uCurrent for oscilloscopes? - Page 1
Forum section: electronics / projects

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 23, 2010, 09:47:06 pm
content: Has anyone considered designing a higher bandwidth version of the uCurrent for an oscilloscope? A low current version would be useful for measuring current ripple and a high current isolated version (along with an isolated voltage probe) would be useful for measuring input power and power factor of a switching power supply.

author: Unknown
author_rank: Unknown
timestamp: November 23, 2010, 09:58:27 pm
content: One issue is that very few scopes have floating inputs, unlike DMM's which are basically always floating. So you really need a differential-to-single-ended converter IMO, not terribly hard at a relatively low frequencies, but not trivial either.Tying one end of the shunt resistor to ground makes it less versatile (you have to pay careful attention to what other points of the circuit are grounded), and can pick up extra noise (eg. ground loops), especially if you're trying to measure tiny voltages (which is the whole point of the uCurrent). Depending on frequency, parasitics may also be an issue. Guess why the probe manufacturers are still selling current probes for $$$.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 23, 2010, 10:13:24 pm
content: A differential amplifier would probably be enough if only low voltage "isolation" is needed. If high voltage, high frequency isolation is needed, I have thought about modulating a RF carrier with the signal, isolating it using a transformer, and then doing synchronous detection.

author: Unknown
author_rank: Unknown
timestamp: November 23, 2010, 10:58:58 pm
content: Quote from: NiHaoMike on November 23, 2010, 10:13:24 pmYou don't need isolation for high voltage, just enough dynamic range (or attenuation to bring it into range), plus safety measures that limit the fault current to a safe level. This is how the commercial high voltage differential probes do it (of course high voltage in this context is anything above ELV, not what power guys call high voltage). It's pretty hard to have attenuation that won't destroy your CMRR, however, since for even 100:1 CMRR (which is pretty bad), the attenuation of both inputs needs to be within 1% over the frequency range.If you do need isolation, you're definitely talking about something much harder. Modulation would probably be the way to go (that's how some commercial designs do it, although some combine it with optical for low-frequency part), but orders of magnitude more complex than the uCurrent. Tektronix did in their TPS2000 series scopes, and I've read a fair amount of complaints about noise and slew rate.There's a Tektronix service manual for their isolation amplifier with full schematics floating around (older than the TPS2000 series, back when they still published schematics), although it's not exactly a design you'd replicate on a breadboard.

author: vivekgani
author_rank: Newbie
timestamp: December 30, 2013, 03:13:28 am
content: Really late reply, found this while looking at Dave's ucurrent kickstarter. I noticed the new version of the ucurrent (ucurrent gold) does 300khz bandwidth now instead of 8khz.While, it's not the speed of a microprocessor clock, it sounds like it's now good enough for using on an oscilloscope.

author: EEVblog
author_rank: Unknown
timestamp: December 30, 2013, 03:15:32 am
content: Quote from: vivekgani on December 30, 2013, 03:13:28 amYes, as I demoed in the KS video. Good enough to see fast changing processor modes etc.

author: philpem
author_rank: Frequent Contributor
timestamp: December 30, 2013, 03:23:16 am
content: I put together a 3.5MHz probe using an Analog Devices inamp, based on a design I saw on HackAday. That works fairly well, I could dig up more information if anyone is interested?

author: nctnico
author_rank: Super Contributor
timestamp: December 30, 2013, 11:26:47 am
content: Quote from: NiHaoMike on November 23, 2010, 09:47:06 pmI actually designed / sell an isolated high current / high bandwidth device like the uCurrent. But it starts to get useful at currents of at least 10mA due to noise of the Hall sensor.Using a differential amplifier is also an option but if you want to be able to handle up to several hundred volts then it becomes very tricky. Even with 0.1% resistors it is easy to get an error of 5% and poor CMMR. Therefore I built my device around a Hall sensor and a current transformer.