analog input SIMATIC S7-200, EM235

hi,

i need some advice, so help me if someone can!

i use analog input AIW0 from analog module EM235. Signal on the input AIW0 is 0(4) ... 20 mA (0...32000), but that signal is unstable! when on the calibrator mA(simulator of mA), which is connected in the analog input AIW0, is set 0 mA, 4 mA or 20 mA on the Status Chart, in the program STEP 7, i see that analog value is unstable. Example:"when on the calibrator mA is set 20 mA (32000), in the Status Charts that analog value is changing within 31998 ... 32018" . or, that is ok!? Calibrator mA is surely O.K., brand new.
Because of that instability, i can't really correctly calibrate analog value with offset (0) and gain (32000).
if You have some more advices about analog inputs and outputs, please send.
thank You very much!

sarac

Hi Sarac

A couple of questions first, have you shorted out the inputs to the other unused channels, check the manual Appendices / Analog Expansion modules on how to do it.

Second, assuming you are using Micro/WIN, what setting have you got in the 'System Block' / 'Analog Input Filters'. You can vary the Averaging and Deadband, if you are getting sudden steps between 31998 and 32018 you probably need to reduce the Deadband setting.

The an observation:
In the real world if you were measuring temperature with a maximum of 100 degrees, using your figures, the output would vary between 99.993 and 100.056 degrees. I don't know about your process but a 0.063 degree variation wouldn't even be noticed on the machines I design.

So in answer to your question unless a very high degree of accuracy is needed I wouldn't worry about it.

Bryan