4-20 ma measurement and PT100 connection

[mesuty]

HI ;
Does anybody have some experience on
a ) measuring 4-20 ma via analog input of PIC 877
b ) measuring temperature via PT 100.

thanks.

[rnielsen]

I know that if you put a 250 ohm resistor in series with the 4-20ma signal it will give you a 1V-5V signal that you can measure with the analog inputs.

Ronald

[Ttelmah]

For reasonable accuracy though, you'd probably want to use a 2.5v reference, and I'd suggest 120 ohms. Many '4-20mA' interfaces, use an overcurrent as an 'error' indicator (typically 21mA), so designing to handle this is useful, as is being able to detect current below 4mA (which is normally taken as a 'link failed' indication. Remember to include clamping to trap spikes on the line, and you may need to consider opto-isolation (look at the Siemens application notes for the IL300 optocoupler for a way of handling this). HP also do a coupler that will directly substitute in this circuit, and is more repeatable, removing the need to 'tweak'.
Generally however it is more common for the output to be isolated, and the input to be 0v relative.
Remember that few circuits will be accurate, so being able to calibrate your 'concept' of the zero and full scale values may be worthwhile (on the instruments I have built with this interface, you can select a mode that shows the input in 'percent' from 0 to 100, corresponding to 4-20mA, and either adjust the source to agree with your measurement, or feed a value in, and tell the unit this is the 'zero' point, and then another current and tell the system this is the 100% value. These adjustments are stored in EEPROM.
You will also find quite a few systems using 0-16mA, and so the ability to adjust your concept of the zero point, and full scale, allows these systems to also be used.
For higher accuracy, you can offset the zero point using an OP-amp, to use the entire range of the ADC. Whether this is worthwhile depends on the actual accuracy available, and considerations like detecting the 'link failed' situation...

Best Wishes