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filtering signal noise

hip36

Feb 28, 2014
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I am really having a hard time getting low voltage signals to trigger and smooth on this Digilent. Used to say I could trigger any signal, then they took my knobs away, and these buttons don't work the same,I can't figure how to run a sync cable either. I scrolled through the buffers, added capacitance to ground, scrolled wildly at the trigger numbers, tried syncing on the output, Read through the help files, re-watched what tutorials I could find, and even tried Emailing Digilent's support site.
So what do the pros say? Just where are the waves on my wave coming from? Shouldn't my Input offset voltage/currents and bias currents all remain a constant dc? Could I be getting this much noise through my rails? I am only powering pins 4 &7 of a uA741 with +-12V from an old Heathkit, could this noise be coming from there? Just what are the effects of changing my rail voltage, and what are the datasheet characteristics that apply to pins 7 and 4?
Could this be usb jitter? I know I have a hard time at high frequencies, that is a given limitation, but nothing told me I would hit problems like this at small amplitude.
One more idea I had was to add another op-amp with gain of 1 in series with my signal to cancel out some of the noise. I am pretty sure I could make that work, but this lab is all about studying micro amps in op-amp circuits, and I would bet they would start growing and shrinking exponentially.
 

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Harald Kapp

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It would help if you showed us your circuit and your measurement setup. Chances are that the noise comes from improper connections of the scope probes.

one simple test you can perform to check the scope is to connect the probe to a single isolated battery. The scope should show a clean horizontal line (use auto-trigger).
From that you can work forward:
- check the noise on the power supply
- check the ground connections of the scope probes:
-- make the loop between ground clip and probe tip as small as possible
-- connect all grounds to the same common ground point (I know this contradicts the previous tip. You'll have to compromise)
-- check that there is no ground loop involving e.g. any point connected to earth.
 

hip36

Feb 28, 2014
7
Joined
Feb 28, 2014
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Thanks

Thanks that was a good step by step troubleshooting procedure, and in fact the noise is traced back to the input signal. I am still not sure what to do about it. Here is the circuit, but I can't lose any more time on this lab. An experienced guess, says cables or connections. This hurry hurry pace is killing me. Professionally I never left a job until it was finished correctly, now I go to school and the standard seems to be to half ass as much work as you can as fast as you can to keep the zeroes to a minimum. No wonder I can't make in the corporate world.
 

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Harald Kapp

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The schematic looks o.k., The fault is most probably in the physical setup.
 
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