RTD and Thermocouple problem?

[ud23]

HI all i have some problem now i am using the some reference design for temperature measurement.I attached the signal conditioning ciruit part which i am using for rtd and thermocouple.

this is working fine in different input connector(screw terminal) ,no I want to modified it with single input connector,means user can connect both RTD and Thermocouple in same connector.

As of now if i connect that way Thermocouple voltage divider resistor comes in a RTD case and it will change the RTD resistance.

I am using rtd PT100 and K.j type thermocouple.

 

[FvM]

re: RTD and thermocouple problem?

Not clear, what your actual signal condition circuit is, but you'll usually would need analog switches or a switchable current source to change the functionality of the temperature input

 

[ud23]

re: RTD and thermocouple problem?

is there any circuit or voltage divider then op amp stage which can be use for common pin connection for thermocouple and RTD type sensors?

 

[FvM]

re: RTD and thermocouple problem?

You'll need at least a switchable current source for RTD mode. To operate the RTD at a higher signal level than the thermocouple (less offset errors and noise influence), you'll also would want a variable gain amplifier.

 

[ud23]

re: RTD and thermocouple problem?

here circuit i am working on now i tested with it thermocouple and pt 100 rtd its working fine but i want to add also pt 1000 with same signal circuit.I need to change the value of 2.2k(R1) resistor for pt 1000 rtd but i dont know how i can do it?


https://obrazki.elektroda.pl/7139088800_1366010829.jpg

 

[FvM]

re: RTD and thermocouple problem?

What's the exact problem? You'll either need switchable series resistor/current source or switchable gain to support both RTD variants.

 

[fouwad]

re: RTD and thermocouple problem?

hi, i am experiencing the same problem, i want to ask whether i can use adg732 analog multiplexer/demultiplexer to demultiplex 1mA from my single current source to 32 RTDs? Please do reply

 

[FvM]

1 mA is no problem as ADG732 is rated for continuous switch current of 30 mA.

I presume you budget a separate MUX for current source and voltage sense? Otherwise the temperature accuracy would be seriously reduced, even with a low ohmic switch like ADG732. On the other hand, with separate MUX, you don't necessarily need very low switch resistance, even 100 ohm range won't be a problem.

 

[fouwad]

first of all thanks for your reply,
and yes i am using 2 seperate MUX for current source and voltage sense and shorting the 3rd wire of all the 32 RTDs with ground,
i tried to demultiplex 1mA current using ADG732 to its 32 channels (ofcourse 1 channel active at a time) but it was unable to do so, while voltage was demultiplexed on the same ADG732 ; i am unable to understand this phenomenon, please let me know if you think of a way out

 

[FvM]

I don't see a problem with ADG732 properties (provided you are keeping the permitted voltage range). I guess it's an implementation fault.

I have implemented similar temperature measurement frontends with regular CD4000 and 74HC analog multiplexers.

 

[fouwad]

i have seen CD4067 doing the same job that i want to implement but ADG732 divided the 1mA along its channels while i was selecting only 1 channel, ill try to post schematic and detailed attachment from my work pc