Multiplexing Thermocouples.

Mevlut Aydogdu

Other Parts Discussed in Thread: ADS1114

In my desing, there is 16 thermocouples. The TC's have common constantan wire and other wires is made from copper. I am useing 16bir ADC ADS1114 of TI.
My probelem is multiplexing the TC. What kind of multiplexer i must use. I tested 74HC4051 standart and low cost multiplexer and it is works good on the desktop but what will hapen in the long therm.

Is there more reliable multiplexer? or can i use the 4051?

Mevlüt AYDOĞDU

over 13 years ago

0 Collin Wells over 13 years ago

TI__Guru 62055 points

Hello Mevlut,

This sounds like an interesting design. I'm not an expert on analog MUX's but I can likely help if I have a little more information.

1.) Are the 16 thermocouple outputs buffered or un-buffered before they go to the MUX?

2.) What is the typical output level (voltage) of the signal before it goes to the MUX?

3.) What is the temperature range that the MUX would be operated in? (15-40C), (-40 - 125C), (0 - 80C)?

Below is a link to some of the TI analog MUX's that I'm familiar with. The first link is for parts with with a signal +5V and less, and the second link is for parts for signals +12V and less.

http://focus.ti.com/paramsearch/docs/parametricsearch.tsp?family=analog&familyId=520&uiTemplateId=NODE_STRY_PGE_T

http://focus.ti.com/paramsearch/docs/parametricsearch.tsp?family=analog&familyId=1509&uiTemplateId=NODE_STRY_PGE_T

If you can help with some of the questions asked above I may be able to aid you in your selection.

Regards,
Collin Wells
Precision Analog
Linear Applications

0 Mevlut Aydogdu over 13 years ago in reply to Collin Wells

Prodigy 20 points

Hi Collin

1- The thermocouples outputs un-buffered before go to the Multiplexer.

2- The voltage level is the point what i worry about it. Level between -5mV and + 5mv. With your ADC, i can read the analog signal 8uV steps.

3- Oparations temparature is -25C - 80C.

Thanks.

0 Collin Wells over 13 years ago in reply to Mevlut Aydogdu

TI__Guru 62055 points

^{Hello Mevlut,}

^{Sorry for the delay getting back with you on this.}

^{From what I found, only three of TI's analog MUX's will accept negative signals with a single-supply power-supply. These are below:}

^{TS5A22362
TS5A22364
TS5A22366}

^{All of the other analog MUX IC's that I found only accept analog input voltages from GND to the VCC voltage of the part. Since the TC signal range is -5mV to +5mV you will need a MUX that accepts negative signals.}

^{Therefore, either use a very large combination of the SPDT MUXes listed above or consider using an op-amp to level-shift the signal before sending it into the '4051 or other MUX.}

^{Out of TI's newer series of analog MUXes(TSxAxxxx), the TS3A5017 (2x SP4T) , may be the best fit. The '4051 has significantly higher ON resistance and poorer cross-talk specs, but would save parts because it is an 8:1 MUX.}

^{If you have higher voltage power supplies available you may also want to look at the MPC506 (16:1 MUX). It is an old part and doesn't have great performance compared to the newer series of MUXes, but it is the only part we have that can achieve the 16:1 MUX in a single package.}

^{Hope this helps.}

^{Regards,
Collin Wells
Precision Analog}

0 Bob Burdon over 12 years ago in reply to Collin Wells

Prodigy 10 points

Gentlemen, forgive my intrusion.

I am an inventor who has found a new form of geometrical lattice. I am wondering if the above arrangement would work as a thermopile. The lattice appears to be the same as STM images of graphite. I wish to build a prototype and am seeking effective methods of joining the whole area of the hexagons to the constantan sheets for maximum contact area.

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Multiplexing Thermocouples.