Isolated differential probe - Page 1
Forum section: electronics / projects

author: johnwa
author_rank: Frequent Contributor
timestamp: March 24, 2017, 11:49:04 am
content: Seeing Dave & Dave talking about designs for current probes reminded me that I was going to write about a little project that I designed recently. I had been wanting a differential voltage probe for a while, but not enough to justify shelling out for a prebuilt one. I saw Jim Rowe's design in Silicon Chip, and thought about building one of these. This is quite an impressive design - 80MHz bandwidth - but has the unfortunate disadvantage of being AC coupled. It is really designed for high frequency, small signal work, while I was more after a device for general purpose work where safety isolation is desirable.After looking at various design options (a proper differential design like Jim's, digitising in front of an isolation barrier, linear optocouplers (basically everything the Daves talked about)), I settled on the HCPL-7520-300E optoisolator. This has the ADC and DAC all built in, and basically only requires a power supply on each side. Although the bandwidth is only 100kHz, this should still be adequate for quite a few situations, and it is not too expensive at ~ AUD $7The power supply for the module presented a few issues. I was initially going to use batteries for both power supplies, but couldn't fit them it the case I wanted to use. Then, I decided to try Jim's idea of using USB power from the scope, with an isolated DC-DC converter for the input side. However, I ran into trouble finding a converter with a suitable insulation rating. Eventually, I decided on a combination of the two approaches, using a battery for the input side, and external power for the output. A second optocoupler switches power to the input circuit when the output circuit is powered up, so the input is shut off automatically when the attached oscilloscope is powered down, preserving the battery.Unfortunately, the module ended up being a bit more complex than I had first hoped - a negative rail generator and some op amps were needed for span and offset adjustment, and I also decided that a clipping indicator was necessary. But it all seems to work OK. I don't have any plans to manufacture these in quantity at this stage, but I can probably publish some schematics and board files if people are interested in building one up.

author: vealmike
author_rank: Regular Contributor
timestamp: March 24, 2017, 12:16:18 pm
content: Interesting.Is there a forum thread about DIY current probes? I searched and couldn't find one.I have literally (10 mins ago) finished throwing together a small PCB for a current probe. The boards have just been ordered. Essentially I got fed up with air wiring one of these every time I needed to measure a current.It's based on the INA21x series from Texas, so it is not isolated, but can handle a common mode of +26V (IIRC.)By fitting the right chip, voltage gain goes from 50 to 1000, a single 2512 sense resistor can be fitted to the board, or you can leave it off & tap a resistor on the DUT.The Ref input of the INA21x is driven by a precision vref, and a buffered copy is available to drive a second 'scope channel. This should allow me to accurately set the zero current, and avoids the need for a split rail.Not sure how this will perform. The INA series are not precision parts. Gain will not be perfect, frequency range may be limited, but the board should be able to measure mA dropped across very low mOhm sense resistors.Happy to share when I have confirmed it works!

author: johnwa
author_rank: Frequent Contributor
timestamp: March 25, 2017, 07:54:42 am
content: Hi,I don't know if there is anything on the forum relating to current probes - I didn't see a thread about the latest video, but there might be something older about the micro currents, I haven't really been following that closely.The INA219 is not a bad little part - we have got some test gear at work that uses these. I haven't used them for really low current measurements, but I would think they should work OK given a suitable shunt resistor.

author: MagicSmoker
author_rank: Super Contributor
timestamp: March 25, 2017, 11:21:25 am
content: Usually one designs either a differential probe or an isolated one, as they are two different approaches to the same problem: making safe/meaningful measurements when neither probe input is common with the oscilloscope's ground. Generally speaking, differential probes are preferred when measuring wide bandwidth/small signals in the face of a much large common mode voltage while isolated probes are preferred for things like motor drives, power supplies and the like, where signal amplitudes are large and/or low frequency (<1-2MHz) and the main problem is a lack of common ground, rather than the need to make a true differential measurement (ie - between two points in a circuit which are both offset from ground).

author: johnwa
author_rank: Frequent Contributor
timestamp: March 25, 2017, 11:38:57 am
content: Yes, I was more thinking of motor drives and inverters when I designed this. It doesn't actually have a differential input stage as such - only the intrinsic differential measurement ability by virtue of the floating input. One good thing about this approach is that you don't have to muck around getting the complex gain (magnitude and phase) of each channel to match, though this is of course less of a problem at 100kHz than at higher frequencies.

author: Kalvin
author_rank: Super Contributor
timestamp: March 25, 2017, 03:28:46 pm
content: You could possibly get a wide bandwidth solution by combining an optocoupler and a transfomer stages. The optocoupler signal path would handle the common mode DC-component and low-bandwidth signal path up to 100 kHz, and the transformer signal path would take care of the high bandwidth signal path from 100kHz up to tens of megahertz. I haven't built nor simulated this, just an idea.

author: alm
author_rank: Super Contributor
timestamp: March 25, 2017, 04:15:42 pm
content: I believe that is how Tektronix did it in the A6902 isolator. What generally limits the high-frequency performance is capacitive feedthrough to ground, which limits the isolation at higher frequencies. But the impedance to ground at low frequencies can be very high. This is unlike differential amplifiers where the performance is often limited by a limited common mode rejection at higher frequencies.

author: Kalvin
author_rank: Super Contributor
timestamp: March 25, 2017, 04:28:56 pm
content: Quote from: alm on March 25, 2017, 04:15:42 pmYes, true. The PDFhttp://w140.com/tek_a6902b.pdfpage 81 and onwards has the schematics.

author: David Hess
author_rank: Super Contributor
timestamp: March 26, 2017, 09:08:29 pm
content: I was going to mention the Tektronix A6902 but alm beat me to it.An isolated probe could also be made by driving a current into an AC/DC current probe.

author: johnwa
author_rank: Frequent Contributor
timestamp: March 29, 2017, 07:38:57 am
content: Quote from: Kalvin on March 25, 2017, 04:28:56 pmThanks for posting this Kalvin, it is good to see how a 'proper' one is made. Interesting to see that they used two (presumably matched) optocouplers, rather than something like an IL300. I remember seeing something similar in an oscilloscope Z mod circuit, where the DC was used to offset the ground of an HV supply, but a capacitive bypass was required for the high frequencies.I was a bit concerned about the isolation capacitance of my design, but it measures at around 20pF, which is actually better than the spec for the Tek unit. Of course, you still need to be a bit careful about hanging these probes on something flapping around at really high frequencies.

author: David Hess
author_rank: Super Contributor
timestamp: March 29, 2017, 11:25:43 am
content: Quote from: johnwa on March 29, 2017, 07:38:57 amI assume that either the IL300 and similar linear optocouplers were not available then or lacked the required performance. The parts list says that the optocouplers were screened and matched.QuoteTransformer and charge pump based DC restorer circuits all work that way simply because they require an output capacitor to filter their ripple. Even simple level shift circuits often have an AC bypass capacitor to preserve high frequency response.

author: Ghizz02
author_rank: Newbie
timestamp: December 26, 2018, 01:36:04 pm
content: Hi, I really like your work.I was wondering if it was possible for you to share some informations about your differential probe.I would really like to build a couple for personal use and after seeing all other designs your is the best option.Thanks.

author: eric appla
author_rank: Newbie
timestamp: November 18, 2019, 09:28:28 pm
content: Hi, I hope youre still around on EEVblog, this is pretty much ideal probe I'm after, would you mind sharing gerber file and it looks like you 3D printed the enclosure I would most welcome if you could share the file too. Im even haoppy to pay for it, its spot on beautiful small and simple yet it has quite good parameters