On Thursday 03 February 2005 01:46 pm, Ake Hedman wrote:
> my main problem appeared to be a ground problem just as Jim pointed out.
> I still need to follow you  advices though for he final card.

Good to hear.

> I have no decoupling on the sensor lines today. Only on the power to the
> sensors. The sensor in this project gives out a square wave of 1 kHz - 4
> kHz and having a cap. on it will ruin the signal.

A)does it have to be a perfect square wave?   Not likely
B)is it okay to degrade the square wave? Probably. Is 10% okay? Probably.

...shoot for an RC or LC at about 40khz. That shouldn't affect your signal by 
very much.  If you can make your RC or LC closer to 20khz, then even better.
Go for a combination of value RC or LC that only affects your circuit up to 
1/10 at let's say 8 or 12 khz  (for example, the fluorescent lights are 
running at approximately 90khz).

> Would something like Murata BLM21AG102SN1
> (http://www.murata.com/catalog/c31/es0100.pdf) do this job?

Well, look at the values on the spec sheet. You have an inductor of about 
10ohms at 100mhz, or 1 ohm at 10mhz or 0.1 ohm at 1mhz. in other words, when 
you are dealing with maybe 100khz or 50khz, those beads really aren't doing 
more than costing you money. Leave the beads for cell-phone circuits at 
900mhz or higher. What you want will be better served with 10 ohm resistors, 
easier to find, cheaper too.

Can your circuit handle 100 ohms instead of 10 ohm resistors? Then you will 
have better immunity.
What type of circuit do you want to aim for? You mentioned you had to send 
vcc to the sensor too, so you may want to aim for something like this:

vcc--100R-+-100R-->to sensor vcc+
          |
         0.5u
          |
      chassisgnd

gnd--100R-+-100R-->to sensor gnd-
          |
         0.5u
          |
      chassisgnd

input-100R-+-100R--<input
           |
          0.25u
           |
       chassisgnd

chassis---------->shield connection(only connect one side, not both)

chassis is the metal box you put your circuit inside of.

Perhaps the above is way overboard versus what you plan on doing, but gives 
you a general idea of things.

note, if we substituted a 390uH inductor, it would be roughly 100ohms at 
40khz yet neglible at 4khz and far higher at higher frequencies.


> I know I can 
> test this but just now I feel I need to hold the hand of someone who has
> done this more than once....

There is no hard/fast/rules I can point you towards.
I don't know anything about your sensor or your circuit.
I don't know if you are running on a battery circuit or an AC circuit.
I don't know if you are going to run this in the middle of a forest with no 
local noise, or if you are going to run it within a setting full of 
hi-voltage motors generating lots of spark-noise spikes.
I don't know if you will be running this for your personal use, or for dry 
arid lighting-frequented areas.
I don't know if you will run it in a noisy environment such as a car 
ignition, boat ignition or general house noise or industrial factory 
location or close to a radio tower of one sort or another.
Far easier for you to work it out on your end versus me multiple guessing 
here.  ;-)
Since you are in Europe (I think), then one or more of these will give you an 
idea of what you are guarding your circuits against:
----------------------
Immunity: IEC 1000 
 EN55024 per EMC Directive 89/336/EEC, including 
 IEC 61000-4-2 Electrostatic discharge immunity test 
 IEC 61000-4-3 Radiated, radio-frequency, electromagnetic field immunity test 
 IEC 61000-4-4 Electrical fast transient/burst immunity test 
 IEC 61000-4-5 Surge immunity test 
 IEC 61000-4-6 Immunity to conducted disturbances, induced by radio frequency 
fields 
 IEC 61000-4-8 Power frequency magnetic field immunity test 
 IEC 61000-4-11 Voltage dips, short interruptions and voltage variations 
immunity tests 

Line Distortion: EN 61000-3-2 per EMC Directive 89/336/EEC 

Voltage Fluctuations and Flicker: EN 61000-3-3 per EMC Directive 89/336/EEC 
----------------------
These people would like to have their components installed in your circuit.
Some of the info is useful (TThe IEC61000 info is real-world problems to 
consider... perhaps not useful where you live, but will certainly be useful 
in a dry-static-lightning environment) this doc seems to give you a general 
feel of IEC61000
http://www.semtech.com/pdf/an96-07.pdf

(they also have example circuits).
perhaps a page to start with.... look at the circuits here:
http://www.semtech.com/products/products_line_familydisplay.asp_Q_FamilyID_E_7_A_LineID_E_1

here is another circuit (see top of page 4):
http://www.semtech.com/pdf/stf701.pdf


> Another path in on the board is the CAN bus which has a very long cable.
> Should I do the same thing there?

I have not looked at CAN, however, I would believe some CAN chip 
manufacturers will supply real sample circuits which you can model your 
circuit on while others may have sample circuits which have never gone 
beyond a "paper" example (in other words, not tested in real life).
You'll have to ask someone else with experience in this area then.

Maybe the above is more info than you want, but hopefully useful :-)

cheers!
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