Re: Input resolution ?

"Thomas" <EvilTool> wrote in message news:<[EMAIL PROTECTED]>...
> Can someone explain in layman's terms with a mix of technical terms the
> meaning of Input resolution regarding a building HVAC DDC control system.
>  I am in the process of doing a functional performance test on some new
> Digital controllers and am a bit confused on this subject. Some of sensors
> are Thermistors and some are using 4 to 20 MA . Any help, advice, or links
> would be appreciated.


There are 4 parameters you need to worry about.

1 Resolution.  A 13 bit plus signe digital to analog convertor divides
a 0-10v singnl into 8192 steps.  (ie about 1.25mV).  If measuring a
load cell mass in the range 0-100kg you could resolve down to +/-12.5
gramms.

2 Analog Linearity.

All that accuracy will do you not necessarily be of use you if the
analog linerity of your front end is not up to it.

An ordinary  Siemens 14 bit A to D card for an S7 has a linearity of
1.0% with 0.3% losses due to temperature coefficient, linearity and
hysteriesis etc.
(Its generaly much much better than that but that is what they quote)

So if the card is used to measure a weight in the 0-100Kg range it
could be of by +/-1kg. Due to the high resolution it would however
detect a 12.5 gram change in weight!

A high accuracy siemens analog card has a linearity of 0.05% to 0.1%
instead of 1% and it has 15 or 16 bit resolution.

The high accuracy cards cost about 60% more.

If you are trying to control a process: eg dosing or saving of
material you may need the higher accuracy.  In an oild refinery saving
0.5% material can be a lot.