Review of Real Time Current Monitor Rev 2 please - Page 1
Forum section: electronics / general

author: EarthLord
author_rank: Contributor
timestamp: September 12, 2015, 01:09:12 pm
content: A lot of people are nowadays developing solutions that have micro amps or less sleeps current while needing many magnitudes higher processing current requirement. The Real Time Current Monitor Rev 2http://www.ee-quipment.com/collections/all/products/real-time-current-monitor-with-usbseems to be a low cost solution to current measurements in this kind of situation. How about a review (Yes, yes, a teardown) of this please? Cheers!

author: codeboy2k
author_rank: Super Contributor
timestamp: September 13, 2015, 04:41:15 am
content: Disclaimer: I don't own it, I've never touched it, so I'm completely un-qualified here to give a real review. Instead, here are my initial thoughts after reviewing what it does, looking at other reviews online and downloading and reading all the manuals.For $200 you don't get a proper BNC for your scope, it's not isolated from the DUT, and it's max input common mode voltage is only 5.5V, which is way too low for a bench instrument. It seems to be available in a plastic box, is probably not shielded, and thus might pick up noise from the bench. It is so cheap they couldn't be bothered to put in one more 30 cent op-amp to invert the oscilloscope voltage output. Instead you get a negative voltage (proportional to the log of the voltage across the sense resistor) so you must invert the oscilloscope channel to see it correctly. That's just lame and immediately shows me that the developer is not interested in making a truly high-quality measurement device, and is willing to cut corners and skip (skimp) on the details that differentiate a so-so device from a great device.At least this 2nd revision has a 1 kHZ bandwidth. That's still too small in my opinion, but better than Rev 1 which only had about 300 Hz bandwidth. The low bandwidth means you might miss the power consumption during a very fast wake-up, read an I/O port and sleep again. Basically it can't capture current spikes with rise times less than 1 ms, so any transient wake-up event that is shorter than 1 ms might get missed completely. Now, you can argue that decoupling caps on the DUT will mask these short-lived transients anyways, but a current monitor with a high-enough bandwidth would be able to let you know when your decoupling is actually too small. For a current monitor that is supposed to be useful for monitoring high-speed modern micros, you don't want it to be missing things and leave you guessing.It still only goes down to 1 uA, which is not going to be useful for many of today's modern processors that have sleep states that consume in the nanoamps. It needs to go down to 1nA to make it useful for monitoring recent and future microcontrollers (especially at that price point!). I want something that will last a few years or more.The USB connection appears as a Mass Storage Device (MSD), and 2 serial ports. The MSD appears to be used only for firmware updates. The serial port sends comma separated serial data, constantly streamed to the terminal as it samples the DUT voltage and current and computes power. (you can change the sample rate via a terminal to the second serial port, which is the command port). If you want to save the data you have to turn on data logging in the terminal software and save it to your PC's disk drive as it streams.Teardown? it's probably a current sense resistor (or two resistors switched via FET's), a log-amp, and a PIC micro with USB. Instrument power is via the USB port, so it probably has a charge pump voltage inverter there as well, maybe done via the PIC too.I would buy it at $50. It absolutely needs to have a higher bandwidth (50-100 kHz) and the nA ranges for me to part with $200.

author: EarthLord
author_rank: Contributor
timestamp: September 14, 2015, 09:50:16 am
content: Thanks @codeboy2k for your detailed thoughts of this device. As good as a review for me.Any recommendations for an instrument that for this task at this price range? For me a much higher bandwidth (500-1000 kHz) would be of greater importance than nA resolution. Or why not both?

author: Bud
author_rank: Super Contributor
timestamp: September 15, 2015, 01:05:48 am
content: There is nothing to tear down, the picture on the web site is a rendering, not a photo of an actual product. Guess they do not have it.

author: codeboy2k
author_rank: Super Contributor
timestamp: September 15, 2015, 07:27:29 am
content: Quote from: Bud on September 15, 2015, 01:05:48 amYeah, I also found that strange, but they had rev1 in 2013 supposedly, and rev 2 in 2015, ... but maybe they've never sold one and thus never produced it

author: EarthLord
author_rank: Contributor
timestamp: September 15, 2015, 09:28:22 am
content: There's this review here by Jack Ganssle who's quite reputed in the embedded world.http://www.embedded.com/electronics-blogs/break-points/4440109/Real-Time-Current-Monitor-Rev-2My interest perked up after reading this. Still strange that they just have a rendering on their site.