Just ta bit of temperature, solder, and iron

as interesting.

Minding your temperature differential is just some of the P's and Q's you watch when you get to 16 or 24 bit measurements. Fortunately almost everything I need to measure is -- or can easily be -- AC coupled; so far I have dodged that bullet.

as interesting.

Thermal islands so both sides see the same thermocouple voltages, cancelled out. The best AC coupled chopper amps are not going to ignore DC thermal inputs when layout care not taken -- same materials and component count in series to inputs, so they stay balanced?

Most thermocouples need cold junction compensation anyway (the high temp ones are sometimes not concerned about the ambient variations ;)

Grant.

I've been doing some work for a startup in New Mexico that is making miniature, ultrasensitive seismometers and other low frequency sensors--needing ppb stability down to the 100-microhertz neighbourhood, in hostile environments. It's a pretty challenging business--their main technical guy is very sharp, and he needs to be. In my cube down there I have a walnut-and-brass plaque that reads,

DC: The Final Frontier.

Which it really is--a whole lot of things just get very hard below a kilohertz or so. Random temperature drift gives you noise that goes roughly as 1/f**2, which dominates the 1/f noise at low frequencies almost no matter what you do. Slow unidirectional drifts due e.g. to materials creeping and to outgassed crud gradually condensing on the optics give you a ramp that you can compensate for, but also have a lot of variability.

If you can find a way to chop the sample, you can turn the DC into AC, which is a big help if you do it right. For example, you can use a flip coil to measure the geomagnetic field--when you invert the coil, the field effectively goes from +0.5 G to -0.5 G, which is easy to measure.

If you do it wrong, you translate all the DC junk up to your modulation frequency as well--for instance, if the flip coil holder is slightly magnetic.

Cheers

Phil Hobbs

(My other main work just now is making thermal infrared antennas, so my current circuits projects span the range from 0.0001 Hz to 40 THz. Consulting is frequently crazy but rarely boring.)

On a sunny day (Fri, 15 Oct 2010 15:36:19 -0700) it happened Tim Wescott wrote in :

as interesting.

This is with copper-versus 60/40: ftp://panteltje.com/pub/copper_solder_thermocouple_img_2352.jpg ftp://panteltje.com/pub/copper_solder_thermocouple_2_img_2354.jpg

Heated the joint up a bit with a lighter.