EEGR6437, Mixed Signal IC Design Project 2

Note:

Please form groups of 2 or 3 for this project (ideally the same group for final project). Note: students in 6000 sections should group together.

Overview

The objective of the project is to design a simple R2R ladder Digital to Analog Converter (DAC). The R2R DAC you are given is not the best design! (It has many problems.)

IN NO CASE may code or files be exchanged between groups, and each group must answer the questions themselves, NO COPYING!

Turn in all requested plots ( Px ). Place answers to all questions ( Qx ) on a separate sheet of paper, with questions consequitively numbered. Keep answers to questions only 2 or 3 sentences long, or 2 or 3 lines of computation, in most cases. 2 or 3 sentences should suffice for most explanations.

Only turn in requested plots ( Pxx ) and requested answers to questions ( Qxx ).

Part 1

BEFORE DOING ANYTHING!!

This project is done in AMI 0.5 micron, so you must run the proper startup scripts. See the instructions on the course website.

Copy the schematic for the DAC (digital-to-analog converter) circuit from ~/tpweldon/pub/mixsigxx/project2 . Make sure the circuit is loaded properly, and is loadable by Accusim.

Apply the following 3 sets of DC forces as inputs, and view the outfile (it may be helpful to use AC Frequency analysis mode in accusim, with DC forces.).

Set (with accusim AddForce) D0, D1, and D2 to 5 Volts DC (logic 1). Since you are doing AC analysis, you must set one of the inputs as an AC source/force with the desired DC value. ( Dont worry about the AC analysis because you never use it, you just want to view the DC results in the outfile). Print out the schematic, view the spice output file (Results:Outfile) and write (by hand) all DC currents and voltages on the schematic. for measuring the DC currents. Turn this in. Take particular note of the currents in R2, R4, R6, and R7. ( P1 )

Print out another copy of the schematic, and write (by hand) all DC currents and voltages on the schematic for the case D0 and D1 at 5 Volts (logic 1) and D2 at 0 Volts. Turn this in. ( P2 )

Print out another copy of the schematic, and write (by hand) all DC currents and voltages on the schematic for the case D0, D1, and D2 at 0 Volts (logic 0). Turn this in. ( P3 )

Do M4 and M13 form a current mirror?

Ideally, M1 and M3 would be identical switches as in Fig. 29.5. Since M1 and M3 are different types (PMOS/NMOS) they may not have the same voltage drops from source to drain, and may have different backgate (or bulk/substrate) effect. Look at the differences in K for P and N on the MOSIS website for Wafer test results on the last run AMI 0.5u. Turn in the values of K in uA/V^2 for P and N FET's as given in MOSIS data for the latest run for AMI 0.5 micron.( Q1 )

Part 2

Do a time domain Accusim simulation and apply appropriate digital inputs (D0-D2) to ramp the DAC through all 8 states, starting at logic 111 and decreasing until logic 000, as shown below. The voltage steps at the DAC output should occur every 6 nanoseconds, and the duration of the simulation should be 100 ns, as illustrated below. Use 0.1 nS rise and fall times on the edges of the input waveforms.

Plot the input voltages (D0, D1, D2 ) and output voltages and turn this in. ( P4 )

What are the output voltages at D2,D1,D0 = logic 000 and 111? (Use the cursor in Accusim to measure voltages accurately) ( Q2 )

Make a table of the 8 output voltages for all possible inputs (logic level inputs, i.e., 000, 001, 010, etc.), AND measure the voltage step sizes from one level to the next. Turn this in. ( Q3 )

Are the stepsizes uniform? ( Q4 )

What is the +/- accuracy of the DAC in LSB, assuming that:

LSB = (Vmaxout - Vminout)/(8 - 1)? ( Q5 )

Tabulate the 3 stepsizes, 000 to 001, 000 to 010, and 000 to 100, and include the ratio of each stepsize to the next smaller one. (Ideally, the ratio should be 2.) Turn in this table. ( Q6 )

What is causing the "glitches" in the output? ( Q7 )
Search the web for "de-glitchers" for DACs , not for ADCs. Analog Devices, Inc. is usually a good place to look. Explain the underlying mechanism, in terms of the circuit, that causes these glitches. (Hint: do the glitches tend to occur at particular bit transitions and are they larger at particular transitions and is the timing perfect, and what added circuits might prevent glitches? Are there timing problems?)

Part 3

Do a time domain Accusim simulation and apply appropriate digital inputs (D0-D2) to step the the DAC from D2,D1,D0 = logic 000 to logic 100, and back to 000. This is exercising the MSB of the DAC. The steps should last for at least 10 nanoseconds, and you must use 0.1 ns risetime. Use a small enough time window (20 ns simulation stop time) to clearly see the rise/fall time of the output pulse. For this simulation:

Plot the input voltages (D0, D1, D2) and output voltages and turn this in. ( P5 )

What is the rise/fall time of the DAC to within 1/2 LSB of final value?
(also, note any time delay in addition to rise/fall time of the edges) ( Q8 )

Based on the previous question, what would you estimate the maximum DAC speed to be? (samples/second)
( Q8b )

Use the create-sdl method as mentioned in the AMI 0.5 micron notes to do the following layout. (Run IC Station, and use the "Create-SDL" pallette command, and select the SDL viewpoint as the schematic source.) DO NOT USE OTHER METHODS PREVIOUSLY GIVEN IN THE MANUALS OR PROJECTS. Place all of your components before you place your ports. Create-sdl is the ONLY WAY to place your ports. You must initially create the cell with "Create-SDL", you cant start some other way and hope to "clean it up." It is hopeless and you will lose all of your layout ...

Do a VERY ROUGH layout of the DAC (basically just pace the parts, then place the ports, and autoroute and compress them). Do not get bogged down, just work quickly to get a rough estimate of size. (If you have extra time try using DRC and cleaning up the layout for practice.) Just make sure all the parts get placed, and do one pass of autoroute. Place the layout in a padframe and turn this in. This part of the project is just to estimate the rough size of the DAC, as you would if it was being used in some larger design.
also, before placing ports, unselect everything in the schematic, then click place-port. (or select one port at a time and place it, but dont select ports plus schematic parts at the same time )

Turn in your layout, inside a padframe so the size can be judged. (For this quick layout, ont worry if the pads dont place and you only get corner pads and green lines) ( P6 )

Part 4

In this final part, you will go through the COMPLETE procedure of layout for a ridiculously simple cascade of 3 inverters.

Create a schematic of a 3-inverter AMI 0.5 micron cascade, where the first inverter is minimum size, and each successive stage is twice as big. (This is how you would design a buffer to drive a pin. DO NOT FORGET TO DO THIS IN YOUR FINAL PROJECT!)

Exactly follow the recipe given in the AMI 0.5 micron notes to do a full layout with schematics, SDL-layout, LVS, and DRC on the whole chip, INCLUDING PADFRAME. DO NOT USE OTHER METHODS PREVIOUSLY GIVEN IN THE MANUALS OR PROJECTS. Create-sdl is the ONLY WAY to place your ports. You must begin with this, you cant start some other way and hope to "clean it up." It is hopeless and you will lose all of your layout ...

any deviation from the known recipe for success almost guarantees failure ... so use the exact procedure as given in the AMI 0.5 notes

Note, you MUST add filler. Filler is necessary in sub-micron process because they polish the substrate between steps of fabrication. The filler prevents "low-spots" or polishing through to the next layer.

To save time, I have a top-level schematic for the 40 pins of a chip in ~/tpweldon/pub/mixsigxx/project2 . It looks like this:

For the 3-inverter layout, turn in:

Top-level schematic with pads.( P7 )

Top-level layout with pads.( P8 )

Main schematic (without pads).( P9 )

Symbol for your main schematic.( P10 )

Layout corresponding to your main schematic.( P11 )

Part 5

There are other intersting examples on the system.

Look at the 1.2 micron DAC schematic and layout. (Under "Speceal" parts in the SDL libraries menu of design architect.)

How many bits is the DAC?( Q9 )

What is the size of the DAC in microns? (Be careful, .... are you using 0.5 or 1.2 micron startup script.) ( Q10 )

Look at the diffamp1u05 ami 0.5 diffamp. In the layout, spot the ground ring that solidly contacts the substrate all the way around the amplifier.

How many ground contacts are inthe ground ring?( Q11 )

Is there filler in the amp?( Q12 )

What is the size of the amp (microns x microns)( Q13 )

Look at the capacitors available (near the easyfets) and see how they are constructed in the layout.

What layers comprise the top and bottom plates?( Q14 )

How many square microns is the area of the AMI 0.5 micron, .125 pF capacitor?( Q15 )

Report

Write any comments or observations you may have directly on the printouts. Type or clearly handwrite. Do not add extraneous pages or put explanations on separate pages unless specifically directed to do so. The instructor will read extraneous pages!

Turn in a separate sheet with answers to all of the specific questions above.

Write a cover sheet plus a 1-page double-spaced summary of the project. Stick to discussion of the results you observed. Include any problems or unresolved issues in the summary.

Only turn in requested plots ( Pxx ) and requested answers to questions ( Qxx ).