Multiplexing Thermocouple Inputs - Page 1
Forum section: electronics / projects

author: eerocketman
author_rank: Contributor
timestamp: December 02, 2017, 08:03:04 pm
content: Hi everyone,I am currently designing a Thermocouple sensor board and I would like to minimize the number of amplifiers used in this circuit and so I would like to useVishay DG409LE:http://www.vishay.com/docs/78084/dg408le.pdfmultiplexer to feed into the AD8945:http://www.analog.com/en/products/amplifiers/specialty-amplifiers/thermocouple-interface-amplifiers/ad8495.html#product-overview.Do you guys see an issue with this design.Any input is appreciated.Thanks!Ali

author: jbb
author_rank: Super Contributor
timestamp: December 02, 2017, 08:40:41 pm
content: I'm not too experienced with precision MUX work, but I would be a little concerned about accidental thermocouples formed across the MUX. I guess that using a differential MUX and being meticulous about balanced layout would help. The leakage currents look nice and low.Also, ESD could be an issue: the +- 2.5kV ESD rating of the DG408 isn't very high, so you'd probably need to add a little more protection (e.g. TVS diode and small series resistors) if the thermocouples terminals are accessible to the user.If you want a minimum-effort, maximum-performance design, LT make some multichannel chips. They areexpensivebut the specs look really good (0.1 deg C) and they're highly integrated. I'm not sure about ESD behaviour. I haven't used them, but LT chips generally do what they say on the tin.LTC2986 (max 10 channels)LTC2983 (max 20 channels)

author: elal1862
author_rank: Newbie
timestamp: December 02, 2017, 08:46:10 pm
content: That's a less-than-great idea: The extra solder joints you're introducing into the signal path act like thermal junctions of their own, so thermal gradients across the board will introduce errors, just like leakage currents from the multiplexer does. And you're taking away the thermal coupling between the thermocouple wires and the AD8945's input pins, reducing the accuracy of the cold junction compensation even further (see page 13 of the datasheet).DL;DR: If you're willing to settle on ±5°C accuracy anyway, it may work for you.

author: eerocketman
author_rank: Contributor
timestamp: December 03, 2017, 12:40:46 am
content: How would I avoid thermal junctions on a PCB, I was thinking about bringing in the thermo-couple inputs with a through hole screw terminal connector.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 03, 2017, 09:11:10 am
content: Those CMOS multiplexers are quite good and generally not a big problem for thermocouples - just make sure there are no significant temperature gradients in that area. For leakage currents and power consumption it helps to use less than there maximum supply. One might even consider a DIP version in a socket, so be able to replace the MUX in case you got one with relatively high leakage or a damaged MUX.The bigger questions are protection and how to bring the wires to the board. If the special amplifier with internal cold junction compensation is used, the terminal kind of need to be on the board, close to the cold junction compensation and thus close to the AD8495. Getting the thermal coupling to the cold junction could be kind of tricky. An extra temperature sensor for the cold junction, a standard ADC and a µC to do the compensation numerical might be an option.Using thermocouple (or extension) wire to external plugs is still possible - the cold junction would be still on the board. Especially the kind of cheap looking yellow plastic ones with flat contacts are surprisingly good (low error).