ENGR338 Lab 2022 Fall
Lab 2 Building R-2R Ladder DAC
Mason Brady
Email: mrbrady1@fortlewis.edu

Designing an R-2R DAC

Introduction: This lab uses a new program, electric, to layout a schematic and then integrate the schematic with LTSpice in order to simulate the created circuit.

Materials: LTSpice, Electric VLSI

Methods: First, electric VLSI was installed and LTSpice was integrated into their simulation. A sample DAC was then copied into electric VLSI and a sample simulation was run confirming everything was working properly.

After everything was confirmed to be working in order, 10k n-well resistors and off page connectors were used to create an R-2R resistor ladder. To simulate the ladder, the aformentioned simulation file was used and the sample DAC was replaced with the R-2R DAC that we created. A simulation was run using LTSpice to confirm the DAC was working properly.

For the final task, every pin except for the B9 pin was grounded and the output was connected to ground via a 10pf resistor. The hand calculations were completed using Therevenin's law and the formula 0.7RC. The simulation was then run to confirm the results of the hand calculation.


Figure 1. Simulation results from sample DAC confirming that Electric VLSI and LTSpice are working as intended and setup properly.

Figure 2. R_2R_Ladder of custom made layout, I struggled to get the sizing right but it worked.

Task1 Voltage
Figure 3. LTSpice simulation result, it is clear this is a less than ideal DAC since the step sizes aren't consistent but it does work as intended converting the values properly.

Task1 Current
Fiugre 4.
LTSpice timing simulation result. The voltage reaches the 50% threshold after 72ns

Fiugre 5.
Hand calculations for Task 3

Discussion: I found this lab much easier than last week, it is becoming much easier to do calculations on circuits and my understanding is growing. It is interesting how R-2R circuits work and how the math works out to divide the voltage in half at each stage. I find all the calculations much more straight forward now than they felt last week.