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DS1054Z voltage differences between channel

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aether22:
I bought a DS1054Z, 2 of the probes were faulty (way way out) so I finally got replacements...

The replacements work fine, but the first 2 channels read 3.04v on the calibration signal, the next 3.12v and the last 3.20v (all vmax)

Additionally this time it is not the probes, as switching the probes does not change what each channel reads.

Sooooo is 0.16v difference between channels 1,2 and 4 enough to be complaint worthy?  A warranty issue?

Also, maybe I am being lame and there is some way I can calibrate it myself?

I tried Autocalibration, but no changes in what each channel read.

Performa01:
3.xx V what? Vpp, Vrms ... ? (I do not have a Rigol DS1000Z, so I cannot know what the calibration signal amplitude is supposed to be)
What vertical gain setting?
Could you provide a screenshot maybe?

Fungus:
I'd say it's nothing to worry about.

Oscilloscopes aren't multimeters, the ADCs in oscilloscopes have very high bandwidth and very limited resolution (8 bit).

If it helps: Imagine they work by looking at the pixels on screen (it's not quite true, but it's pretty close).

Look at the signal on screen. Do you see the problem? Noise. It only takes a couple of pixels of noise to cause a few percent error.

Things you can try:
a) Go to Acquire->Mode and set "High res". This gives you a 12-bit ADC. Now adjust the vertical scale/position so that the wave is as tall as possible on screen (think 'pixels' - the more pixels it covers, the more accurate it will be).
b) Turn on the low pass filter to eliminate some noise (press the channel's on/off button and choose "BW Limit->ON")
c) Turn off all the lights and computers in the room. A 'scope will pick up everything and each channel will be slightly different.
d) Calibrate the probes very carefully. Turn on the display of "overshoot" using the measurement menu on the left. If there's any overshoot on the calibration signal it needs to be calibrated away.

nb. Each probe has to be calibrated to a particular channel. You aren't supposed to swap the probes around randomly (that's what the colored plastic rings are for - so you know which probe goes on which channel).


Fungus:

--- Quote from: Performa01 on March 30, 2016, 01:08:19 pm ---3.xx V what? Vpp, Vrms ... ? (I do not have a Rigol DS1000Z, so I cannot know what the calibration signal amplitude is supposed to be)

--- End quote ---

The calibration signal is 3Vpp.

Fungus:
I just had a look at all the channels on mine. I thought of a couple more tweaks as well.

a) Set "High res"mode
b) Turn on the low pass filter
c) Turn off all the lights
d) Push the vertical scale knob. This allows you to fine tune the vertical scale (I set it to 400mV/div for the readings instead of 500mV).
e) Attach the ground clip of the probe to the ground point underneath the calibration signal output. This seems to reduce the noise a tiny bit.

With these settings I got the following Vpp readings on my DS1054Z.
CH1: 3.02
CH2: 2.99
CH3: 3.05
CH4: 3.05

For comparison, in 'normal' mode with a 1Vpp vertical scale I get these readings:
CH1: 3.08
CH2: 3.12
CH3: 3.12
CH4: 3.04

The values weren't stable though, those are just the numbers I seemed to see most. All of them wobbled around and quite often went over 3.2V (sometimes even over 3.3V).

I also noticed that when the numbers wobbled they always went up/down in steps of 4 (ie. values were 3.04, 3.08, 3.12, 3.16, 3.20, etc. with nothing in between). The resolution of the ADC must be 0.04V at that vertical scale - that's more than 1% of the reading!

Conclusion: DSOs aren't multimeters, they simply don't have the noise immunity or number of bits in the ADC.

(or at least the DS1054Z doesn't... I'm sure the old green screen HP27634837684A will)

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