January 24, 2021
— updated February 2, 2022
In building an ultrasensitive instrument, we’re always fighting to improve our signal-to-noise ratio (SNR). The SNR is the ratio of signal power to noise power in the measurement bandwidth, and is limited by noise in the instrument itself and the noise of any background signals, such as the shot noise of the background light or the slight hiss of a microphone.
If the signal is weak, it will have proportionally more noise, so that the apparatus has to be designed to get rid of as much noise as possible. There are a number of ways to do this. The best is to get more signal or reduce the noise, for instance by increasing the laser power and using a laser noise canceller, but eventually we hit a practical limit. At that point, we’re left with several options, all of which boil down to filtering in one form or another.
Read more…October 29, 2020
— updated January 20, 2022
Thermoelectric (Peltier) Coolers
A thermoelectric cooler is a solid-state device made from two alumina ceramic plates with an array of metallized pillars in between. The pillars are also ceramic–they’re made of alternating p-type and n-type bismuth telluride (Bi2Te) semiconductors, alloyed with antimony telluride (p-type) or bismuth selenide (n-type), and connected in series electrically. The Peltier effect makes them electric-powered solid state heat pumps. (Thermocouples work the other way round, via the Seebeck effect, but the physics is the same.)
Read more…July 14, 2020
— updated January 20, 2022
James Elbert (Jim) Thompson was a well-known chip designer who used to be a regular on sci.electronics.design. He last posted in July 2018. As he was very sick at the time, we presume that he has died, but no obituary has so far turned up. He was born on February 29th, 1940, and used to say that he was looking forward to his 21st birthday in 2024.
Jim had a consulting company, Analog Innovations LLC, and a website, http://www.analog-innovations.com. You can find it on web.archive.org, but those copies are incomplete. A complete archive (minus a bit of javascript) is here at
Read more…February 23, 2020
— updated January 20, 2022
InGaAs heterojunction FETs are magic parts—fast, strong, and extremely quiet. They’re also called pseudomorphic high electron-mobility transistors (pHEMTs), because they use a 2D quantum well to to force the conduction electrons to move in a plane without much scattering. My fave Avago ATF38143 pHEMT was discontinued, but luckily Mini-Circuits stepped into the breach with their very nice SAV-551+ and its siblings, which are similar enough that the ATF SPICE model can be hacked up to work with them. (RF companies like Mini-Circuits never seem to supply SPICE models for some reason.) In one post on the ‘purpose of precision’ thread on sci.electronics.design, I noted that the Avago ATF38143 model I had posted awhile back predicted way, way too much low frequency noise. The real pHEMTs tend to have a pretty accurately 1/f PSD with corner frequencies between 10 and 50 MHz and flatband noise of around 0.3 nV/√Hz, about 10 dB quieter than the best JFETs, as well as being 20 times faster.
Read more…January 30, 2020
— updated November 21, 2022
At EOI, we’ve been building advanced instruments for a long time. One reason for our success is our large inventory of working designs, and another is the way we go about doing it. This post walks through a typical sort of development plan for a challenging customer requirement, in the form of a hypothetical email proposal outline for a fibre-coupled noninvasive glucose sensor similar to the one we did in 2013.
(You can also read about a recent project that went a lot like this, except with a single prototype stage.)
Read more…