Monthly Archives: January 2008

Motorola AN-587: on the design of an op-amp current source


It is true that I hold a high regard for current sources due to their limitless applications in the biosciences. Current sources are at the heart of electroplating systems for electrode manufacture, pilule stimulation of tissue and imaging. This is only a small part of the reason that I find this 1973 application note so appealing. The most important message, to me, is the complete walk-through from the basic governing equations based on ideal op-amps, to non-ideal characteristics, to error propagation.

In this case, the current source becomes less important than the design process. One of the subtle issues that can be seen from the equations is that resistor matching can degrade circuit accuracy just as much as the op-amp quality. For this reason, it may be beneficial to spend an $0.05 on more accurate resistors and save $1.00 on an op-amp. The converse is also true: you can spend an extra $1.00 on a more precise op-amp to save $0.05 on passive components. The standard error from your complete circuit may end up being comparable in the two cases.

( an587-d.pdf )

Sony Semiconductor Reliability Handbook


I have been looking at semiconductor reliability handbooks (provided by most semiconductor manufacturers) and have found that Sony has one that is put together very well. Their overview of the reliability problem is on par with textbooks and features nice illustrations. From an integrated circuit design standpoint, medicine the second chapter was the most interesting as it outlined many of the failure mechanisms that occur in modern ICs. This provides insight into design constraints and criteria which can provide higher yield and over-all reliability. The other three chapters are also interesting, seek however, buy cialis they focus more on a the process engineering approach.

Why was I looking at these handbooks? I recently read an anonymous insider interview, meaning it might be false, regarding failure rates of Microsoft XBOX360 game consoles. Some individuals put average failure rates at around 20-30% which cannot be acceptable for consumer devices. The insider puts most of the blame on poor thermal management leading to mainboard warping, which then leads to solder joint failures (covered in the Sony handbook). Poor design and haste lead to these problems, and in the end, the incurred losses may have justified pushing back the initial deadline.

( qrhb.pdf )

HOWTO: compile and use custom OpenWRT images on your fonera (from start to end)


For what it is worth, the La Fonera is still one of the better deals on basic embedded systems on the internet. I have looked at it before and shelved it for quite some time until I needed it again for a prank (open wifi, http redirection, etc). The available documentation, at the time of writing, is a bit spotty but one can gather enough information to build and test firmware based on the OpenWRT project. This guide will hopefully illustrate the complete process from the very start to actually running the custom firmware.

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Audio application notes


Although Wolfson Microelectronics produces some fine integrated circuits, their application note section is somewhat out of the way and doesn’t like to be linked to directly. This didn’t stop me from looking around and finding some potentially useful app notes:

A.C. Coupling Capacitor Selection

Recommended Output Filters for Wolfson Audio DACs 

Class D Headphone Filter Component Selection

Issues When Grounding D.C. Coupled Headphone Outputs

The main reason that I was looking there in the first place was that I was getting excessive noise when coupling a portable audio player to an audio system I am working on and couldn’t figure out why. When I took everything apart, I found that output stage of the audio device was being pulled up to a higher voltage than expected by the coupling on the input stage and thereby biasing the input stage of the audio amplifier incorrectly. After some careful circuit modifications, the signal integrity was returned with fairly good low frequency response. At this point, my audio circuit experience is still minimal, I hope to post some designs once I get something worthwhile going.

IC Friday: TI’s ADS831


Today we have the ADS830 (8bit, help 80MSPS) analog to digital converter from Texas Instruments. This is a pipelined ADC so it is likely that each of the vertical sections in the center (there are six) probably contains a two-bit flash ADC, DAC and appropriate amplifiers for taking the difference and gain after each stage. It is likely that there is a single voltage reference location that supplies all of the comparators. Given that this is an 8bit ADC that incorporates some sort of digital error correction, and there are six 2bit stages, there may be some stage redundancy to allow for higher certainty of the lower bits.



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Broken hotlink code


Phil, stuff from, diagnosis has let me know that many of the links on here are broken for Camino browser on OSX and resulted in him seeing a hotlink message. I am guessing that this was a problem for other people as well. I would like to apologize for the broken code and the resulting poor browsing experience. I have disabled the hotlink code and will likely keep it off until I start getting close to my bandwidth quota. Thanks again for the information.

IC Friday: TI’s THS4601


As promised, I am posting analog circuits yet again. This time we have the THS4601: a wide-band FET-input op-amp. For clarity, I imaged the whole circuit under 400x total magnification resulting in a large file, which is why two 40x objective stitch files are provided. I like this circuit because it makes it possible to have some understanding of the underlying IC schematics. The last image includes labels for the pads and some of the components, so I will refer to these sections in a short discussion. Hopefully these things will make up for the lack of last week’s IC Friday.

The first thing to notice about this chip is, although it has FET inputs, the active load on the input side (right side) is made up of BJTs. This is to improve the bandwidth of the device as BJTs keep providing high effective load values with increasing frequency where a FETs gm might degrade. Furthermore, the connection pattern for the eight devices is such that complementary devices from each input chain are next to each other. This is to keep good load matching in the face of manufacturing differences with the understanding that adjacent devices will more likely undergo the same process as devices at some distance.

Large BJTs are again used to generate a voltage reference, likely to drive a current mirror. The size of these devices is large to, yet again, desensitize the chip to slight manufacturing differences across the die. Over all, this chip uses a fairly large process so any time a metal layer makes electrical contact with a polysilicon layer, a large number of small vias are used. These vias can be viewed as the small pits that litter many of the metal traces. This is done to prevent metal from forming cracks and entering the polysilicon beyond the specified depth due to strain associated with deposition and bonding.

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