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neuro11

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wrong measurment with PCB circuit
Mar 10^th, 2014, 1:43am

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 receving 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 suggession 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 sholdering on veroboard, so the problem is not related to circuit design, i think .

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aaron_do

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Re: wrong measurment with PCB circuit
Reply #1 - Mar 10^th, 2014, 5:52pm

Hi,

if you previously got the design to work on veroboad, then probably you have simply made a mistake in your PCB design. Did you test the veroboard design with the "real" input? Assuming you did not, then is it possible you haven't taken into account the source impedance? For such a low current, I doubt that trace width is the issue. If you want a better answer, you will probably need to provide much more information. Like a photo of your PCB for example.

regards,
Aaron

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neuro11 Junior Member Offline Posts: 30	Re: wrong measurment with PCB circuit Reply #2 - Mar 10^th, 2014, 9:56pm Hi, I have attached the circuit, pcb layout and trace
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neuro11 Junior Member Offline Posts: 30	Re: wrong measurment with PCB circuit Reply #3 - Mar 10^th, 2014, 9:58pm the schematic
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neuro11 Junior Member Offline Posts: 30	Re: wrong measurment with PCB circuit Reply #4 - Mar 10^th, 2014, 10:00pm input trace picture
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neuro11

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Re: wrong measurment with PCB circuit
Reply #5 - Mar 10^th, 2014, 10:04pm

aaron_do wrote on Mar 10^th, 2014, 5:52pm:

Hi,

if you previously got the design to work on veroboad, then probably you have simply made a mistake in your PCB design. Did you test the veroboard design with the "real" input? Assuming you did not, then is it possible you haven't taken into account the source impedance? For such a low current, I doubt that trace width is the issue. If you want a better answer, you will probably need to provide much more information. Like a photo of your PCB for example.

regards,
Aaron

actually I am using the Vero board circuit quite a long time without problem. I fabricated the pcb for convenience and if it helps reducing more issues.
I think the circuit in pcb is ok because with a DC supply its giving the desired output.

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aaron_do

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Re: wrong measurment with PCB circuit
Reply #6 - Mar 10^th, 2014, 10:25pm

I assume you've already checked for shorts. I don't think the trace width is a problem. If you think it is, then check the IR drop at the pins. As I mentioned, since you say the voltage drop is at the input of the RC filter, then it sounds like your circuit is loading down the source. If you disconnect your circuit from the source, does the source voltage go back to the correct value?

Aaron

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there is no energy in matter other than that received from the environment - Nikola Tesla

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neuro11

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Re: wrong measurment with PCB circuit
Reply #7 - Mar 10^th, 2014, 11:49pm

Yes, I checked for shorts. The resistance drop in the input trace (from input pin to resistance pin) is very low ~2 ohm.
At first I check the voltage of the source, then connect to pcb input pin, then remove source again to measure. The source measurement becomes lower only when connected. When removed they are same and much high then when connected.

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aaron_do Senior Fellow Offline Posts: 1398	Re: wrong measurment with PCB circuit Reply #8 - Mar 11^th, 2014, 8:19pm Hi neuro11, is there any voltage drop across the 2kohm resistors (there shouldn't be any)? Just wondering if the input impedance of the amplifier has dropped somehow...like maybe you have fried the input protection diodes (to be honest I'm not sure how possible that is). regards, Aaron
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neuro11 Junior Member Offline Posts: 30	Re: wrong measurment with PCB circuit Reply #9 - Mar 11^th, 2014, 11:50pm Actually not. Those things I already checked, like identifying where the voltages are vanishing. Moreover the protection diode of the IC if damaged, it would probably give very unstable results, no? With simulation it works fine though.
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aaron_do

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Re: wrong measurment with PCB circuit
Reply #10 - Mar 12^th, 2014, 1:58am

Hi neuro11,

yeah you're probably right. If the diodes had failed then I don't think it would matter which source you're using. Somehow your circuit is loading down the source, but I'm not sure why...if you use an multimeter to check the resistance between the input and ground, and input + to -, I assume you will get a high impedance?

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there is no energy in matter other than that received from the environment - Nikola Tesla

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neuro11

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Re: wrong measurment with PCB circuit
Reply #11 - Mar 14^th, 2014, 4:40am

aaron_do wrote on Mar 12^th, 2014, 1:58am:

Hi neuro11,

yeah you're probably right. If the diodes had failed then I don't think it would matter which source you're using. Somehow your circuit is loading down the source, but I'm not sure why...if you use an multimeter to check the resistance between the input and ground, and input + to -, I assume you will get a high impedance?

yeah...I get high impedance.....
I had another board (same unsoldered) and I just soldered input pin, connected them to the source. then measured the voltage in resistance pin(of the input) vs different GND point of the circuit. Its ok. Then I soldered the IC holder (that holds DIP IC don't know the name) and the same problem!!! so now I just have soldered the input pin and IC holder and even did not place IC. and I can not get the voltage.....What on earth could be the problem with placing that holder. Why without it is fine and with it is wrong.....
Something is seriously wrong with board probably......

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