EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: golivera on March 10, 2016, 09:23:11 am
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Helllo,
I wanted to learn how some op amps works, and I found a small problem with my understanding of "ground" but must be pretty simple.
Between my usb powered board earth ground and my power supply's earth connection there is a voltage drop, I measured 0.1mA flowing in between these two with my multimeter.
So my op amp design uses my power supply to power the op amp and my input signal (DC square wave) comes from the USB powered board. So if I earth ground, my power supply (connect the green and black connector) had the "same" reference as the input source (from USB), but because of the voltage drop I measured something tells me this cant be right. Should I just connect the negative source of my power supply to the earth ground of the USB board and don't give much more thought about this voltage drop across different earth connections?
Sorry in beforehand if the question isn't well described. Thanks!
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Every time when current flows thru "ground" it causes a voltage drop.
For analog stuff, especially when op amps are used for measurement, it is better to use a star grounding point as a reference.
By connectung the PSU and USB ground you could cause an ground loop (in your case an earth loop) which could cause noise.
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When you connected the two grounds together, you made a ground loop. And the USB cable has some resistance, so some of the current will flow other ways, so through your power supply-> mains cabling-> computer ground-> computer USB power supply. Say the resistance of the USB cable is 100 times less than the other way, but you will still have some current going that way.
Disconnect power supply earth, !!!! After verifying that what you are about to do is safe!!!! And it should fix the issue.
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I once made an amplifier when I was a kid and was very prowd of it an my nice earthing with a nnice thick wire from one point to another on my veroboard mess. Well it's turned out i didn't make an amplifier I made an oscilator. when i followed my dad's advice and did a star ground the amplifiers magically worked again.
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Thanks for your answers, I hadn't heard of the term star ground, thanks for that!
So if I understand correctly I shouldn't test my op-amp like this, but could a dirty trick be in this case to ground my power supply to the ground of the USB board directly? Any reasons not to do this? Sorry I cant test stuff until tonight!
And yeah Simon, I was getting very weird behaviour when I got lost in ground :)