Hi Everyone, first post and looking for a little advice or some recommendations.
I'm looking to do a thermocouple (tc) project that has to be able to handle types K, N or R. Each may be used from 0degC upto their maximum temps as shown in the NIST tc tables.(1300-1600degC) My limited background is in
68hc05 assembly but I'd like to transition to using C.If anyone is able to offer any opinions or recommendations regarding the following points I'd be most grateful...
(1) After a bit of reading I'm currently leaning toward either TI's MSP430F147 or Microchip's PIC18F2550 or close derivatives of either. Microchip have a tc reference design based on the '2550'
(2) The TI part is 16bit with a 12bit ADC. Good enough for the tc input. Not real happy with their leadtimes though...18+ weeks. I've even read some posts questioning the long term availability of the MSP430 1xxx series.
(3) The Microchip part is 8bit with a 10bit ADC. Suitable for the cold junction compensation input. It is much more readily available, has seemingly more community support and is a bit easier to play with being available in dip packaging. Microchip have a pretty good record for long term avaialability.
(4) The Microchip part will need external ADC (minimum 12bit) plus an amp for the front end. There's any number of suitable ADC's and AD have the AD8628 op amp with 1uV offset and input offset drift of 0.002 ?V/°C.
(5)I'd like to use the NIST polynomials to resolve temperature readings and my reading indicates I'll need 32bit floating point support to do that. I've seen numerous posts which use lookup tables to avoid the nist equations, but these are usually examples where only one tc type is in use, usually within a limited temperature range and sacrificing some accuracy. I need multiple tc types, pretty much full temp range and good accuracy. Are these nist equations THAT bad???
(6) I'd like to keep the cold junction compensation circuitry seperate because of the multiple tc type requirement. That way the software can handle the voltage differences between the various tc types and the pcb can be left more or less unchanged for whatever tc type is used.
(7) TI's Code Composer Studio vs Microchip's C18 or Hi-Tech C??? Floating point math support???
There may well be better choices than the above so please don't limit any possible suggestions just to these. The aim is to make an informed decision before laying out any major $$$.
Cheers,
Peter
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