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Hardware Design input

 
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rwskinner



Joined: 08 Dec 2006
Posts: 125
Location: Texas

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Hardware Design input
PostPosted: Fri Mar 23, 2007 6:21 am     Reply with quote
I have to build a small industrial board for our own use which will allow several 4 to 20ma sensors and 24 volt logic DIO.

I generally take the 4-20ma and run thru a precision 250ohm resistor to get 1-5 volts and not any other filtering. Does anyone have any suggestions for a decent input conditioning circuit for the 4-20 inputs that is relatively simple and reliable other than what I'm doing. I'm using 12 bit adc and trying to insure I don't get to much noise of flutter. I'll most likely use a max 12 bit 8 channel ADC.

DIO, I plan on using optos with the high side on 24 volts and use the low side for the inputs. Grounding the input turns on the opto. Inadvertantly connecting 24 volts to the opto should hurt it. Instead of using all the pic's IO for this, I'll most likely use some P2S shift registers like the 165's.
165 input would be pulled low, the opto if on would pull the input High.
Neutone



Joined: 08 Sep 2003
Posts: 839
Location: Houston

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Re: Hardware Design input
PostPosted: Fri Mar 23, 2007 8:48 am     Reply with quote
rwskinner wrote:
I generally take the 4-20ma and run thru a precision 250ohm resistor to get 1-5 volts and not any other filtering.

Use a resistor that is temperature and time stable. Precision is less of an issue as you can calibrate the inputs. I would expect you are going to put one side of the load resistor to ground. If you connect the other side to the analog input through a resistor and place a capacitor across the analog input and ground you will have a nice RC filter. This will help protect the input and keep the input stable. The RC time constant should be matched to your sampling rate and input resistance.

rwskinner wrote:
I'll most likely use a max 12 bit 8 channel ADC.

With filtering the PIC can read 12 bits. You could skip the external chip.

rwskinner wrote:

Instead of using all the pic's IO for this, I'll most likely use some P2S shift registers like the 165's.

You could chose a PIC with enough IO.
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