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Regarding "Earthing of isolated circuits" in measurement applications like temperature measurement using thermocouple.

Question asked by PrateekJain-28889 on Oct 4, 2014

Respected Sir,

I am on the verge of completion of an 8-channel analog input module design. Each channel can measure RTD Pt-100, Thermocouple (J,K,R,S,T,B,N),0-10VDC and 0-20mA input types. A 24VDC external supply can power this module. I have used an isolated DC-DC convertor to  provide +-12VDC to my entire circuitry, which galvanically isolates my circuit from external 24VDC supply. All 8- Channels share the same circuit isolated ground reference, there is no channel to channel isolation though, but as mentioned above they are isolated from external 24V supply. All channels are configured in differential mode. I am using transil diodes protection on inputs as well and i am using low pass first order filters as well at each channel input. A 8-channnel diffferential mux (ADG1207) connects the channel to In-amp and the output of In-amp goes to the ADC (ADuC834 microcontroller with on chip 24bit sigma-delta ADC).

After having told you the story what remains is "EARTHING". I have never ever made provision for an earth connection in my previous designs as well. But i have read quite a bit about it. But i don't know significance of earthing.If it is essential how to implement it? I have seen isolated circuits where a Capacitor 1nF/1kV approx connected between "ISOLATED CIRCUIT REFERENCE AND EARTH". Documents related to ACROMAG say that proper earthing in measuring instruments reduce the effect of noise as well. But i have not seen Analog Devices CFTL do that. Can you please throw some light on this.

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