ADN8831

If you use two thermistors you might be best off providing en external opamp. The ADN8831 isn't geared to do that.

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Regards, Joerg

http://www.analogconsultants.com

The ADN8831 is designed to control a TEC in a rather simple-minded fashion.

The Linear Technology LTC1923 chip does the same job, and the Jim Williams application note AN-89 is pretty good.

It isn't as good as the comment I published in Rev. Sci, Instrum. in

2004 - volume 75 pages 788-89, but you've have to e-mail me (at billdotslomanatieeedotorg - my e-mail address is real) if you wanted a copy of the pre-print.

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Bill Sloman, Nijmegen

There is no significant bias current. Pin2 is non-inverting and Pin3 is the inverting input of the first stage. They have similar input characteristics, but still cannot be exchanged, or you have it latching to the rail. I also quite don't understand what you want to do with the second thermistor. Isn't it already built into the Laser-diode or Tec assembly? A second thermistor might measure some other temp, but it cannot be used to regulate the tec. The second opamp stage is hard wired to the controller and is used to filter the control voltage. Read about the non-linearity in thermistors and how to linearize it.

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ciao Ban
Apricale, Italy

is the

A second thermistor can be useful to monitor the temperature at the exhaust side of the TEC. The amount of heat per unit current transferred across a Peltier junction depends heavily on the temperature difference across it, and this can have a dramatic influence on the gain around the thermostatic feedback loop.

Jim Williams Linear Technology application note AN89

discusses this in qualitative way, but doesn't include the quadratic equation relating heat transfer in watts, current through the junction in Amps and the temperature difference across the junction in degrees Kelvin which allows you to deal with this in practice. As mentioned above, I have got the equation into the literature - most recently in

2004 in Rev. Sci, Instrum. - volume 75 pages 788-89.

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Bill Sloman, Nijmegen

But do you use the second sensor to change the gain/dynamic settings of the PID-control loop, so the ripple of the temperature is minimized or the polarity change avoided? This seems to be rather a uP job than those simple controller chips.

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ciao Ban
Apricale, Italy

second sensor to change the gain/dynamic settings of the

We did use a uP to change the gain around the PID loop to keep it fairly close to critically damped over a wide range of temperature differences across the Peltier junction.

It is something that you could theoretically do with an analog multiplier or two, but I've not heard of anyone doing ti that way. The simple controller chips can't manage this, but aren't usually used in circumstances where it is a problem.

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Bill Sloman, Nijmegen

sensor to change the gain/dynamic settings of the

The larger Cypress PSoC might come in handy. You can program the behavior of some of the analog sections on the fly. Should certainly be fast enough in TEC applications.

--
Regards, Joerg

http://www.analogconsultants.com