I am trying to simulate a 4-bit DAC circuit using a R-2R resistor ladder. I am using a 741 op-amp. I selected the R value as 10k and calculated the value of Rf using the equation given on this page – http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/dac.html
The values were calculated with a 5V input and I am trying to obtain -10V output for a digital input of 1111. The circuit I have is shown below.
DAC Circuit http://postimg.com/173000/dac-circuit-172789.jpg
The R values are 10K
2R – 20K
Feedback resistor – 21.33K
These are the values I keep getting:
0000 – 0V output
0001 – 1.33V output
0010 – 2V output
0011 – 2.66V output
0100 – 3.05V output
0101 – 3.88V output
and so on.
As you can see there is no linearity with the output voltages. Could someone tell me what I'm doing wrong?
Additional Link to Image if it doesn't show above – http://postimg.com/image/173000/dac-circuit-172789.jpg
Best Answer
The problem is that your bit inputs must be at either a logical 1 (Vcc) or a logical zero, (GND) as they would be if they were connected to the outputs of, say, a counter like an HC191.
You've shown them floating when the switch is OFF, which won't work properly if you're using an R-2R ladder.
This works, and here are the LTspice files so you can play with the circuit if you want to.