oscope problem, CP2102, ground loop?

[Admin]

My expensive tektronix oscope is doing something really odd . . .

I am trying to measure the 3.3V pin voltage coming out of my CP2102 (the USB-> serial adapter on the sparkfun breakout board). Its being powered by USB.

Without attaching any wires, the oscope is already measuring 0.18V.

Then, attaching just my positive wire, it jumps to 3.44V. The ground wire isn't doing anything - its just floating!!! This is reliable with an instant voltage change, no slow drifting . . .

So then I think, why don't I measure the voltage between the 5V power pin and the 3.3V pin. Logic would say I would get 1.7V, no? Well connecting the ground oscope lead to 5V caused a short, with sparks and a power reset of my CP2102!!!

Is there some kind of ground loop problem I am experiencing?!? How do I stop this?!

[Kohanbash]

Hi
Seeing 0.18v is a bit high but I would still consider it normal being that you have the ground floating. Is the scope set to AC or DC coupling?
3.3V + 0.18V (of DC bias) - Small Losses in the scope +/- tolerances of adapter = 3.44V.

Try measuring the voltage with a battery powered multimeter and see if you still get that problem, also check the continuity between the ground pin and the power source.

The battery multimeter will let you determine if the "problem" is the device or the scope, and if it is the scope the continuity test is a good initial test for finding a ground loop.

[Admin]

hmmmm ok the sparks stopped happening . . .

and I determined what was causing that .18V minimum . . . the scope is apparently lower quality than I thought . . . that minimum voltage changes as I turn the scale knob:

500mV => 30mV minimum
1V => 110mV
2V => 140mV
5V => 200mV
10V => 1V

I barely used this scope . . . something broken? Because I can't get a reliable voltage if the scaling is going to do that . . . Should I call up Tektronix?

Now here is another test that makes no sense to me:

Test 1
oscope Ground - floating
oscope Power  - connected to multimeter power
multimeter Ground - connected to USB ground
result: nothing happens

Test 2
oscope Ground - floating
oscope Power  - connected to multimeter power
multimeter Ground - connected to USB power
result: multimeter reads -2.9V, and oscope went up +300mV

Test 3
all grounds floating
oscope Power - connected to USB power
result: oscope reads 3.2V

Test 4
multimeter across USB
result: 3.2V

check the continuity between the ground pin and the power source.

1Mohm for the oscope leads (a bit low in my opinion), infinite resistance between oscope ground and USB ground

[Admin]

one more test that is confusing me:

oscope ground to multimeter ground
oscope power to USB power
multimeter power to USB power
result: multimeter reads 3.2V

how does that happen if not a single wire is connected to USB ground?!?

[Kohanbash]

Hi
When you use a scope you should always use ground it. If your using a probe than there is usually an alligator clip attached for ground.

I barely used this scope . . . something broken? Because I can't get a reliable voltage if the scaling is going to do that . . . Should I call up Tektronix?

Are those scaling changes happening when your probe is grounded?

All four tests seem to make sense.
test 1 - nothing should happen
test2 - your meter is taking the reading in reference to USB power (instead of to USB ground) producing the negative voltage, while the scope is referenced properly since its ground is zero potential (it should be grounded to insure zero potential) so  2.9+0.3= 3.2V
 

how does that happen if not a single wire is connected to USB ground?!?

Internally the USB is tied to ground, that same ground is used for the scope which is connected to the meter.
For example if a robots chassis (assuming metal)  is grounded, measuring between battery ground and battery power is the same as chassis to battery power.

[Admin]

Are those scaling changes happening when your probe is grounded?

The results are the same whether the leads are floating or directly connected to each other. Oh and I kinda told you wrong, I was measuring the high . . . so I redid it with mean values:

10V -> 260mV
5V -> -60mV
2V -> -12mV
1V -> 10mV

More acceptable . . . but still pretty bad . . . this just shows how much noise is in the system . . . ~5% starting error . . .

Internally the USB is tied to ground, that same ground is used for the scope which is connected to the meter.

I admit that I don't really know what goes on inside an oscope . . . but I always thought both ground and power were left to float and only the voltage difference mattered . . .

Moral of the story: don't use an oscope on mains power when measuring other stuff on mains power without being very careful of ground loops . . .

[Kohanbash]

The results are the same whether the leads are floating or directly connected to each other.

The ground should be tied to the USB ground not the meters ground.

Moral of the story: don't use an oscope on mains power when measuring other stuff on mains power without being very careful of ground loops . . .

Actually with many scopes if you try measuring the AC power from the same power source as the scope is plugged into very bad things can happen (picture: exploding scope with the smell of burnt capacitors). If you need to do this than you need to put an isolation transformer between the wall and the scope (isolation transformer is a 1:1 transformer).

[Admin]

The ground should be tied to the USB ground not the meters ground.

No effect/difference.

Interestingly though, the mean voltages are different than before . . .

at 10V its now 350mV off . . . 2V -> 40mV . . . so odd . . . I guess its slowly slowly drifting . . .

[Kohanbash]

Another way to get around this is to set the scope to GND coupling (instead of AC or DC) than move the horizontal line to zero followed by putting the scope back to AC or DC coupling

[Admin]

Hmmmm my scope has four coupling options:
DC (default)
Noise
HF Reject
LF Reject

In Source, a few more options:
Ch: 1 (default), 2
Alt
Ext
Ext/10
AC Line

[Kohanbash]

I have never seen a scope that dident have it, what model do you have?

[Admin]

Tektronix TDS 3012B
http://www.tek.com/site/ps/0,,41-12482-INTRO_EN,00.html
http://www.tequipment.net/TektronixTDS3012B.asp

[Kohanbash]

http://www.tequipment.net/pdf/tektronix/TDS3000_TrainingManual.pdf

see the training manual on page 28 (1-16) and 96 (2-12).

[Admin]

oooooooohhhhhhhh

There are TWO coupling menus . . . the trigger coupling, and the 'Vertical' coupling . . . I was looking at the trigger coupling, but the Vertical coupling has it . . .

Another way to get around this is to set the scope to GND coupling (instead of AC or DC) than move the horizontal line to zero followed by putting the scope back to AC or DC coupling

Ok I pushed buttons for like half an hour and couldn't figure it out. I put it into GND coupling, and I move the horizontal line (using vertical position knob) until the Ch1 Mean goes to ~0V. Then I swap it back to DC Coupling . . . but nothing changes, the prob is still there . . .

What am I doing wrong?

[Kohanbash]

Hi
Set your regulated benchtop supply to a fixed voltage and try measuring it multiple times with your scope(in DC coupling mode) to see if it is repeatable, if it is it might just be noise that wont show up in your measurements.

I don't really know what else to try other than that 
(maybe try another probe)