PCB problem

Discussion in 'General Electronics Chat' started by legolas11, Mar 10, 2014.

  1. legolas11

    Thread Starter Member

    Feb 26, 2013
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    I fabricated a PCB board first time and used trace width of 20 mil for power traces and 10 mil for all others. When i check the circuit with dc input voltage (ex:50 mv- 600 mv) everything is perfect.
    When i connect it to a real input (also a dc potential in the same range, ~30 uA current), the circuit receive signal much lower than original.Other parts of the circuit are working fine, just i am receiving very low signal as soon as it is connected to circuit input.
    Can it be due to the trace width. Or if you have any other idea or suggestion please help.

    NB: A little more about my circuit, the input pass through RC filter and then amplifier. The filter has nothing to do with the low signal, since i am getting the low signal even before filter. I had already been using same circuit with soldering on veroboard, so the problem is not related to circuit design, i think .
     
  2. pwdixon

    Member

    Oct 11, 2012
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    Perhaps your signal source impedance is high
     
  3. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    i guess you should post a schematic and a picture of your board.
     
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    How did you make your board with 10 mil traces. Using the toner transfer method can result in some rough edges and some very narrow (high resistance) areas. Double-check the board quality with a magnefying glass or microscope. If you don't have one of those, some digital cameras have good close-up capabilities (flower setting) - set the camera on the table with the pcb leaning against something (or use a tripod). Stability is key to close up microphotography.
     
  5. legolas11

    Thread Starter Member

    Feb 26, 2013
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    i don't know whether the problem with the narrow trace, but the same signal can be measured well with breadboard or soldered in Vero board.
     
  6. legolas11

    Thread Starter Member

    Feb 26, 2013
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    Attached!!!
     
  7. legolas11

    Thread Starter Member

    Feb 26, 2013
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    Sorry for my poor English. I did not fabricate myself. I sent the design to company(OSH park). The picture of input trace is attached though it can not be seen due to its covered with resist.
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    I doubt if the problem is caused by narrow trace.
    There is a simple test. Connect jumper wires where you suspect that the trace is too narrow.
     
  9. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    What is the input source and what are your power supplies?
    You might need to add some high resistance like 1meg from each input to ground.
     
  10. pwdixon

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    Oct 11, 2012
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    So your breadboard/vero circuit is exactly the same as your pcb circuit?
     
  11. legolas11

    Thread Starter Member

    Feb 26, 2013
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    I also don't think traces are too narrow, but nothing else come to my mind...
     
  12. legolas11

    Thread Starter Member

    Feb 26, 2013
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    the input is the potential from a solution measured vs 2 electrodes.
    But that additional resistor was not necessary for the same circuit used in vero board.
     
  13. legolas11

    Thread Starter Member

    Feb 26, 2013
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    exactly.
    I first placed it on breadboard. Then used it for long on Vero board. I made the PCB circuit only for more convenience and less noise.
     
  14. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Well you can use a multimeter and check that the traces have very low resistance like they should, and then you can try powering up both the veroboard and the pcb circuits and measuring voltages and comparing them. Maybe that will get you closer to tracking that error.
     
  15. legolas11

    Thread Starter Member

    Feb 26, 2013
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    Yeah, the trace resistance is very low. ~2 ohm. I already checked the real result and the simulated result in both the boards. while the simulated result is exactly same in both board, the real results are getting stuck to the pcb. complete mystery to me!!! I am asking about trace issue because then i can fabricate another and try if that seems to be the faulty one. for the present board it seems i have no other tests left.....
     
  16. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Also check what resistance you measure when you short the leads together, it will likely be the same 2 ohm. So unless you got a short there between adjacent traces, the board should be ok. Maybe you got some trouble with soldering? The picture you last posted didn't seem to have one of the resistors soldered in.
     
  17. pwdixon

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    Oct 11, 2012
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    "used it for long on Vero board" sounds like this has been in use of a long time, if so then as most will have experienced things get changed or memory loses changes and actually things are not what you remembered. I would have another check that the vero version is what you think it is, also you might find that some difference in say the amplifier chips because they were bought at a different time. Recheck everything and don't let your memory make assumptions.
     
  18. legolas11

    Thread Starter Member

    Feb 26, 2013
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    Sorry, which 2 pins you asked to short?? the two different input pins??

    My skills of soldering is not very good, but just enough to connect them on board. Since the simulated input is working well, bad soldering should not be an issue here, no??

    i soldered the mentioned resistance on the bottom side of the board.
     
  19. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Well in post #7 you posted a picture of a bit of that board and the resistor lead on the right doesn't look like it is soldered in.

    I meant the leads of your ohm meter, when you connect them together you will see some resistance, that is the resistance of your leads which you need to subtract from what you measure on the board to get accurate results in the low ohms range.

    Well you got two boards - one on veroboard and one pcb. Connect them to the same source and measure voltage on every point in the circuit, write it down and compare with the other board. There has to be some difference somewhere.

    Also, what exactly is the input singal?
     
  20. legolas11

    Thread Starter Member

    Feb 26, 2013
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    ok....
    yeah, after subtraction resistance almost does not exist.
    I will try the troubleshooting method.
    The signal coming from a solution measured vs electrode (similar to a battery cell)
     
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