Ad595 and K type thermocouple oscillation problems

Hi , I am making a data acqusation system with 20 K type thermocouples.my connection are are thmermocouple>maleconnector>female cooncteyor>sheldedwire of 15-20 cm >wire end soldered on pcb>muxdg407>ad595cq>muxDG406>adc ad574>89C52>pc . I have got the prolem of fluctions/oscillations.when only one thermocouple is connected i get small data varation of 2-3 oC(continuous oscillations :'() but if 2 thermocouples are connected the tmep get out of control to 200 to 500 .even at room temp.also when i put my thermocouple in hot water. I have earth the the body of thermocouple and grounded pin-1 of ad595.if i donot ground it my output goes to zero even if thermocouple body is earthed. plz if any one can suggest a solution. or tell me what and where the probem is.Thanks :'( :-/. I spent a lot of time on this but no use i really need help thanks.....

It looks like the problem might be due to lack of decoupling on the supplies. see:- http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html

Hi,
Thanks a lot 4 ur responce . I have deouples all my supplies. I think the problem is due to lack of isolation among parallel thermocouple channels that are connected to same mux. the controllel read all the 20 channels continuously one by one . can u suggest any method of how to isolate the channels or how to reduce this cross talk and interchannel noise :-[ . plz if u have any suggestion . thanks

Thermocouples cannot be wired with random wire types, connector types, etc. Each type of thermocouple is a very specific pair of two different metal wires. If your wires/connectors aren't matched to the thermocouple metal type, you are introducing noise. Thermocouple outputs are small signals, so even a little noise is very bad.

I suspect to make things work correctly for 20 thermocouples, you will either a) have to learn a fair amount of electrical engineering to multiplex them, or b) use 20 AD595s, one for each t-couple.

-j

when only one thermocouple is connected i get small data varation of 2-3 oC(continuous oscillations

That is a decoupling problem, if you have some decoupling and it doesn't help then you have not got enough. It is not just simply a matter of putting the capacitors on they have to be the right type and in the right place.

Where is the oscillations taking place? i sit on the input of the A/D or the multiplexer. Incidentally in the chain:-

thmermocouple>maleconnector>female cooncteyor>sheldedwire of 15-20 cm >wire end soldered on pcb>muxdg407>ad595cq>muxDG406>adc ad574>89C52>pc

I can't see an Arduino.

What is the muxdg407, this is happening ahead for the ad595cq I am not convinced you can do that.

dg407 = 8 channel analog mux. On the THERMOCOUPLE INPUT? hmmm.. the 595 needed to be "attached" to the thermocouple!

In my opinion, (and mikes too) the analog switch "WILL" change the characteristics of the VERY small signal he thermocouple produces.

Have you tested with the DG407 switch IN and OUT Of the circuit path of the AD595?

Pete

Here is an application note from Analog on the AD595.

http://www.analog.com/static/imported-files/application_notes/34661261AN369.pdf

The last page has a description of how to multiplex inputs and compensate for the copper junctions.

I did a PID controller for my smoker and was going to try this but I chickened out and used two AD595's. :)

Hi. all thanks a lot 4 ur suggestions . I am using shielded thermocouple wire and green colour k type connectors but still the problem is there. I wana show u ppl my sechematic but it is not being attached here. I wonder why?. U can see it on the link http://www.electro-tech-online.com/electronic-projects-design-ideas-reviews/92778-thermocouple-conditioning-fluctuations.html#post763112 The microcontroller send adress to select mux channels one by one and then send the data value of all TC's to pc serially. If i directely connect a TC to ad595 the noise problem is solved by connecting 8 TC to one ad595 through a mux is creating problem. Plz if u ppl can suggest some thing. the green connnectors are mounted on the box and the card is attached inside the box.