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Picoscope 3404D MSO strange vertical step size

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Picoscope 3404D MSO strange vertical step size

Postby kc55 » Sat Jul 16, 2016 5:31 pm

Hi, I just got my new scope but I observed a strange behavior. Someone please help me on this. I need to know if the unit I just received is faulty.
I applied a square wave at 1 kHz 4 V p-p (from the unit's function generator output) to channel A. All other channels are off. Channel setup: 10 V full scale, 200 us/div, number of samples at 1MS, no resolution enhancement, probe and software both at 1X.
I zoomed in to take a look at the noise. Due to the scope's 8-bit vertical resolution, the step size is (10/(256-1))*1000 = 39.2157 mV. But, when I measured the actual step size (it looks like a step to me), it was 47.4 mV, about 20.9% more than the expected step size How can this be possible?
I own an old (discontinued model) Picoscope 3204. With the exact setup, I get 39,64 mV, very close to the expected value.
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3404D MSO
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3204
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Re: Picoscope 3404D MSO strange vertical step size

Postby Martyn » Wed Jul 20, 2016 1:24 pm

Can you check what happens when you have all four channels enabled.
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Re: Picoscope 3404D MSO strange vertical step size

Postby kc55 » Wed Jul 20, 2016 3:03 pm

It's 39.83 mV with all channels switched on, with nothing connected to the other channels.
I tried some combinations and found that I need to switch on at least two other channels to get this value.
With only one other channel switched on, I get approximately 44.7 mV.
Hmmm, my older 3204 scope does not exhibit this behavior.
Attachments
07.png
3404D MSO - all channels switched on
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Re: Picoscope 3404D MSO strange vertical step size

Postby Martyn » Wed Jul 20, 2016 8:16 pm

That is the expected behaviour, it is due to the use of digital gain, applied to the ADC values to ensure that the readings in the software are within the published specifications for the unit. Digital gain was not present on the older units.

The reason it varies depending on number of active channels is due to the setup of the ADC chip cores and the loading this creates.
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Re: Picoscope 3404D MSO strange vertical step size

Postby kc55 » Fri Jul 22, 2016 2:48 pm

Hi Martyn, thank you for your reply.
I was told this is due to ADC interleaving.
The specification states when up to 4 channels are used, the max sampling rate is 250 MS/s. Does that mean each ADC is able to sample at up to 250 MS/s?
If so, is it possible to disable this interleaving when only 1 or 2 channels are used, and when sampling rate per channel is <= 250 MS/s? This should bring back the almost 255 vertical steps.
With interleaving active, I visually counted the number of vertical steps is only 212, which is consistent with calculated result with a step size of approximately 47.4 mV (10,000 / 47.4 = 211).
So, the number of vertical steps is reduced by 255 - 212 = 43, or almost 17%. Loosing 43 steps for an 8-bit scope seems a little too much.

Regards,
kc
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Re: Picoscope 3404D MSO strange vertical step size

Postby Martyn » Mon Aug 01, 2016 1:10 pm

Unfortunately it will not be possible to do this at the present time due to space limitations in the available code space on the device.
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