Measuring Ambient Temperature

[MostlyCarbon]

Hi there,

I'm working on a project, essentially a humidity and ambient temperature sensor. I'd like it to run on batteries and the standard LM35 needs 4V. I'd like something that can run on 3V and will work all the way down to (preferably) 2V.

I'm wondering about the suitability of an IC like this: http://datasheets.maxim-ic.com/en/ds/MAX6607-MAX6608.pdf

How does an IC in the SOT23 vs. TO packages ability to measure the ambient temperature vary? The SOT will get more of the heat from the board... which should be negligible?

Any other tips to getting a good ambient temperature reading?

Thanks,
MostlyCarbon

[ivan747]

For me the LM35 is a PITA for digital electronics. It is analog so if you are using a DC-DC converter you should use some heavy filtering. Depending on the board layout, the DC-DC converter might introduce some heat to the system. I like the TO-220 version, it can be mounted on a big surface like a metal chassis. If you are using this with a digital circuit please do yourself a favor and get a humidity sensor IC with integrated temperatur sensor. Go for something that supports I2C, SPI or at least RS-232.

[MostlyCarbon]

Fortunately the Humidity sensor section at DigiKey is small (53 options).

There is also this thingy from Parallax:
http://www.parallax.com/Store/Sensors/TemperatureHumidity/tabid/174/CategoryID/49/List/0/SortField/0/Level/a/ProductID/94/Default.aspx

It does a nice digital output of both temperature and RH (relative humidity) but it's also $35 a pop.  Almost double the cost of implementing them separately.

I plan on using a PIC18 to do the A/D and then ZigBee the info to the master. I'll have multiple units.

Mainly I'm just interested in ensuring that the temperature info accurately reflect the ambient rather than the internal/pcb temperature. And the layout/design issues that I should consider to help me get the best accuracy I can (or at least avoid overly in-accurate results).

As it turns out the lowest Vs I could find in a RH sensor is 2.7v. So I'd like to use a Temperature sensor with Vs minimum ~ 2.7v. 

MostlyCarbon

[ivan747]

Go for the TO-220 or something that sticks out of the board. You may want to have the hot parts away and do some sort of thermally resistive path for the ground plane. I can't recommend any thecniques because I am not into real EMI stuff yet. (If you have a high speed circuit with RF components, then you have to be careful about the layout).