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ADS1118 used for Kiln Controller

Other Parts Discussed in Thread: ADS1118

Warning - Digital guy wandering into the dark side of Analog design...

I am considering the ADS1118 and a K-Type thermocouple for a glass annealing kiln I am designing. The temperature range could go from ambient to 1100° C. From the literature and YouTube tutorials I believe I understand the input network, and intend to use the Omega PCB to quick connect socket to improve the odds of getting a cleaner signal from the thermocouple. I have not seen specific designs using the ADS1118 that use the full temperature range of a K-Type thermocouple.

My question is one of optimal PCB layout. Is it recommended to use an opto-isolator on the SPI interface? What would be the most effective method of isolating the ADS1118 supply rail? I am not trying to over think the design, but rather start with the best layout possible to improve the odds of a useable design first pass. The only thing I remember from analog design is "Ground" is a place you grow roses.

Any help from the "Analog Gods" would be appreciated, even if it were an admonition to RTFM, as long as the requisite FM reference was given.

Cheers!

David W, Bennington VT

  • Hi David,

    Welcome to the forum!  One of the reasons you haven't seen a design or application note using the full range of the K-type TC is we have no good way of testing it.

    It is a good idea to use isolation.  This includes power isolation as well as the communication.  Depending on how this is all connected, the TC may be at a vastly different potential relative to your communication (microcontroller) supply which may damage the input to the ADS1118.  So one determining factor is how large is the possible difference in potential?  Here there are two concerns.  One is related to device protection and the other includes personal saftety requirements.

    You mentioned opto isolators.  These are commonly used.  Sometimes digital isolators will work as well, but they generally don't have as much isolation voltage range.  These would be the ISO devices from TI.

    For the supply isolation, transformers usually work well.  On one side (controller side) a modulated current runs through the transformer primary.  One the secondary side the voltage is created by rectifying the current and filtering.

    You should be able to find examples of this type of system if you internet search temperature transmitters or PLC systems.

    Best regards,

    Bob B