Analog Discovery Studio / BNC Probe Compensation Problem

[rkruetli]

Hi

Got a new Analog Discovery Studio. I have some trouble to compensate existing BNC probes from Keysight (200 MHz rated, 10MOhm, 15pF). I tried it with my friends Probe Master probes too (250 MHz rated). Somehow we can not compensate the signal the right way. The probes work without any problems with a Keysight scope in 10 x mode.

A simple square wave generated 1kHz signal (from Wavegen as well as from a dedicated signal generator) looks quite horrible in 10x mode -  in 1x mode in looks quite perfect. Both BNC scope channels behalf more or less the same - completely undercompensated. Followed some pics/screenshots (last pic shows the best result I got after tuning using the compensation screw on the terminal end). Any suggestions? Am I doing something wrong? Any suggestions? Thanks!

Cheers,
Raphael

[JColvin]

Hi @attila, do you have some thoughts on this?

I tested this on my own Analog Discovery Studio, but was also unable to get a clean compensation at 10x attenuation using the probes Digilent provides. However, when I switch the setup to the Analog Discovery 2 (with the BNC adapter, DC coupled, 0 Ohm on the AWG) without otherwise changing the probe setup or workspace, I get a clean view of the square wave in both 1x and 10x attenuation.

Thanks,
JColvin

[attila]

Hi @rkruetli

You should try using probes with higher compensation range, at least 35pF.

The P6100 probes rated for compensation range of 10-35pF should be able to compensate with the ADS.
https://store.digilentinc.com/bnc-oscilloscope-x1-x10-probes-pair/

The N2140A probe specifies 15-30pF

 

Hi @JColvin

I only have a prototype of ADS, revB. On this I can't compensate the P6100 probe but on the release revD version it can be compensated.
Which ADS rev have you tested?

Here I tried to measure the input impedance, to compare the ADS with AD with BNC Adapter, using AD Impedance Analyzer.
The ADS shows approximately 33pF and AD2 BNC adapter 21pF.
This requires unpowered device, so on ADS I had to short solder the relay. This way the capacitance is increases a bit since the signal goes to both the header and to the BNC.

 

Here you can see the solder effect on the ADS compensation rage.
The R1/R2 are min/max compensation having the short solder, shows higher input capacitance. The R3/R4 without solder has slightly lower cap.

 

ADS revB compared to AD BNC:

[vicentiu]

@rkruetli, what is the input capacitance of the Keysight scope?

[rkruetli]

First of all: thank you all very much to have a look into this issue (if its really one)!

@vicentiu, its a DSOX1102A (https://literature.cdn.keysight.com/litweb/pdf/5992-3484EN.pdf?id=3019454) with about 1 MΩ ± 2% / 16 pF ±3 pF. The scope probes I have access to have the following specs:

• Keysight N2140A (https://literature.cdn.keysight.com/litweb/pdf/N2140-92002.pdf?id=2803394) with 15pf - 30pf comp range - tested, undercompensated.
• Probe Master 4902-2 (https://probemaster.com/4900-oscilloscope-probe-basic-kit-150-300-mhz/) with ~10pF - 50pF comp range (according to the page) - tested, undercompensated.
• Rohde & Schwarz RT-ZP05 (500Mhz) - not tested yet. Should it be needed, I have also access to a higher end scope (1Ghz) and AWG.

I used the 1kHz square wave signal coming out of the WG (& test clips) of the Keysight scope as well as with Discovery Studios own generator. No luck with compensation.

My Analog Discovery Studio seems to be Revision D (sticker on the back, got it from Mouser).

I ordered the "original" scope probes @attila mentioned, they should be here (Switzerland) from a local distributor tomorrow. Btw: I'm not an electronics engineer (MSc in mathematics/computer science and sometimes no clue what I am doing), just a so called "Maker".

//Raphael

 

[rkruetli]

Hi there

Got the P6100 probes today and... I'm now able to compensate the probes in junction with ADS properly!

Channel 1 is slightly undercompansated, on channel 2 I'm even able to overcompensate. I am still not sure way it is not possible to compensate the other probes properly (according to the specs they even have a wider comp range).

Thanks!