At 07:58 AM 3/25/99 +0200, you wrote:
>      Hi all,
>    I am looking for simple ( low cost) circuit to multiplex 4 (four)
>thermocouples
>  to one input amplifier.
>Any tips or hints for the same would be helpful as well.
>Thanks in advance.
>Romeo Dimitrov
>

The lowest cost solution would be to use a CD4066 quad analog switch to do
the multiplexing - this would take 4 PIC pins to control. A more expensive
(and lower offset/leakage) addressable analog switch would only need 2 pins
for control. Check the Maxim and Linear Tech websites (www.maxim.com and
www.linear-tech.com) for their analog switches.

For the amplifier you need low input offset voltage and drift because of
the low microvolts/degC output from a thermocouple - I would suggest a
Linear Technology LTC1049 chopper-stabilized amp. This costs $4 in the
Digi-Key catalog (www.digikey.com). The LTC1049 datasheet shows an
application for a thermocouple amplifier; you can download this from the
LTC website.

The app note shows an LT1025A being used for cold junction compensation --
however, probably the simplest way to do this is to measure the temperature
of the board using a separate temp sensor IC and compensate in the
calculations. This would take an extra analog channel but might not need an
extra amplifier depending on the temp sensor output voltage.

Maris