Multiple Thermistor Inputs

[pacr_284376]

Hi,

   

I am sure I saw a document/example for measuring multiple thermistor inputs, using (digital) output signals to multiplex between the channels. Can someone link me to the place I saw this ?

   

Theoreticly I know how to do this, but I remember seeing a whitepaper/application note which showed some advantages/disadvantages using this method. 

   

My goal is to make a 16-channel thermistor measurement system.

   

Thanks

   

Patrick

[ETRO_SSN583]

This should help -

               

http://www.cypress.com/?rID=49052     AN66477

           

Regards, Dana.

[ETRO_SSN583]

And this user module, but for PSOC 1 -

           

          http://www.cypress.com/?rID=72585

       

Regards, Dana.

[pacr_284376]

Thanks for the answer. I was thinking more about something like the attached diagram.

   

Using one reference resistor and 16 analog inputs that are pulled to GND one-by-one. 

[ETRO_SSN583]

You are adding mux Rdson to Ntc value, so you would have to

   

do an error analysis to see if performance meets your goals.

   

If you use ohmmeter tool you would be able to see delta values

   

between various thermistor paths, a contributor to chan-chan error.

       

Regards, Dana.

[pacr_284376]

@Dana

   

That is exactly what I have in mind. That is why I hoped to find the application note i found it in.  

   

I hope there is a way to switch the analog input from these pins in such a way that I can measure the analog voltage OUTSIDE of the digital output. That is also why I use digital outputs because they have a lower output resistance than analog outputs.

   

Attached - better version of my diagram. I use the Output Enable to switch between channels

   

[ETRO_SSN583]

Looking at GPIO specs, Vddio 3.3, 3 mA sink, Vol  <= .4 V. So inferred

   

Rdson <= 13.3 ohms.

       

So thats an appreciable error in light of "typical" characteristics below.

   

And of course there is a strong T dependence of Rdson.

       

You could always dedicate a pin and using current source measure its

   

Rdson and subtract that out from reading. Analysis of all errors would be

   

prudent.

       

Food for thought. Regards, Dana.