analog inputs

Hi,

We read analog inputs of 0-20mA by converting them to 0-5 volt to bring them to the plc. The A/D converter of the plc is using 10 bits.
The problem we have is that this is fluctuating a lot when we monitor this input of the plc. In real, when we put a multimeter (read out by 0,00 mV, we only have fluctuations at the digits who are indicating the values less than 1 mV. The mV themselbves are realy stable in real. As we need to use this to read out a weigher, we need to find a solution to avoid that we have those fluctuations in the plc (by using some kind of formula or supplementary hardware filter).Does anybody have experience with this?

Well, first of all you should provide more information.
Are you sure you are looking into right input (fluctuations
are not unusual on floating inputs). Can't you setup
little test circuit to provide known current to input (5, 10
and 15mA for example)? Battery and wirewound potentiometer
will do. Put one multimter in series with battery to confirm
current. Use another meter to check voltage output of the
converter. See if it scales correctly. By the way, what kind
of voltmeter you use to measure 0.00mV?

What PLC and analog card you have? Many cards can be configured
to read current directly without converter. You can test
PLC's voltage input by bringing known voltage to the input...

right, what PLC is it and what card type ?
Many analog inpt cards have filtering built-in.

Alternatively you can use this simple formula:
(to be triggered with a cyclic interval (0.1 sec f.ex.), but not necessarily every scan).
FilteredVal := ((RawVal - FilteredVal)/DampeningFactor) + FilteredVal

The Dampening factor can be between 3 - 10.
You might have to use floating point math, to avoid rounding errors, especially if the dampening factor is large.

Apart from that, I would look into possible overlaid noise on the analog signal.
Are the cables screened ?
Grounding OK ?
Are there sources of electrical noise ?
etc.

edit: Why do you convert from 0-20mA to 0-5V ?
0-20mA is more robust against noise.
You should consider to have an analog input card with 0-20mA range.

It sounds like you are using a Moeller smart relay. They are also put out under the name AB- pico and seimans. The only analog input they have is volts. We use these with 4-20ma xmtrs by bridging a 500-ohm resister across the two wires from the xmtr to get a 2-10VDC signal (or a 250-ohm for 0-5VDC). If this is the case or something like it you must get 1-% tolerance resister
or you can experience problems like you are experiencing.

If you are getting flustuations your meter may not pick it up. Digital meters can be quite slow due to the dampening factor. I have found Fluke to be particularly slow. I use a TES from Taiwan. It has very little dampening and is very fast. Sometimes too fast.
beerchug

Instead of trying to filter the signal, what is the amplitude
of the swings? What is that in % of the normal signal?
If your input range is 0-5V and normal signal is ca 2-3Volt
and your fluctuation is bellow 1-2mV you are chasing ghosts.
This is in range of 1-bit difference of your analog signal.
Consider this:

(0.001Volt/2Volt)*100=0.05%

Fluctuation of 0.05% is not an error and honestly it is
damn good result. You will hardly find anything better
unless you get into research lab equipment.
If your ego insists on perfectly stable signal, you can round it or
put one or two rungs of code to create deadband. Adding
ca. 1uF capacitor parallel with the input will stabilize
it enough. Beware, what ever you do artificially
"improve" your signal it will only make it look better,
but actual signal quality will decrease.

Use metal film resistors of 250 ohm across input. and use a capacitor of about 1 uF.
first only connect the resistor and a voltmeter. put input as close as possible to half way so 12 mA and read the voltage on your meter. switch the meter to AC and look at value.
if ac higher then 5 mV problem is the low imedance of the input. connect a capacitor.
and do the same again.
check if reading is same as plc reading.

if problem gone leave it like it is.
if not connect all zero volt input together and ground them.
check again.
you must be aware the plc scans one input and not all on same time. this is called sample and hold sequence.
i have seen problems when using internal zero offset it made input jump 10 bits just because timing was incorrect.

connect a simple battery to the input tnow the input should be rocksteady not even one bit should change.
on all inputs should be exact same if not problem is not correct plc for this purpose.
if still problems come back to us and we will talk further.