SDS2000X+ Caution - Page 1
Forum section: electronics / testgear

author: mawyatt
author_rank: Super Contributor
timestamp: January 13, 2022, 05:09:38 pm
content: While preparing for some tests with our SDS2102X+ (1.3.9R6) and using a direct BNC cable input on Channel 1 without a probe, the scope indicated DC1M input when in fact the input impedance was 50 ohms!! Cycling the Impedance corrected the issue and returned to the correct 1M impedance as indicated.We caught this by observed waveform loading effects at lower voltages, before ramping up to higher voltages which would cause damage to the DUT and scope. Don't know if this was a one-off or potential reoccurring issue, but confirms paying close attention, especially when using potential damaging voltage/currents, with equipment & setups helps avoid disasters!!Best,

author: 2N3055
author_rank: Super Contributor
timestamp: January 13, 2022, 05:46:23 pm
content: Was that immediately after power up of the scope?

author: H.O
author_rank: Frequent Contributor
timestamp: January 13, 2022, 05:55:09 pm
content: I tried to reproduce it here (on 1.3.7R5) but no "luck" so far. I set channel 1 (tried 3 as well) to 50ohm and cycled the power but it correctly came back up with with the actual input impedance matching the indicating. Have done a few times.Do you remember if you shut down the scope normaly the last time you used it?However, I did learn two things:A) When the scope is turned off the inputs are open circuit, (unlike for example my Rigol DS4k) andB) During the first 20 seconds of the boot process the inputsdoesswitch to 50ohm - something to keep in mind if one leaves powered DUTs connected to the scope which aren't supposed to see 50ohm.

author: 2N3055
author_rank: Super Contributor
timestamp: January 13, 2022, 06:14:31 pm
content: Quote from: H.O on January 13, 2022, 05:55:09 pmKeysight 3000T will leave inputs in 50 Ohm even when OFF when you shut it down while doing 50 Ohm measurements. Very bad oversight. It will disconnect inputs when in 1 MOhm mode.That was my first thought: maybe something wasn't saved properly on shutdown. Like you hit shutdown, and before it goes down, you press 1M and then it saves something partially..

author: Martin72
author_rank: Super Contributor
timestamp: January 13, 2022, 08:52:38 pm
content: Hm-hm..Will test it tomorrow on my siglent, same firmware.Nice thing on our waverunner/hdo scope from lecroy:When powering on and booting, a warning message pops up, reminding that you got 50Ohm on the last online state - keeping it or changing to 1M....

author: bdunham7
author_rank: Super Contributor
timestamp: January 13, 2022, 09:07:43 pm
content: It would be nice if they would have the 50R input resistor automatically disconnect upon overload like my Tek 2445 and 2465B. I would think this would be possible in firmware using the existing hardware.

author: Martin72
author_rank: Super Contributor
timestamp: January 13, 2022, 09:24:36 pm
content: This plus/or automatically switch back to 1M when restart/power it on.Burned 50 Ohms...We got this situation several times on elder scopes.Repairs were always really expensive, so a protection would be a real benefit for the customer.

author: mawyatt
author_rank: Super Contributor
timestamp: January 14, 2022, 02:47:44 am
content: Don't remember the details of when the scope was turned off and what we actually did turning turn on (was preparing for some complex system level tests). However it DID NOT indicate 50 ohms when it was turned on, but was in fact 50 ohms!! Discovered after noting the waveform RMS value on the DMM that was connected in parallel with scope, was very low. Thank goodness we weren't trying to pump 70VRMS into the scope, this would have tested the Amplifiers Over-Current and Over-Thermal Protection as well as the sacrificial source resistorAgree with bdunham7, some of the old Tek analog scopes sense the input voltage magnitude and alert by a LED and trip OFF the 50 ohm resistor if the input voltage is above 5 volts I recall. This is a very nice feature that is highly beneficial in saving 50 ohm front ends from overload, as well as likely saving some DUTBest,

author: tautech
author_rank: Super Contributor
timestamp: January 14, 2022, 08:24:09 am
content: Quote from: mawyatt on January 14, 2022, 02:47:44 amBest you get to the bottom of this and find out what set of circumstances led to the scope booting up so.By any chance did the scope get a hard power OFF ?If so it wouldn't remember it's last usage setting and instead revert to the previous power OFF settings.Still, even if in 50 Ohm input mode it should display such.