Thanks Ruben, for taking the time to write this information down. Being mostly a=20 software guy myself this is very hard subject to get a path in to. Most=20 people you ask However I have decide to learn this now. Your info was =20 *very* valuable. Thanks again. /Ake Ruben J=F6nsson wrote: >Hi Ake, > >Identify the current path of the noise through your board and either blo= ck it=20 >or make it as easy for it as possible to pass through your board. It onl= y=20 >becomes a problem if the current is converted to a voltage as seen by yo= ur=20 >circuits (through an impedance of some sort, perhaps a bad ground plane). > >Remember that the noise (or transient or surge) wants to go back to it's= source=20 >and it probably does so through earth. Either in your powersupply (direc= tly or=20 >capacitively coupled) or capacitively coupled directly from your board t= o an=20 >earthed groundplane. > >One effective way to keep the noise currents away from your board is to = put it=20 >in a metallic enclosure and have shielded cables where the shield is dir= ectly=20 >connected to the enclosure through a very good connection such as an EMC= cable=20 >gland. Connect the shield at both ends. Block the power supply with indu= ctors=20 >and decouple with capacitors to the enclosure (not the ground plane) in = order=20 >to make the impedance for the noise much lower through the enclosure tha= n=20 >through your board. You could also decouple the sensor signal to the enc= losure. > >If you don't have a metallic enclosure, mounting the board on a chassis = plate=20 >where the shield is connected to the plate and the powersupply decoupled= to it,=20 >could have the same effect. > >If you want to filter the signal directly you have to do this through a = good=20 >groundplane. Knowing what frequency your board is most sensitive to the = noise=20 >will help you pick the right filter components. An EMC test will tell yo= u that.=20 >Either way, a good very ground plane is always helpful. > >Good luck / Ruben > > =20 > >>Jose, >> >>my main problem appeared to be a ground problem just as Jim pointed out= .=20 >>I still need to follow you advices though for he final card. >> >>I have no decoupling on the sensor lines today. Only on the power to th= e=20 >>sensors. The sensor in this project gives out a square wave of 1 kHz - = 4=20 >>kHz and having a cap. on it will ruin the signal. >> >>Would something like Murata BLM21AG102SN1=20 >>(http://www.murata.com/catalog/c31/es0100.pdf) do this job? I know I ca= n=20 >>test this but just now I feel I need to hold the hand of someone who ha= s=20 >>done this more than once.... >> >>Another path in on the board is the CAN bus which has a very long cable= .=20 >>Should I do the same thing there? >> >>Regards >>/Ake >> >>Jose Da Silva wrote: >> >> =20 >> >>>On Thursday 03 February 2005 09:35 am, Ake Hedman wrote: >>>=20 >>> >>> =20 >>> >>>>I have two identical boards on my desk and have some noise problems >>>>with one of them. Both have a sensor input and a cable of about two >>>>meters attached to the input plus are connected to a CAN bus. The=20 >>>>sensors are decouples on the board with a 0.1uF cap. Above my bench I >>>>have a fluorescent lamp and one of the boards get very upset (watchdo= g >>>>reset) when this lamp is turned on/off. >>>> =20 >>>> >>>> =20 >>>> >>>If it is not a shielded cable, then you have a "nice" antenna to colle= ct all=20 >>>that RF noise. The 0.1uF caps by themselves are not enough. You need t= o=20 >>>think of adding either a resistance or an inductor on the sensor line = before=20 >>>the cap and, preferably you add them to both lines (assuming 1 pin for= =20 >>>gnd/vcc and 2nd pin for sensor input). >>> >>>=20 >>> >>> =20 >>> >>>>First it is strange that only one board does this but my question is = how >>>>do one approach a problem like this? >>>>I have also tested a piezo gas lightning device close to the boards b= ut >>>>it does not disturb the systems. >>>> =20 >>>> >>>> =20 >>>> >>>As you have found out, not all frequencies appear to affect it, or aff= ect=20 >>>every board the same way however, you are still dealing with an RF sou= rce=20 >>>and an antenna. If you look at some of the tests done to pass FCC or E= U or=20 >>>other electrical/electronics agencies, you will note that the tests ha= ve=20 >>>some very high voltages which will affect cables like yours of only 2 = meters=20 >>>length. >>>1st, you specify that the sensor should be on a shielded cable to make= your=20 >>>circuit more immune from noise, 2nd you design your board so that peop= le who=20 >>>seem to know more than they think and still use plain wire won't affec= t your=20 >>>circuit.... so basically, you make your sensor input a low-pass filter= =20 >>>input. RC might be cheap, but LC may do nicer. >>>=20 >>> >>> =20 >>> >>--=20 >> --- >>Ake Hedman (YAP - Yet Another Programmer) >>eurosource, Brattbergav=E4gen 17, 820 50 LOS, Sweden >>Phone: (46) 657 413430 Cellular: (46) 73 84 84 102 >>Company home: http://www.eurosource.se =20 >>Kryddor/Te/Kaffe: http://www.brattberg.com >>Personal homepage: http://www.eurosource.se/akhe >>Automated home: http://www.vscp.org >> >> >>--=20 >>http://www.piclist.com PIC/SX FAQ & list archive >>View/change your membership options at >>http://mailman.mit.edu/mailman/listinfo/piclist >> >> =20 >> > >=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D >Ruben J=F6nsson >AB Liros Electronic >Box 9124, 200 39 Malm=F6, Sweden >TEL INT +46 40142078 >FAX INT +46 40947388 >ruben@pp.sbbs.se >=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D > > =20 > --=20 --- Ake Hedman (YAP - Yet Another Programmer) eurosource, Brattbergav=E4gen 17, 820 50 LOS, Sweden Phone: (46) 657 413430 Cellular: (46) 73 84 84 102 Company home: http://www.eurosource.se =20 Kryddor/Te/Kaffe: http://www.brattberg.com Personal homepage: http://www.eurosource.se/akhe Automated home: http://www.vscp.org --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist