Viewing posts for the category SED

Temperature Measurement is Hard

Measuring temperature is surprisingly subtle.  There are lots of sensors out there; Digikey sells thermistor sensors interchangeable to +- 0.1 C from several vendors for about $3 in onesies.  IC sensors tout good accuracy and linearity, and come in both analogue and digital versions for way under a buck.  So what's the issue?

The issue is: temperature sensors measure the temperature of the sensor, whereas what we want is the temperature of something else: air, fluid, or some solid object we're trying to control.  So the problem is to get the sensor temperature to track the temperature we actually care about.   IC sensors are especially bad, because they have stout leads made of copper (400 W/m/K thermal conductivity) and small packages made of plastic (0.1 W/m/K).  Thus they basically measure the temperature of their leads, and are horrible at measuring air temperature, for instance.

National Semiconductor used to put out a Temperature Measurements Handbook, but since TI bought them, it seems to have disappeared from the web, but here's the  2007 edition. Not much has changed about the properties of plastic and metal since then, so it's still very current.

Decap Photo of Terabeam APD Photoreceiver

Decap picture of a Terabeam CD3109 APD/TIA module, taken with a lens glued to a cell
phone camera

Thermal Runaway Found Useful

This odd circuit  is an on-chip temperature balancer that uses thermal runaway to force all the transistors on an array to run at constant temperature. BJT dissipation goes up at low temperature, with very high gain.  Here's its step response.

Sine Wave Generation with TANH Wave Shaper

Sine wave generation is a perennial problem.

Direct-digital synthesis (DDS) uses a bunch of counters, lookup tables, and DACs,
but that's a relatively heavyweight solution that doesn't fit all problems.
BJT differential pairs naturally have a hyperbolic tangent (tanh) characteristic, which can be used to round off a triangle wave into a very passable sine.  I'm not old enough to have invented this technique, but here are a couple of illustrations of how it works: TANH Sine Wave Shaper (PDF) and TANH Sine Wave Shaper (Mathcad).

Care and Feeding of Tantalum Capacitors

Solid tantalum capacitors have a lot of advantages: very low inductance in surface mount packages, ESR low but not so low that your LDO regulators start oscillating; and good capacitance per unit volume.  Unfortunately they're also prone to burn up when mistreated, which makes many engineers wary of them.  This war story, entitled What a Cap-astrophe! talks about how to treat them properly, and how a bit of TLC after soldering can restore their full performance.