Rigol DS1054Z calculates with 10 divisions vertically?

[Payne]

Hi all,

I have been testing (or playing ) with my new DS1054Z lately and found a strange issue, which may have been discussed before in this forum.

The issue is that if i view a full screen waveform (all 8 divisions), and I make a single trig in order to stop the triggering after one trigger. Now, if I zoom in, I don't have a 8 bit resolution (full scale divided by 256), Instead I have just about 160 counts resolution.

Example: full scale signal of 8 divisions, 200mV/division = 1600mV. Now if I zoom, I would expect a step size for the AD conversion at 1600mV/256 = 6.25mV. The problem is that I have a step size of 10mV which is only 1600mV/10mV = 160 counts.

Now, If I make the same signal (1600mV) and turn the vertical vernier so that the signal fills 5 divisions (320mV/divison), the signal of course looks perfect.

Now, with the same signal (1600mV), I half the vernier to 160mV/division so that the signal actually fills 10 divisions (this means that the signal is more far out than the screen borders), and make a single trigger. Now, I go back to 320mV/division, and I can see that I still have my full, undistorted signal.

Now, if I zoom in at the signal captured at 160mV/division (but out of borders, still not destorted), I can see that the I have a step size of 6.25mV, which is exactly 1600mV/256, so now I suddenly have a 8 bit resolution. Hooray.....!!

My conclusion:

Based on this, I calculate that the DS1054Z calculates with 10 divisions vertically internally in the AD conversion algorithm.

If you/I want the best possible resolution, make the signal 5 divisions tall, and hereafter half the vertical vernier and do a single trigger, this will make the best possible vertical resolution.

A full screen signal (as close to 8 divisions as possible) will not (as many people say) give the best resolution. This is instead 5 divisions on the DS1054z

Now, my question;

Why is my Rigol DS1054z calculating with 10 divisions vertically instead of 8, which is how my screen is divided, and I would expect that a full screen is divided into 256 steps (8 bit resolution)?

I don't understand this 

Do we have a bug here?

Thank you for your reply


Rigol DS1054z
SW: 00.04.04.SP3
Board: 0.1.4

[zbyr]

It is known issue.

https://www.eevblog.com/forum/testgear/rigol-ds1000z-series-buglist-continued-(from-fw-00-04-04-03-02)/msg1329189/#msg1329189

[Payne]

Hi Zbyr,

Yes, thank you, you are right.

Hope Rigol will change this in a future update since I dont see any reason to hide that much headroom outside the borders

[Fungus]

Why is my Rigol DS1054z calculating with 10 divisions vertically instead of 8, which is how my screen is divided, and I would expect that a full screen is divided into 256 steps (8 bit resolution)?

Well spotted, but I think you'll find the ratio isn't exactly 10:8, it's really 256:200. 

The DS1054Z has 400 vertical pixels and I suspect it maps the values by multiplying by 2 and throwing away the rest instead of doing a mathematically correct scale operation.

Hope Rigol will change this in a future update since I dont see any reason to hide that much headroom outside the borders

It's probably a hardware limitation.

Bottom line: Oscilloscopes are for looking at the shape of wiggly lines, not for making very precise measurements.

(the spec is only about 5% accuracy).