RIGOL DS1054Z - voltage calibration off - feature or looming issue?

[Lorenzo_1]

Was using RIGOL to look at some circuit voltages recently and found this apparent difference between two nodes that should have been common (one on a socketed opamp and the other on a connected transistor).  First thought was perhaps a problem with the socket, but substituting LeCroy for RIGOL using same probes/same connections showed no voltage difference.  What surprised me was the difference in voltage between the two lines - around 0.16V.  Anyway, decided to do an auto recal (for the first time in many months) through the system menu and this cured the problem immediately.  Lines came into vertical alignment. 

My question is whether this amount of vertical separation between channels is commonplace on this model and within the typical range for autocal correction.  Or does it look too high to be normal and perhaps an indicator of something going awry?

[james_s]

What happens when you place both probe tips together on the SAME node?

What you're seeing could easily be drop across the resistance of a trace depending on how much current is flowing.

[Fungus]

My question is whether this amount of vertical separation between channels is commonplace on this model and within the typical range for autocal correction.

Yes.

Was it a very hot day when it happened?

Or does it look too high to be normal and perhaps an indicator of something going awry?

Don't think so.

Anyway, decided to do an auto recal (for the first time in many months) through the system menu and this cured the problem immediately.

There you go!

[Lorenzo_1]

Thks for the quick replies.  Tested same node readings for both probes - same readings.  And it was pretty cool when I was doing this - perhaps 21C.  I've not seen such differences on my LeCroy (though I think that one does regular autocals while in use). So was a bit surprised by this instance with the RIGOL.  The voltage offset was enough to be material in trying to work out what was going on in the circuit.   Good to know it's normal and not atypical. 

[james_s]

That doesn't seem right, 0.16V discrepancy with both probes on exactly the same point in the circuit? When I do that with my Tek the two traces are bang-on, right on top of each other. While I don't expect Tek performance out of a low cost entry level scope, I would expect it to be significantly better than what I see there.

[Fungus]

That doesn't seem right, 0.16V discrepancy with both probes on exactly the same point in the circuit? When I do that with my Tek the two traces are bang-on, right on top of each other.

So was the Rigol after pressing auto-cal. 

[james_s]

Ooh, I thought he meant that discrepancy was *after* auto-cal.

[Lorenzo_1]

It was indeed out of whack before the recalibration as Fungus says. Came good but I wasn't sure whether this amount of 'drift' was normal.  Pleased to know it's not unusual. I don't need any more gear that needs fixing just now.

[Fungus]

It was indeed out of whack before the recalibration as Fungus says. Came good but I wasn't sure whether this amount of 'drift' was normal.  Pleased to know it's not unusual. I don't need any more gear that needs fixing just now.

If it's months since you did it then it could be a summer/winter thing.

FWIW: The manual recommends doing an autocal whenever the temperature changes by 5 degrees Celcius (or more).

[David Hess]

That amount of drift would be unusual with an older DSO but if it is within the automatic calibration range, then I would not worry about it.  Good designs can achieve a warm-up drift of 500 microvolts or less but this is less important when automatic calibration is available.

Looking at Dave's reverse engineered DS1000Z schematic, the input offset could potentially drift at least 1 millivolt over 10 degrees.

[Fungus]

Looking at Dave's reverse engineered DS1000Z schematic, the input offset could potentially drift at least 1 millivolt over 10 degrees.

Turn one on and watch the offset from the zero marker over the first five minutes of warmup. It's very noticeable.