Taking differential measurements w. USB scope + laptop - Page 1
Forum section: electronics / testgear

author: Lomax
author_rank: Frequent Contributor
timestamp: November 25, 2017, 01:52:25 pm
content: For various reasonsI've been wanting to get a USB oscilloscope for some time, and Picotech's 20% off "Black Friday" offer finally pushed me over the edge, thus I'm now excitedly expecting the delivery of a Picoscope 2206B MSOWhile I'm waiting for it to arrive, I've spent some time brushing up on scope safety - lest I swiftly damage or destroy my new toy, or the equipment under measurement - or indeed myself! The 2000 series Picoscopes have several draw-backs compared to desktop instruments, not only the lack of physical buttons, but also a rather weak +/- 20V max input voltage, which means I'll have to be extra careful. So I thought I'd double check here; have I understood correctly if I assume that a Picoscope connected to a laptop which is running off its battery (charger disconnected) can safely be considered to be "floating", thus negating the need for a (pricey) differential probe?

author: nctnico
author_rank: Super Contributor
timestamp: November 25, 2017, 05:26:44 pm
content: No because all metal parts will be connected to whatever you clip the ground of your probe to. The best option is to get a differential probe. The one from MicSig offers extremely good value for money.https://www.telonic.co.uk/Micsig-DP10013-High-Voltage-Differential-Probe-p/mi-dp10013.htm

author: MrW0lf
author_rank: Frequent Contributor
timestamp: November 25, 2017, 05:32:01 pm
content: I have several Picos and they all just fine. Oldest ones were used even on low power HV up to 15kV. But I never played with concept of floating the scope using laptop for other purposes than mitigating mains interference etc.For HV stuff did use x100 probe. Important is to keep it always on DC coupling if operating above x1 safety limits. Internal AC cap may charge to full voltage not minding the probe division ratio.For dangerous power levels diff probe only option, this is very sane practice not only for Picos.I did purchase mine to complement 25MHz scopes so it was Pico TA041 x10/x100 differential.There are much more options now.Overall do not worry. Keep it sane and it is very unlikely that you burn it.

author: SparkyFX
author_rank: Frequent Contributor
timestamp: November 25, 2017, 06:43:35 pm
content: Quote from: Lomax on November 25, 2017, 01:52:25 pmI wouldn´t rely on it, any other equipment you connect to a battery powered laptop could supply ground to it ... e.g. USB device with external power, network cable (?), external monitor and so on. The system is not guaranteed to be isolated.A differential probe is therefore the way to go if you want to measure > +-100V DC+AC peak on the analog channels or +- 50V on the digital channels (as stated in the spec).Otherwise you can still short to ground and damage the probe and various other parts by having ground reversed to a low impedance power supply. As it is USB powered the circuit includes the PCs USB host controller.To be sure about what is appropriate, measure the voltage with a multimeter between BNC ground and ground point of device under test before making a connection. The tricky part is estimating the current needed to bring both to the same level without destroying one or both. The differential probe takes all of that out of the equation.

author: Pitrsek
author_rank: Regular Contributor
timestamp: November 25, 2017, 09:41:41 pm
content: It's floating till you touch it.... IMHO not really good idea, imagine being electrocuted while unplugging a usb drive..Isolation transformer/diff probe is way to go.

author: Lomax
author_rank: Frequent Contributor
timestamp: November 27, 2017, 11:34:00 am
content: Thanks everyone - that certainly clears things up!

author: Lomax
author_rank: Frequent Contributor
timestamp: December 02, 2017, 10:28:18 am
content: Ok, so I've spent a few days looking at differential probes, and I'm disappointed to see that they're invariably large and expensive devices. The Micsig DP10013 is the only one I've been able to find with a price that doesn't compete with my Picoscope for being the most expensive piece of measuring kit in my toolbox, but it'shugeand has a 50x minimum attenuation. I have never dealt with anything above mains voltage, nor do I have any need to, and I value portability more than most. My main interest in getting a differential probe is for looking at low voltage differential signals, and taking non-ground referenced measurements in low voltage equipment (e.g. RS-485 comms, audio amplifiers). Does anyone have suggestions for a <$200 differential probe which is small in size and has a 1x, 5x or 10x minimum attenuation? Bonus points for AAA battery or USB power and removable test leads!

author: SparkyFX
author_rank: Frequent Contributor
timestamp: December 02, 2017, 02:07:49 pm
content: There used to be schematics, plans and pcb layouts in electronic magazines for making differential probes yourself (the requirements vary with the task of course, but the typical SMPS frequencies < 1 MHz don´t have very high requirements unless you do EMI testing). But if you are new to this and just need a working, good probe... better go with the commercial solution. You get the isolation values and bandwith you pay for, and have a safe working environment.Anyway, depending on the task you could also try to do as many measurements feasible with a handheld multimeter, it is isolated, differential by design and does provide some useful information about a circuit as well.Older, slower scopes used to have a 400V input capability and you could connect the mains neutral input of the circuit to scope ground (or rely on it being connected).All in all there is no one size fits all, you may need to change your test strategy based on the circuit you want to measure and what signal you want to analyze. As soon as there is an inverter in there or an isolation transformer mains GND may not be circuit GND and you run into the problems for which to get a differential probe for.

author: nctnico
author_rank: Super Contributor
timestamp: December 02, 2017, 02:11:52 pm
content: Quote from: Lomax on December 02, 2017, 10:28:18 amSearch for isolation amplifier. AFAIK Siglent has one but the bandwidth isn't stellar.

author: Lomax
author_rank: Frequent Contributor
timestamp: December 02, 2017, 05:41:31 pm
content: Thanks guys. My scope only has a 50MHz bandwidth, so anything higher speced would a bit wasteful.

author: ADT123
author_rank: Regular Contributor
timestamp: December 02, 2017, 06:26:57 pm
content: If you are measuring non hazardous (ie not high enough voltage to hurt you) differential or signals such as RS-485 (and many audio) then you can use 2 scope channels and maths to produce a differential measurement. Have a read herehttps://electronics.stackexchange.com/questions/76514/how-do-i-measure-differential-signals-like-rs-485-or-dmx-on-an-oscilloscope

author: Lomax
author_rank: Frequent Contributor
timestamp: December 07, 2017, 06:22:21 pm
content: This turned out pretty well; a sturdy & waterfproof case for my Picoscope & accessories. Still room for some coax cables and adapters - but definitely not for a differential probeIn case(!) anyone wants to know, it's aMAX003S from Plastica Panaro. Cost ~£20 delivered - a steal compared to Peli etc.