Oscilloscope input noise comparison

[Performa01]

We typically recommend the same method Daniel suggested (smallest volt/div setting, AC RMS or std deviation or a vertical histogram) with one addition (which I think Daniel would agree with) - you should really figure it as a percent of full scale as some oscilloscopes have 10 vertical divisions and some only have 8.

This is why I usually express the noise in LSB instead of µV in my graphs.

Of course, for this you need to know how many LSB/div. a certain DSO has - and keep in mind that this number will be far less for the zoomed (not full resolution) gain settings that some scopes have.

[maxwell3e10]

Here is a compilation of noise spectra for several more oscilloscopes:
R&S RTB2004 thanks to Hydron
Instek GDS2204E thanks to nctnico
R&S RTM3004  thanks to nctnico
Rigol DS6104

I grouped them by the sampling rate, 1 GHz and 5 GHz. It looks like Rohde & Schwarz scopes have the lowest noise among the scopes tested so far.

[Andreas]

Hello,

how many samples are you actually using for calculation for the 1 GS/s sample rate?
the minimum calculated frequency seems to be ~500 kHz with 500 kHz step resolution.
So it seems that you are using 2048 points for spectrum calculation.
(that would be a number which can easily uploaded as CSV here against the 1MS limitation from above).

with best regards

Andreas

[Performa01]

Yes, for these graphs the data can easily be uploaded in this forum and that has been done in that other thread, linked in the opening post. Just have a look there.

The problem only arises when we shall provide data suitable for analysis down to 10Hz.

[nctnico]

It would be interesting to see what the new DSO/MSO7000 from Rigol looks like when it comes to noise.

[maxwell3e10]

how many samples are you actually using for calculation for the 1 GS/s sample rate?

I am using all the samples, about 1M, but I set the linewidth of the FFT to 1 MHz. One can use smaller linewidth, but then the scatter is bigger.

[Keysight DanielBogdanoff]

Thanks for doing this! I'm also interested in how you measured noise. We generally use the smallest hardware V/div setting and measure V RMS (NOT Peak-Peak). This makes sure you don't have any issues with the quantity of acquisitions/update rate.

I'm wondering if that is the best way of doing it on an oscilloscope which uses decimated data for on-screen measurements.  Also the RMS peak measurement may be affected by any DC offset in the signal. Using the actually sampled data and doing a noise spectrum analysis seems like a much better way to me.

We typically recommend the same method Daniel suggested (smallest volt/div setting, AC RMS or std deviation or a vertical histogram) with one addition (which I think Daniel would agree with) - you should really figure it as a percent of full scale as some oscilloscopes have 10 vertical divisions and some only have 8.  Of course the other key thing to keep in mind when measuring noise using this method is that noise is a function of BW.  So you may need to use filters to get an apples to apples compare.

-Rich

Everything I learned about measuring scope noise I learned from Rich, back before he went to the dark side 

I second this comment.

Interesting, Daniel. Have you ever seen any manufacturers skew or butcher RMS measurements that could lead to incorrect results?

What we've seen some manufacturers do is measure pk-pk and not have the same number of captures. Generally as your quantity of captures go up, so will your max pk-pk measurement.

[Rich@RohdeScopesUSA]

Everything I learned about measuring scope noise I learned from Rich, back before he went to the dark side 

  LOL

-Rich

[Andreas]

how many samples are you actually using for calculation for the 1 GS/s sample rate?

I am using all the samples, about 1M, but I set the linewidth of the FFT to 1 MHz. One can use smaller linewidth, but then the scatter is bigger.

Ok,
now I think I found a way: if I remove the time-stamps from the data (only the voltages) it compresses down below 1 MB.
Per default my output is in mV: is this a problem or should I scale it to Volts.

with best regards

Andreas

[maxwell3e10]

Instek scopes make the best csv files from space point of view. They save only the waveform data in bits, so its typically only a single integer number for shorted input. It compresses down to 147 kB for 1 M data points. The only problem is that the preamble doesn't specify the volts/bit conversion in a very clear way. Its easy enough to scale the y axis and make the x axis.

[nctnico]

Instek scopes make the best csv files from space point of view. They save only the waveform data in bits, so its typically only a single integer number for shorted input. It compresses down to 147 kB for 1 M data points. The only problem is that the preamble doesn't specify the volts/bit conversion in a very clear way. Its easy enough to scale the y axis and make the x axis.

Some clarification here: I choose the 'fast CSV file' method when creating the files on the GDS2204E. There is also a slower version which likely has the data in Volts but is also more bulky and probably takes more time to create.

[Andreas]

Hello,

first measurements of PicoScope 5444A
8-Bit Mode with 1 GS/s and 1 Ms of data.
first with 200 MHz (full bandwidth)
and 2nd with 20 MHz (hardware filter).

Best y-resolution is 2mV/div (+/-10mV)
so I should vote for the comparison relative to the range.
Due to the low y-resoution it will not compare to devices with 0.5mV/div.

20 fold Y-Zoom in the diagram shows that the step width is 110 uV/step
so for the +/-10 mV range only 70% are used for the display (28.3 mV total range.)

Measurements are done with 50 Ohms terminator on one input.

the 200 MHz measurement shows 125.7uV AC rms (std dev).
with 200 MHz bandwidth this computes to 8.9 nV/sqrt(Hz) in average.
the 20 MHz measurement is around 11.7 nV/sqrt(Hz). (mostly quantisation noise)

from the .CSV data the timestamps have been removed so that 1Ms of data could be packed within the forum upload limits.
Also the mV output of the scope has been converted to Volts.

Next will be 16 Bit measurements in 16 Bit mode of the scope.

with best regards

Andreas

[lordvader88]

can someone tell me and others that don't know, the basics here, whats the graph mean ? whats good/bad ?

[nctnico]

The easiest way is to send it by Wetransfer or another file sharing service. There is no need to jump through all kinds of hoops to attach a file to a forum message.

[Carrington]

The easiest way is to send it by Wetransfer or another file sharing service. There is no need to jump through all kinds of hoops to attach a file to a forum message.

Absolutely, edited deleted. I was bored, sorry. 

[rsjsouza]

can someone tell me and others that don't know, the basics here, whats the graph mean ? whats good/bad ?

lordvader88, there are many references around on the web.

A training:
https://training.ti.com/ti-precision-labs-adcs-high-speed-SNR-NSD

An article:
http://www.ni.com/white-paper/3304/en/

[Hagrid]

Hello,

first measurements of PicoScope 5444A
[...]

Thanks a lot! I was looking for exactly this data. 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.

I would really appreciate it if somebody could explain this to me, thanks .

Greetings, Hagrid

[MrW0lf]

I would really appreciate it if somebody could explain this to me, thanks .

Picos just do not have low input ranges.

2000 ±20mv to ±20v
4262 ±10 mV to ±20 V
5000 ±10 mV to ±20 V

If think in divs then 2mV/div is lowest for 5000.

Is it a problem? Depends on application. When can average, do FFT etc then noise drops quite good:
https://www.eevblog.com/forum/testgear/fft-spectrum-analysis-reviewed/
https://www.eevblog.com/forum/testgear/cheap-chinese-ad584-voltage-reference-legit-cal-data-let_s-find-out!/msg1154930/#msg1154930
https://www.eevblog.com/forum/testgear/picoscope-2000/msg1155735/#msg1155735

[egonotto]

Hallo,

@Hagrid:
"I would really appreciate it if somebody could explain this to me, thanks"

You are right, the noise of the 5000 serie is not that good.
https://www.picotech.com/support/topic16031.html

But in some cases you can lower the noise.
https://www.eevblog.com/forum/testgear/looking-for-a-dynamic-signal-analyzer-with-extended-bandwidth/?all

You can limit the bandwith and lower the noise. To limit the bandwith with the 5000 series you have different ways.

From PicoScope 6 help:
"Resolution enhancement is a technique for increasing the effective vertical resolution of the scope at the expense of high-frequency detail. In some scope operating modes, PicoScope may reduce the number of samples available to maintain display performance.

For this technique to work, the signal must contain a very small amount of Gaussian noise, but for many practical applications this is generally supplied by the scope itself and the noise inherent in normal signals.

The resolution enhancement feature uses a flat moving-average filter. This acts as a lowpass filter with good step response characteristics and a very slow roll-off from the pass-band to the stop-band."

You can limit the bandwith to 20 MHz

With math you can average to lower the noise.

Best regards
egonotto

[Andreas]

With math you can average to lower the noise.

Hello,

you can also directly configure the low pass frequency of a software filter in the menu.

 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.

I guess with a 0.5mV/Div range and only 8 Div instead of 10  the factor of 5 can be explained.

At least for that what I measure I do not need high bandwidth and high resolution at the same time.
With a software filter of 1 MHz or below (besides the hardware 20 MHz filter) the noise decreases significantly.

with best regards

Andreas

[Andreas]

Hello,

and a 16 Bit noise measurement with .csv data (had to split it into 2 parts)
Again 2mV/Div 62.5 MS/s maximum sample rate in 16 Bit mode and with 20 MHz hardware bandwidth limiter.

Further Pictures:
Software low pass with 1 MHz, 50kHz, 1kHz additional to the 20 MHz.
and 20 Bit enhanced resolution out of 20 MHz.

with best regards

Andreas

[Andreas]

Hello,

and again 16 Bit values, this time without bandwidth limit.
the 16 Bit values are only 625000 samples because of upload limit.

with best regards

Andreas

[David Hess]

Thanks a lot! I was looking for exactly this data. 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.

I would really appreciate it if somebody could explain this to me, thanks .

The Picoscope details I have seen show an integrated CMOS transimpedance buffer intended for DSOs from TI that by itself has 100 times the noise of a discrete front end at low frequencies.  I mean literally 1000nV/SqrtHz where a discrete design could be 10nV/SqrtHz.

So a broadband RMS noise of 125.7uV over 200MHz does not surprise me at all and that is about 5 times worse than my 40+ year old 200MHz analog oscilloscopes.

[Hagrid]

Thank you all for sharing this Information.

Thanks a lot! I was looking for exactly this data. 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.

I would really appreciate it if somebody could explain this to me, thanks .

The Picoscope details I have seen show an integrated CMOS transimpedance buffer intended for DSOs from TI that by itself has 100 times the noise of a discrete front end at low frequencies.  I mean literally 1000nV/SqrtHz where a discrete design could be 10nV/SqrtHz.

So a broadband RMS noise of 125.7uV over 200MHz does not surprise me at all and that is about 5 times worse than my 40+ year old 200MHz analog oscilloscopes.

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.

[MrW0lf]

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.

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.