Many thanks for replying.
We setup same kind but different made sensor befor and run without any problems. Sensors are highly sophisticated, these 0-5v linear outputs are mostly used on independent displays. We do not expect any spikes or similar errors (of course there is always be possible and if this happen it should have to our sensor circuit spesific).
No we do not use AREF in any kind.
I set the
and remove it after your advice nothing changed
We measure voltage both arduino and multimeter.
Arduino is Olimex 328https://www.olimex.com/Products/Duino/AVR/OLIMEXINO-328/https://www.olimex.com/Products/Duino/AVR/OLIMEXINO-328/resources/OLIMEXINO-328-schematic.pdf
Sensor is custom made wideband reader. It has 2 ground one for sensor one for sensor heater.
We power arduino from usb and power sensor from 12v adaptor. Both sensor grounds plus arduino grounds was connected to adapter ground. Anykind of ther ground setup generate veird readings from analog input. Arduino input was a5, (also I change it for testing nothing changed)
The other device has one ground and when we connect the arduino ground it works withouth any problem.
Also I maybe can't explain problem clearly.
Sensor generates 0 to 5 volt linear output. when we connect to arduino it works 5 volt to 2.24 volt and does not go bellow. under normal circumstances it should go to 0v or 0.1v.
However it swings 5v to 2.24v