switching input on DAQ

[TanisHalfElven]

Hi guys.
I'm building a system to measure temperatures from Thermocouples. I have a DAQ capable of a 100 channels (lets reserve 10 for overhead cuz my main daq is gonna be sending some input on to it).

However, i want to measure about 300-500 thermocouple points. I've considered a multiplexer based system, but i can't get myself to trust them because of the cross talk (i'll have to build it myself and i'm not and electrical guy so...).

I've considered using a 200 conductor flat twisted pair cable and just plugging and unplugging manually to do a scan.

Is there a better alternative. Something i could control electronically that would switch inputs (and be scalable) without manual intervention.

Thanks.

 

[uclabachelor]

However, i want to measure about 300-500 thermocouple points. I've considered a multiplexer based system, but i can't get myself to trust them because of the cross talk (i'll have to build it myself and i'm not and electrical guy so...).

Crosstalk across differential pairs (assuming you are sampling a differential signal from the thermocouple) appear as common mode at the receiver.

In other words, if careful layout considerations are excercised, crosstalk will not be an issue for the differential signals.

Also, the sampling rate will help determine whether you can use mechanical switches. If you're sampling just once a day, that's fine. If you're sampling every second, those relays wouldn't last long.

 

[Modelworks]

What is the voltage range that is being input to the DAC ? The problem with connecting a DAC over long distance is you will have to factor in the wire resistance and changes with temperature as room temp will effect the wire resistance. If I had to measure lots of inputs over a distance greater than a few inches from the DAC then I would look at using a 2 or 3 wire system to transfer the data from the DAC to the CPU doing the interpretation of the data. You can get some very good DAC chips that will output data on a 3wire bus, so you only have to run SDI,SDO,SCL back to the host cpu . Eliminates cross talk and you can put 1000 DAC on the one 3 wire bus to do as many readings as you want and they can be hundreds of meters from the host cpu.

Would look like :
thermocouple --- DAC -3wire conversion--------------------------> host cpu

 

[schenley101]

I would use a multiplexer. TCs are pretty immune to interference and cross talk. it sounds like use would only have to multiplex 4 or 5 together which isn't very much. Ive done systems where we multiplex 12 or 15 together. What DAQ are you using and how often do you need to read each temperature?