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Frequently Asked Questions:Devices/ Design:What
is the difference between a custom and a semi-custom device? Simulation: What
is Monte Carlo analysis? Layout/ Breadboarding: How
do I install the commands for ICED into SIMetrix? Frequently Given Responses (Please excuse the in-process answers):What is the difference between a custom and a semi-custom device?A custom device or ASIC requires a full set of masks. Masks are like negatives that define where the circuitry goes. This means that for a full CMOS process, you need to buy a full set of masks, or about 16 different masks. For a 0.18 micron process, this can cost as much as $500,000. For a semi-custom, you only make masks for the metalization layers to customize the device. All other layers are already there. What is the normal design flow?Normally, a customer and DEI will colaborate on a specification for a chip. Then, schematic capture is done to capture a representation of what the circuitry will do. Simulations verify that the circuit will operate over temperature and process variations (this is the reason that DEI suggests the use of SIMetrix monte carlo analysis - to simulate process variations of separate components). Breadboarding of critical circuits will be completed in many instances of newer schematics or changes. Once this is complete and the circuit is operational, layout is performed by either DEI or yourself using ICED from IC Editors, Incorporated. A GDSII, which is basically a Gerber file for an IC, is created and sent to the fabrication facility to build a metal mask (one mask only). Ceramic prototypes are back within three weeks. What is Monte Carlo analysis?In Monte Carlo analysis, the parameters of components are varied independantly to simulate process variations. There are typically two types of process variations in semiconductors: process variations on a particular wafer and process variations from run to run. As an example, a resistor in the Bipolar Analog Arrays will vary in its initial tolerance by +/- 20%, however, any two resistors on the same die differ by less than 2%. The Design Manual also shows that the variation between several resistors ganged together will be less than 1% (between 10 resistors combined for one total resistance and 10 resistance combined for another resistance). Example: Suppose you have a resistor divider between two base resistors as shown (the value "1" means one 750 ohm resistor.
The nominal output at VOUT is 2.5V. The nominal output impedance is half of the base resistance or 375 ohms. The tolerance on the output resistance is +/- 20%, which results in a range from 300 ohms to 450 ohms (half of 600 ohms to 900 ohms)! The good news is that the ratio will vary by much less than that, or 2% matching. This gives an output voltage of 2.45 volts to 2.55 volts. If at least ten resistors are used to make up the two effective resistors, statistics will yield you a 1% matching, or a variation from 2.475 volts to 2.525 volts. To see this variation, you must run a Monte Carlo analysis in addition to your normal runs. Monte Carlo analysis will vary RB1 and RB2 separately, giving you an output something like the following. This will show you if your circuit will really work over process variations on a given die. Note that it depends on how many runs you do as to how close you will get to the actual variation. The circuit will typically operate closer to a least means square type of rule than linear variation.
How do I install SIMetrix support for the 700 Series Bipolar Analog Arrays?An applications note is coming soon. Information is also available in the Design Manual. For a preliminary copy of the applications note, please contact us. We can also provide applications support. How do I install the commands for ICED into SIMetrix?An applications note is coming soon. Information is also available in the Design Manual. For a preliminary copy of the applications note, please contact us. We can also provide applications support. How much digital circuitry can the 700 Series Bipolar Analog Arrays fit?The Bipolar Arrays can implement digital, but it is a small amount. An example is a NAND gate as shown below. This circuit requires three transistor sites (all three input transistors are in the same island) and 7 resistors. There could be a maximum of nine of these devices in one 712 array.
How high/ low is the input impedance?The input impedance can be set up to be either high or low. The high input impedance would be on the scale of 100 K-ohms (darlington pair). For the low input impedance application such as measuring a low current, the input impedance can be on the order of 500 ohms. To calculate input impedance, use the following: Input impedance = Rpi = Beta/gm, where gm = Ic /(kT/q) gm = Ic / (26 mV) at room temperature How low of a current can be measured with the 700 Series Arrays?We can measure very low currents if necessary (around 1 uA or lower). In fact, the lower the input current, the lower the operating current for a differential stage, and the higher you need for input impedance. In order to measure a high input current and use low power techniques, the designer can attenuate the input and run at lower currents. Which 700 Series Bipolar Analog Array fits my design?Getting a quick idea of which array to use is relatively easy. If you want, Device Engineering Incorporated will size the design for you and give you a price. We can also make an Excel Spreadsheet available with sizes of some common circuits from the Design Manual in it. This will allow you to make informed decisions. One thing that you must be aware of is the use of opamps. On a board, you use an opamp with an "infinite" gain because semiconductor companies can make these in volume and you can adjust the gain for your actual needs. In an IC, you build a "gain stage" with just the gain that you actually need. Since a gain stage is not a closed loop device, your chances of oscillations are greatly decreased. Also, an opamp takes between 14 and 20 transistor sites, while a differential gain stage alone will take about 5 transistor sites. So, for the fact of stability and size, it is usually better to use a gain stage in an IC instead of an opamp with a couple of resistors. Where do I get information on sample circuits and sizes?Example circuits and functions are available for download in a library format for SIMetrix and for other formats. Please see our download page if you want the actual schematics. A table is available in HTML format that will tell you how many components these circuits will use. The table of circuit examples contains some full circuits and some examples on how to acheive certain designs such as a Widlar current source. How do I install the tools?Please see the applications note TBD on this subject. If this appnote has not been published yet, contact DEI and ask for tech support on arrays. How can I find out what kind of performance is possible in the arrays?The best way to estimate performance is to do a simulation. After that, it is a good idea to do a breadboard with kit parts that are available free of charge from Device Engineering. Since respins are also easier than a full custom IC, your time can be spent where it is most important - system level design. Can I do a DAC in the arrays?Device Engineering is working on a 5-bit DAC. This is the most bits acheivable with 1% matching. A 6-bit DAC would require better than 1% matching, but we are trying some different techniques to make this happen. Is there added noise in the circuitry due to the architecture of the arrays?No. This is not a switched capacitor topology. The only noise sources are normal noise sources for electronics. What operating voltages do the arrays support?0.9V to 20V How can I make sure that my circuit will function in the arrays before I build my design?Simulation is a great tool, but it is often not enough. Device Engineering offers kit parts free of charge for prototyping circuits before they are manufactured. Please see the list of kit parts page. How long is the design cycle for a 700 Series Bipolar Analog Array?The design cycle is greatly reduced because of all of the circuits available as starting points. Also, since the arrays are a single level of metal, the time from when the layout is completed (GDSII generation which is like a Gerber File) to the time where ceramic prototypes are ready is three weeks. Plastic prototypes can be available in four weeks. What are the quick limitations of the arrays?Ah, the Achiles heel. The biggest limitation is the ability of the arrays to handle higher voltages. The breakdown of the arrays is 20 V, which means that you would do a supply of about 18V with a 10% tolerance as an almost maximum. The only device that has a breakdown below 20V is the capacitor, which has a breakdown of around 9 V. The arrays are capable of small amounts of digital, but as already mentioned, this is limited. Speeds are limited for design. The NPN has an ft of 800 MHz, so for a gain of 20 dB, you would need to be at least a decade in frequency away from this point (80 MHz). A semicustom device is not designed for all applications, but when it fits an application, it usually makes sense (and cents) since the NREs can be much lower. The decrease in cycle time for getting to production also decreases risk. What if I want to view other files from SIMetrix (such as other LVS files)?If you want to add functionaility into the SIMetrix pull-down menus, you can edit the icedmenu.txt file to add in a line. For instance, to be able to open up other LVS files, you can add in the line: DefMenu "Shell|&File|ICED (for LVS)|View Results\other" "let resname=splitpath(getfile('Select Result File To Edit\lvs'));if resname[0]='' then ;unlet resname;scriptabort;endif;shell {'Notepad.exe '&resname[0]&resname[1]&resname[2]&resname[3]}" "SchemOpen" This should go on all one line. What it does is:
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SIMetrix
is a product of Catena Software
Ltd; PSPICE is available
from Cadence; The 700 Series was created by Array
Design, Inc. |
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Engineering, Inc. 385 E. Alamo Dr. Chandler, AZ 85225
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