I have noticed a funny problem with my two-channel digital scope. It looks like the two probes are not measuring voltages very well when one probe is measuring a voltage that is the opposite polarity to the other one!
The scope is a
Keysight EDUX1052G, about 3 years old or so and has had an easy life without ever seeing voltages larger than +/- 30V DC. Hasn't had many hours on it either, nor much of a temperature swing (in the UK).
To measure the average voltages, I'm not relying on my failing eyesight to read the waveform, I'm using the built-in "
Measurement: Average Full Screen" or the "
Analyse: DMM" features. Both of which are pretty good when I'm using unipolar voltages.
Investigating further...
Lets say both probes are measuring the average voltage between GND and
POSITIVE 10V; the scope reports the same results with a small discrepancy of perhaps +/-0.05V which I'm quite OK with.
Same thing goes for both probes are measuring the average voltage between GND and
NEGATIVE 10V +/- 0.05V.
However, when I have one probe on
+10V and one on
-10V, there is suddenly a roughly
0.5V discrepancy between the measurements! So when I'm expecting to see +10V and -10V respectively, I'm actually seeing +9.8V and -10.3V. The same discrepancy exists when I swap the probes and measure the other way.
So I checked the obvious stuff...
- Made sure both probes were set to "X10" mode on the physical probe switches.
- Checked that the menu was also set to X10 for each probe.
- Ran the self probe check on both probes (they passed). But this is using the positive-only DC square wave on the front of the scope. Hmm.
- I'm using both GND clips on the probes connected to the same GND point.
- Three separate multimeters (Fluke 117, Aneng 8008, Uni-T Clamp meter) all agree that the scope is at fault!
Is my scope faulty? Or is this something to do with the way scope front-ends work? 0.5V is close to a diode drop. What's goin' on?