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REVIEW - Rigol DS2072 - First Impressions of the DS2000 series from Rigol

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marmad:

--- Quote from: devitrify on June 13, 2013, 07:17:52 pm ---Marmad, Rigol claims 12 bits of resolution for the DS2000 series in their posted specs at:  http://www.rigolna.com/products/digital-oscilloscopes/ds2000/
but I understand you to say that these additional 4 bits are interpolated from the actual 8-bit measurement.

I am disappointed. I wish that manufacturers would simply state the resolution of their ADCs and separately indicate the means with which they achieve any additional resolution, much like optical vs digital zoom on cameras.

--- End quote ---

As far as I've been able to tell, Rigol has always been quite honest about their specifications (although sometimes you need to dig a little), unlike other brands I can think of. The High-Res mode is a commonly used 'trick' among manufacturers at the moment to 'simulate' 12-bit resolution with sample-averaging (perhaps started in lower-cost models by Agilent and their X-Series). But the DS2000 User Manual clearly states:

High Resolution
This mode uses a kind of ultra-sample technique to average the neighboring points of the sample waveform to reduce the random noise on the input signal and generate much smoother waveforms on the screen. This is generally used when the sample rate of the digital converter is higher than the storage rate of the acquisition memory. Note: “Average” and “High Res” modes use different averaging methods. The former uses “Waveform Average” and the latter uses “Dot Average”.

Again, this is something being done ON sample memory in it's transformation to display memory (and the screen). You won't be able to save this except as the 1400 bytes of display memory.

marmad:
Some cut & paste info about the technique:

High-resolution mode

The second method of averaging in the digitising oscilloscope that does not require a repetitive signal is high-resolution mode.

These types of oscilloscope provide 8-bit vertical resolution in normal acquisition mode, like most other digitising oscilloscopes. However, high-resolution mode on the oscilloscope offers up to 12 bits of vertical resolution in real-time mode, which reduces noise and increases vertical resolution.

Instead of averaging points from multiple acquisitions in a single time bucket like normal averaging mode, high-resolution mode averages sequential points within the same acquisition together.

In high-resolution mode you cannot directly control the number of averages as you can in averaging mode. Instead, the number of extra bits of vertical resolution is dependent on the time/division setting of the scope.

When operating at slow time base ranges, the oscilloscope serially filters sequential data points and maps the filtered results to the display. Increasing the memory depth of on-screen data increases the number of points averaged together.

High-resolution mode has no effect at fast time/div settings, where the number of on-screen points captured is small. It has a large effect at slow time/div settings, where the number of on-screen points captured is large.
 
How to achieve 12 bits of resolution

For the oscilloscope in high-resolution mode, vertical resolution varies as the time base changes as below (at 1Gsample/s sample rate).

Time base   Bits of resolution
<10ns/div          8
50ns/div            9
200ns/div         10
1µs/div             11
> or =5µs/div   12
 
To get up to 12 bits of resolution in high-resolution mode, the oscilloscope averages together at least 16 consecutive samples.

Therefore, to achieve 12 bits of resolution, we add 16 consecutive samples together, then divide the total by 16. This process is commonly referred to as decimation. This results in 12 bits of useful data. Notice that these are bits of resolution, not accuracy.

The effectiveness of high-resolution mode depends on the characteristics of the dominant noise sources, which come internally from the oscilloscope or from external circuits measured by the oscilloscope.

In other words, you only get more “resolution” in the presence of white noise. You may not be able to get more resolution for averaging noise-free samples.

Most A/D converters used in digitising oscilloscopes are 8-bit converters, with 8-bit differential non-linearity (DNL) and 8-bit integral non-linearity (INL). DNL error is defined as the difference between an actual step width, and the ideal value of 1LSB (least significant bit).

The INL error is described as the deviation, in LSB or percent of full-scale range (FSR), of an actual transfer function from a straight line. High-resolution acquisition on a noisy signal will tend to improve DNL characteristics, but not INL. The scope probes and pre-amplifier in the front-end of the oscilloscope are calibrated to a few per cent of accuracy, so those are the dominant component needed to improve the integral non-linearity accuracy.

Teneyes:

--- Quote from: marmad on June 13, 2013, 07:43:16 pm ---High Resolution
Again, this is something being done ON sample memory in it's transformation to display memory (and the screen). You won't be able to save this except as the 1400 bytes of display memory.

EDIT: Actually, perhaps I'm wrong about this and the results of the averaging end up in sample memory - I haven't tested saving High-Res mode so I'm not sure. But in any case, it will be as 8-bit data, not 12-bit.

--- End quote ---
  Average is over frames
I found this video a good explaination of sampling, averaging, and High resolution

marmad:

--- Quote from: Teneyes on June 13, 2013, 08:05:34 pm ---  Average is over frames

--- End quote ---

Normal Average is of waveforms.
High Resolution Average is of sample points.

marmad:

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