2R it should be ok... I have seen example circuits that have a similar configuration to what I have done.... anyway I am probably gonna go breadboard this circuit...

The relais with transistors are not needed and the R2R network you also missunderstand. Check this were I took for R 10kOhm and for 2xR 20kOhm http://everycircuit.com/circuit/6107860747681792

I understand what you are explaining and it makes sense. However your circuit only caters for a single 5V reference which you cannot adjust in the circuit without adjusting the gain of the Op-Amp... I used the relays cause I do not have logic selectors that switch between GND and VREF. I have seen circuits that use the 8, 4, 2, 1 multiplier for the resistors and so I decided to give it a try as well. To say I don't understand the circuit is incorrect as my problem is not on the logic side it's with the way the reference voltage is wired up to the resistor network... I have changed it back to the single R-2R config.

You're driving the FETs wrong. For R2R to work, at any given moment one FET of each pair should be on. As it is, you're leaving some inputs to the resistor chain floating, which is not how R2R is meant to work. Also, EC's defaults for FETs give them a rather high resistance when driven at 5V/0V. It's quite likely you'll want to increase the width of the FETs or increase your resistor values.

Maybe 3-4% of 20k isn't that bad. http://everycircuit.com/circuit/5635090486132736