μblog: engineering from the trenches

This weeks entry is the 20-bit delta-sigma DAC from TI.

dac1220-10x-stitch.jpg

del.icio.us | digg

Today’s entry is the LMC6042 dual-op amp from National Semiconductor. The layout of this device is a bit different from a standard op-amp due to the inclusion of some periodic components. It can also be noted that this die is mostly the same as the LMC6044 except that the two op-amps on the bottom-right side of the chip don’t have most of the metal deposited.

lmc6042-10x-stitch.jpg

del.icio.us | digg

This weeks entry is the INA2143 differential amplifier from Texas Instruments. This is a dual diff-amp and the partitioning is pretty apparent. It is also apparent that most of the device is constructed using a bi-polar process, which matches well with the decreased quiescent current with increased temperature in the datasheet.

ina2143-10x-stitch.jpg

del.icio.us | digg

While looking through Make Magazine’s blog, I noted a link to the Great Internet Migratory Box of Electronics Junk project and quickly realized that it would be a great opportunity for me to off-load some of my accumulated electronics junk treasures before I move to a different place in State College, PA. The rules are pretty simple: add some components if you keep some components; document your box on your site/blog or flickr; choose an individual who wants a box; send it off after two weeks! I am going with a theme of microcontroller related treasures so I will include the following contents:

• P16PRO40 PIC programmer, parallel port version
• Set of PIC16F84A chips and a pair of crystals (10MHz, 4MHz)
• 2×16 LCD display
• My personal Atmega 8051 board with battery-powered NVRAM, RS232 and built-in PAULMON
• A (UNTESTED!!, may break something) USB-PIC board I built that needs to be programmed
• An original Nintendo Entertainment System cartridge that I modified to accept standard flash chips
• A power/eject button assembly from a Sony Playstation 2
• A numeric keypad
• A rotary potentiometer with a digital readout
• A mouse from a Digital Equipment Corporation VAXStation
• Assorted passive components
• Assorted logic gates
• Solderless breadboard
• 5V DC wall adapter

I will pick someone from the list of individuals on the Box Request page, so add yourself there if you are interested. Also, keep in mind that this may be a good opportunity to do some spring-cleaning yourself.

See: the microbox!

del.icio.us | digg

Today’s IC Friday features the DDC114 from TI. This is a quad ADC, which makes sense given the partitioning of the device. I think a set of sample-and-hold circuits can also be identified.

ddc114-10x-stitch.jpg

del.icio.us | digg

When looking at the application notes section of Fujitsu’s site, I came across their FRAM memory guide book. I was surprised as I did not know what FRAM really was and so I flipped throug. Basically, a film deposition process was developed, which is compatible with standard CMOS processing, that introduces films who can maintain their polarization after the applied electric field is removed. We are all familiar with ferromagnetic devices, these are the pieces of metal that can be magnetized when placed in a constant magnetic field. Thanks to some nice electromagnetic research, we can do something similar with thin films and thereby create ferroelectric capacitors that are capable of retaining data without applied power while being as fast as SDRAM. It is clear that half of the Fujitsu guide is a sales pitch for their ICs, however, the other good is a fairly good introduction to the FRAM technologies. The basic technology is discussed along with some typical ferroelectric substrates. A reference list is also attached.

( mn05-00009-5e.pdf )

del.icio.us | digg

This weeks images are of the LMC6442 dual op-amp from National Semiconductor. The devices organization makes it somewhat easy to read with the top and bottom halves dedicated to the two op-amps. Looking at the bottom half, the two inputs are at the bottom center and the op-amp output pin is in the bottom right corner. The circular structures that look like BJTs near the in/out pins are not for driving, but for ESD protection. It looks like the main differential transistor pairs are above the input pins around the center of the chip. The left portion of the device looks to be biasing/current-source circuitry while the right side two-three gain stages. Two metal capacitors can also be seen per op-amp which are likely used for frequency compensation.

I am going to try to focus the next few weeks on more analog ICs as well as an ADC and DAC, these will hopefully be easier to “read” and will give some insights into IC layout.

LMC6442-20x-stitch.jpg

del.icio.us | digg

One of the circuits that I am working on has an optically-isolated sub-circuit which runs on batteries. This is an analog device where the non-isolated input uses LOC110 optocouplers to send some program voltages to the isolated side which then drives a load accordingly and sends back the applied voltage and current back to the non-isolated side. One of the problems with the initial design is maintaining proper signal gain and offset across the optocoupler while the batteries on the isolated side drain.

The LOC110 is a nice device that has an infra-red LED that is driven and a matched pair of photo-resistive elements so that one can be used to control the LED and thereby set the current through the second device. One solution to the problem above was to use a pair of matched voltage references and track the current through the photo-resistive elements while driven at the reference voltage. This proved to be mostly successful, however, it did entail some research regarding voltage reference design.

The most simple voltage reference design involves only one diode operating in a controlled reverse breakdown mode. The exact reverse breakdown voltage depends on the device design (see this post about breakdown mechanisms), however, the general idea is that the current through the device increases very quickly when a certain reverse voltage bias is achieved. One only has to put a resistor in series with the reverse-biased diode that will ensure that the reverse-breakdown criteria are met given the power supply and a voltage reference is born. The devoted fan of μblog will surely exclaim: But Nick! Didn’t you use a reverse-biased diode as a temperature sensor once? What good is a voltage reference that varies with temperature? I would agree completely and exclaim that datasheets for voltage references display the equivalent circuit model of a diode in reverse breakdown, however, the actual designs are a bit more complicated.

This application note from Analog Devices provides some good insights into designing single-technology voltage references. The basic idea in many cases is to create a differential voltage by building mismatched devices or by introducing resistances, such as R3 above, and then using an amplifier to generate a reference voltage. To maintain good stability over a temperature range, the amplifier can be designed to increase gain in order to compensate for the beta degradation at higher temperatures. It is still possible to do all of this using a bi-polar process by designing biasing current sources that change appropriately with temperature. It is also possible, although more difficult, to introduce FET devices which can have opposite thermal effects as compared to BJTs. In this case, it would be fairly straight forward to increase the gain of the difference amplifier through increased FET trans-conductances with increased temperatures.

In the end, I still have some drift and offset problems that are not associated with temperature, however, the design process has been a good introduction to and has developed an appreciation for voltage reference designs.

del.icio.us | digg

The chip on today’s IC Friday is Sony’s CXM 4000, an unknown IC inside the PlayStation 2 game console. Nothing too surprising so I tried to capture as many of the processing layers as possible in a series of images.

Regarding the spring cleaning giveaway, all of the chips finally got sent out yesterday and will hopefully start arriving at their destinations soon. Any oily residue on the slides is just microscope oil which was used to limit light scattering at higher magnifications. This can be washed off with soap and water. The writing is typically with a sharpie marker and can be washed off using alcohol.

cxm4000-4x-stitch.jpg

del.icio.us | digg

Like all component manufacturers, Analog Devices is eager to get their devices into the hands of engineers and into products on the market. They seem to want to push their instrumentation amplifier section so they are offering a free, printed copy of their in-amp design guide. The product can be selected from Analog’s Technical Bookstore which offers the book for free (two day shipping included). Although I mainly use Burr-Brown chips (from TI), I am open to using Analog’s devices as long as they offer some advantages. I am particularly looking for low-power in-amps to replace the INA2332s that I use as a certain batch seems to be prone to latch-up in the presence of a noisy DC power source. I will update this post when I get the book (on 5/21) so you can either order it now or wait until next week to see if it is really worth the time.

del.icio.us | digg