Decent isolated probe with good enough CMRR for high side FETs - Page 1
Forum section: electronics / projects

author: uer166
author_rank: Super Contributor
timestamp: December 20, 2020, 12:38:15 am
content: Does anyone know of a decent cheap-ish, <$500 isolated probes (ideally with a coax connection to the DUT) that can be used for high side FET Vgs measurements? I'm looking at something at least 400V isolation rated cat 2, that would measure a 15V gate swinging over 300V with about 20-100ns rise time.I know there's been plenty of discussion and recommended units, but I wonder if something more up-to-date exists. Ideally I'd get a Cleverscope but this is for personal projects..

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 20, 2020, 01:24:19 am
content: Looked for the same recently, but still only find the TA041http://www.farnell.com/datasheets/2207207.pdfAt 25MHz, it is good for rise times of 0.35/25MHz = 14nshttps://www.picotech.com/library/oscilloscopes/rise-timeI suppose the only other way for <500$ would be a cheap battery scope.

author: uer166
author_rank: Super Contributor
timestamp: December 20, 2020, 01:40:00 am
content: Quote from: opampsmoker on December 20, 2020, 01:24:19 amInteresting idea, but I'm not quite sure if I'm ok with the entire scope, including the touch panel, buttons, and connectors swinging at 300V at 50kHz relative to the GND I'm standing on..

author: BrianHG
author_rank: Super Contributor
timestamp: December 20, 2020, 04:49:42 am
content: You can get this 4 channel 1kv isolated true 14bit scope:https://www.eevblog.com/forum/blog/eevblog-1119-designing-a-1kv-isolated-oscilloscope/msg1804895/#msg1804895Oooops, sorry, you said <500$, not <10000$.

author: uer166
author_rank: Super Contributor
timestamp: December 20, 2020, 06:24:01 am
content: Quote from: blueskull on December 20, 2020, 05:04:06 amGood advice, I think a dedicated HV scopemeter (not just a random battery powered scope) can be a good compromise. $500 was mostly arbitrary, if there's anything easy to use that solves the issue in under $2k that would be considered as well.

author: uer166
author_rank: Super Contributor
timestamp: December 20, 2020, 07:07:17 am
content: What about Tektronix TPS2000? We have a few at work I can borrow, but it's designed for mains in mind. Also, I don't necessarily need more than one channel here, i.e. channel-to-channel isolation doesn't matter, even getting one Vgs traces would be good enough.

author: alm
author_rank: Super Contributor
timestamp: December 25, 2020, 09:19:54 am
content: How much capacitance to ground can your DUT tolerate without affecting the measurement, or causing fireworks? Isolated scopes will generally have a quite high capacitance between the low/reference lead and ground. The TPS2000 is specified as 37 pF between reference lead and ground, which is not horrible, but is a lot more than a differential probe.So with an isolated solution, be mindful where you hook your reference lead to!

author: David Hess
author_rank: Super Contributor
timestamp: December 26, 2020, 05:10:06 pm
content: Quote from: uer166 on December 20, 2020, 12:38:15 amIsolated probes do not really have a CMRR in the normal sense but they do have common mode dV/dT limits. Their biggest limitation is common mode capacitance to ground which means that the ground/common side needs to be connected to a low impedance point but in gate drive applications that is available at the source so not a problem.The least expensive option may be a DSO which has isolated inputs. Siglent and Fluke have handheld DSOs with isolated inputs, and Tektronix has the TPS2000. The Siglent SHS806 can be had new for less than $500; maybe someone can common about it.Using one of the tiny and cheap battery powered DSOs even without real isolated inputs might be an acceptable option in this case albeit not the safest.