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Oscilloscope input noise comparison

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nctnico:

--- Quote from: MrW0lf on November 01, 2018, 09:53:40 am ---
--- Quote from: Hagrid on November 01, 2018, 09:35:46 am ---Very interesting. I really wonder why they made this design decision. In my opinion it looks like this contradics the whole purpose of a high resolution mode.

--- End quote ---

Maybe it's related to their main business background - automotive. High resolution is not only for looking at very low voltages. Would be useful with x100 probe etc.

--- End quote ---
The problem is that the input attenuator will attenuate the signals before they hit the amplifier so you'll get the noise also at lower sensitivities. A high resolution ADC doesn't make sense with that much noise. You also have to read the datasheet for the Picoscopes very carefully. On some models the high resolution mode is nothing more than oversampling an 8 bit ADC while other models do have an ADC with more bits.

MrW0lf:

--- Quote from: nctnico on November 01, 2018, 02:48:57 pm ---The problem is that the input attenuator will attenuate the signals before they hit the amplifier so you'll get the noise also at lower sensitivities. A high resolution ADC doesn't make sense with that much noise.
--- End quote ---

Cannot agree, it does makes very much sense, just as stated, when poking LV & HV:

Reference, x100 setting, 100V/div, input shorted with 50Ω:

PicoScope 2205, 195.3kSa/s
8bit AC RMS = 1100mV
12bit (software) AC RMS = 80mV

PicoScope 2408B, 125MSa/s
8bit AC RMS = 2400mV
12bit (software)  AC RMS = 250mV

Analog Discovery 2, 320kSa/s
14bit decimate AC RMS = 990mV
14bit average (hardware*) AC RMS = 250mV

*ADC is sampling at full speed (100MSa/s) in background

Pico TA041 probe at x100 setting, 100V/div, 10x H/V zoom windows, observing mains:

PicoScope 2205:



8bit trace fairly stepped, 12bit (software) trace deforms shape due to bw hit. Instrument not very suitable for the task.

PicoScope 2408B:



Much higher sample rate & analog bw, 8bit trace drowned in noise. 12bit (software) processing works because of much larger recordset hiding bw hit. However response becomes a bit sluggish because PC has to crunch the numbers.

Analog Discovery 2:



Here can see why 14bit hw is superior despite very poor AC RMS noise performance (see reference). Darker green trace is decimated one. Noise is visible but UI response is very good because of native 14bit and no software processing. When turn on hardware averaging (bright green) trace clears up w/o noticeable performance hit.

So can conclude that native high bit scopes are very useful, even if their frontend is not low noise.

Performa01:

--- Quote from: Hagrid on October 30, 2018, 07:18:59 pm ---Please someone correct me if I am wrong, but is the noise of the 5444 about five to ten times higher than the noise from the cheap Chinese scopes from the first page?
This makes little sense to me, since it has modes up to 16 bit resolution.
 
--- End quote ---
Well, these tests have been with just 8 bits resolution.

We need to be cautious to not compare apples with oranges. I have used the best of the “cheap Chinese scopes” and tried hard to resemble the test scenario for the PicoScope 5444 as close as possible, i.e. 1GSa/s, 1.4Mpts, 100µs/div and 2mV/div:


Siglent SDS1104X-E_100µs_2mV_100MHz_T50

Yes we get lower noise (63µV), but bandwidth is only 110MHz for this scope. The average noise density over the full bandwidth is about 6nV/sqrt(Hz) and I cannot see how this could be 5-10 times better than the ~9nV/sqrt(Hz) of the Pico 5444.

For the true high resolution models, like the 4262, we get much lower noise (4.33µV) because of the limited bandwidth, but also a lower noise density of <2nV/sqrt(Hz) at 2mV/div (zoomed to 100µV/div):


Pico 4262_Noise_10ms_2mV_5MHz_T50_Z100µV

The noise spectrum of the 4262 from 5Hz – 5MHz captured at a RBW of 14Hz looks like this:


Pico 4262_Noise_10ms_2mV_5MHz_T50_FFT

1/f noise is not very pronounced down to 50kHz and noise floor is below -156dBV (16nV) at 50kHz and even lower above. The strongest spurious signal measures -149.5dBV (32nV).

A closer look at the low frequency spectrum finally shows 1/f noise quite clearly:


Pico 4262_Noise_10ms_2mV_5MHz_T50_FFT_Z50kHz

We can see -156.5dBv (15nV) at 50kHz and -135.7dBV (164nV) at 100Hz. Given the FFT bin width of 4.768Hz and the flat-top window, we can expect a noise bandwidth of some 14Hz for the measurement.

This would result in 4nV/sqrt(Hz) at 50kHz and 44nV/ sqrt(Hz) at 100Hz, yet all these figures have to be taken with a grain of salt, because even at 5MHz, where we get -160dBV (10nV) the calculated noise density would still be ~2. 4nV/sqrt(Hz), which does not conform to the total noise density of <2nV/sqrt(Hz) as calculated earlier. I would tend to rather trust the FFT and believe that the RMS measurement in y-t mode might have given a slightly low reading.

maxwell3e10:
Here is the comparison of PicoScope modes from Andreas as well as Rigol DS4014 data I got from rsjsouza.

As one might expect, at low mV/div setting the noise does not depend on the bit resolution, it is simply limited by the input amplifier noise.  The advantage of high resolution modes is that they would give the same noise even at higher mV/div setting (for example Owon 12 bit scope has similar noise on 5mv/div setting).

Also, small details, like how many divisions are in the full screen, do not matter when noise is limited by the input amplifier.

The 16 bit 200 MHz BW noise is higher due to aliasing with 62.5 MHz sample rate. I am surprised it is even allowed by the firmware.

_Wim_:
Hi Maxwell3e10 and Performa01. Thanks for the plots above. Very interesting!


--- Quote from: maxwell3e10 on November 02, 2018, 03:33:20 am ---I am surprised it is even allowed by the firmware.

--- End quote ---

Yes, I am surprised about this to, I always thought that for reduced sample rates a corresponding hardware aliasing filter would have been activated. If only a single 20MHz filter is available, then only the 12-bit mode makes sense for the 200Mhz model.
Makes me wonder how they do the bandwidth limiting on the lower bandwidth models like mine? I always thought it was done in hardware, but seeing this that makes no sense. If it done in software, I normally should be able to create aliasing effect on the higher resolution modes. I will give that a try later today.


I wonder now if this is common on other scopes also. If four example you enable all channels, samples rate drops below nyquist for the lower end scope (ex. "upgraded" Rigol ds1054z has only 125MS/s for 100Mhz bandwidth"

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