Looking for affordable isolated optical voltage probe - Page 1
Forum section: electronics / testgear

author: KlausKragelund
author_rank: Regular Contributor
timestamp: August 21, 2025, 09:42:39 pm
content: HiI need to measure the switch nodes in a power supply, and need a voltage probe with good HF CMRR.I have the Migsic differential probes, DP10013, but being cheap, they have horrible noise and HF CMRR.I have looked at the optical probes:https://www.batronix.com/shop/measurement/probes/Micsig-MOIP100P.htmlBut they are at 4000 USD and up, and I cannot afford that right now.Any alternatives?Used Tek probes?ThanksKlaus

author: bateau020
author_rank: Frequent Contributor
timestamp: August 22, 2025, 06:50:17 am
content: This comes up a lot. Just some thoughts:* 2 channels + math on your scope?* USB scope + a good USB isolator? The USB isolator will be handy anyway. (Intona, Alldaq,.. but be aware that support for some USB speeds may be lacking)* battery powered scope (native or via UPS) + thick gloves?* float the power supply? (but this may not work out with HF noise)

author: BH3XON
author_rank: Contributor
timestamp: August 22, 2025, 07:01:13 am
content: Migsic SigOFIT MOIP seriesand Cybertek OPL6000 series are both laser-powered,optically isolated voltage probes.

author: KlausKragelund
author_rank: Regular Contributor
timestamp: August 22, 2025, 11:33:07 pm
content: Quote from: BH3XON on August 22, 2025, 07:01:13 amThanks, I did look at the MOIP probes, quite expensive, which is why I was looking for alternatives

author: KlausKragelund
author_rank: Regular Contributor
timestamp: August 22, 2025, 11:40:02 pm
content: Quote from: bateau020 on August 22, 2025, 06:50:17 am2 channels -> could be an option, but then need perfect matchingUSB scope -> There's a significant capacitance over the USB isolator link, so will pushing current into the PCBattery power -> A lot of CM capacitance, so the measurement will affect the DUTFloating -> Same problem

author: bateau020
author_rank: Frequent Contributor
timestamp: August 23, 2025, 06:32:25 am
content: Quote from: KlausKragelund on August 22, 2025, 11:40:02 pmJust measured some:Alldaq USB3.0 superspeed: 3,3nF@100kHz => this is not OK of courseIntona 7055-D: 30pF@100kHz => could be doableYou should probably power your USB scope from a battery, or use a medical grade PSU.Quote from: KlausKragelund on August 22, 2025, 11:40:02 pmPure battery power? That should be even less than the above (unless you hook up a boat anchor).Don't quite know your needs, but Micsig has portable scopes, some people here have managed to adapt some of the newer scopes (ex: siglent) to battery power, and there are tools like the ZOYI ZT-703s.

author: Fungus
author_rank: Super Contributor
timestamp: August 23, 2025, 08:22:57 am
content: Quote from: KlausKragelund on August 21, 2025, 09:42:39 pmCan you afford a battery powered oscilloscope?

author: KlausKragelund
author_rank: Regular Contributor
timestamp: August 23, 2025, 01:39:33 pm
content: Quote from: bateau020 on August 23, 2025, 06:32:25 amThanks for the measurementsSay we have 100pF across the barrier, a switch node at 10V/ns will produce 1A of current during the transition, and if its a PSU switching at 100V/500kHz, the added loss will be 0.5W. This can be significant part of the loss of a converter.Quote from: bateau020 on August 23, 2025, 06:32:25 amA floating scope can easily have 100pF capacitance to earth, simply due to the metal block size in free space

author: KlausKragelund
author_rank: Regular Contributor
timestamp: August 23, 2025, 01:40:32 pm
content: Quote from: Fungus on August 23, 2025, 08:22:57 amYes, but it will not solve the issue (see other post)

author: slugrustle
author_rank: Frequent Contributor
timestamp: August 23, 2025, 03:12:25 pm
content: Do you need to measure the switch node itself or do you need to measure something riding on the switch node like a high side gate drive? For the latter, an optical probe or special isolated input scope (like the Cleverscope CS548, also maybe Tek TPS2000B series) is the only solution I know of.If you're just looking at the switch node itself and can earth the negative reference of the switch node, then a passive scope probe with an appropriate voltage rating should work fine. The other day, I measured a 400V PFC switch node with ~18ns rise and fall times (~22V/ns) using, in turn, a 100x passive probe and a 10x passive probe, the latter after I realized the voltage was within the probe rating. Tip capacitance is 8pF or 10pF. I was able to earth the negative reference because I was powering the unit from a programmable AC source with an isolated output. There was only 14mA of ground current, measured with a 50MHz clamp. I earthed the negative reference through a fuse and measured the ground current before creating a solid connection, haha.If your switch node negative reference is riding on some other signal, a differential probe should work as long as that other signal doesn't have very high dV/dt. Before I realized I could ground my primary side negative reference, it was riding on the programmable source AC output, so up to 240V 50Hz. I found a Pico TA043 (100MHz) diff probe worked fine. IthinkI looked at the PFC switch node with this probe. I definitely looked at an LLC half bridge switch node with this probe, and that also has fast 0V↔400V transitions.Quote from: KlausKragelund on August 23, 2025, 01:39:33 pm0.5W doesn't sound bad to me. Obviously don't run efficiency test with the probe connected to the switch node.

author: nctnico
author_rank: Super Contributor
timestamp: August 23, 2025, 03:50:46 pm
content: Another way out is to use a generic probe with a high resolution oscilloscope. Nowadays there is a whole flurry of low noise 12 bit scopes out there. These might have enough resolution to resolve the switching node's signal with the probe grounded at the negative / ground of the circuit. Capacitive loading shouldn't be that bad. Maybe use 2 channels to subtract the signal and create a differential probe mathematically. 8 bit DSOs run out of steam quickly when using 2 channels to measure differentially, but having 12 bits opens up more possibilities.Yokogawa has floating input modules for their oscilloscopes. Like the 720212 module (40MHz / 14bit) for the DL950 scopecorder. Maybe this can be found on the used market for a reasonable price.

author: Brad O
author_rank: Regular Contributor
timestamp: August 26, 2025, 02:40:29 pm
content: I think you realize this, but isolated probes are so expensive because they offer those unique measurement capabilities. A used probe (or a rental?) might be your best bet if you need the performance.Some Tek resources on this:IsoVu probe v. differential probe and why lead length matters:Basically a text version of that video:https://download.tek.com/document/51W_60956_0_IsoVu_TB.pdfApp Note that covers some of the physics behind high-side switching:https://download.tek.com/document/53W_60779_0_IsoVu_Vgs_AN.pdf

author: KlausKragelund
author_rank: Regular Contributor
timestamp: August 27, 2025, 10:33:08 pm
content: Quote from: slugrustle on August 23, 2025, 03:12:25 pmI need to measure, riding on the switch node. The problem is the CMRR at higher frequencies, which get really bad at +10MHz, so bad measurements can not be trusted with standard differential probesQuoteYeah, 0.5W is not that bad, and can be removed for efficiency measurements :-)

author: KlausKragelund
author_rank: Regular Contributor
timestamp: August 27, 2025, 10:34:58 pm
content: Quote from: nctnico on August 23, 2025, 03:50:46 pmI have a 12 bit Rigol scope, so I will try it out. Needs very good matching of probes to work at higher frequencies, but it's worth a try :-)

author: KlausKragelund
author_rank: Regular Contributor
timestamp: August 27, 2025, 10:42:14 pm
content: Quote from: Brad O on August 26, 2025, 02:40:29 pmGood information, and yes, the CMRR is the problem. I guess I someday will get lucky and find a IsoVu probe at Ebay

author: Phil1977
author_rank: Frequent Contributor
timestamp: August 28, 2025, 10:05:21 am
content: A high quality isolated probe is of course the most versatile and reliable option.Sometimes it´s possible to workaround by measuring the DC and LF with two channels of a 12-Bit-scope using the subtraction mathematics. For higher frequencies I sometimes used a RF transformer for galvanic isolation:+ o---OOO---o | o---o Scope Signal25Ohm | | || | |330pF = | || | |25Ohm | | |- o---OOO---o | o---o Scope GNDI hope the "ASCI-Art" is understandable... Of course it has no perfect CMRR, but it´s orders of magnitudes better than all experiments with floating scope GND.