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You are here: Home / Guest Appearance / #527 – Measuring Current with Matt Liberty

#527 – Measuring Current with Matt Liberty

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Welcome Matt Liberty, creator of the Joulescope!

  • The Joulescope solves the problem of measuring current with a high dynamic range.
  • There are very different sense resistors when measuring active current vs sleep current
  • Chris learned this when working on electrometers at Keithley.
  • The key thing is keeping the burden voltage low so it does not brown out low voltage systems
  • Chris is planning on using the Joulescope to measure the ABC board
  • Matt recommends starting with a power budget
  • It’s also important to understand how to set up triggers for the joulescope, such as a GPIO
  • Joulescope sits in the middle of the device under test (DUT) and the power supply of the system.
  • The front panels can be swapped out. It comes with USB and binding posts by default.
  • Default view is multimeter view, but the real magic is looking at the power profile.
  • There are many pitfalls in low power electronics, such as backpowering a pin
  • Some people are using Joulescopes in opearations/deployment to test devices are performing as expected.
  • There is an open source Python library
  • Matt describes why some elements are open vs closed source
  • GPIB
  • LXI
  • Most traditional test equipment doese a capture and transfers the buffer (either to a computer or a screen)
  • The Joulescope and other headless equipment streams the data. This is similar to the HackRF (episode) and Saleae (episode).
  • Streaming vs Buffering
  • Isochronous timing
  • USB limitations means you cannot have too many Joulescopes that are plugged in to a single system.
  • Competition
    • High end test equipment from Keysight
    • uCurrent
    • Current ranger
    • Nano ranger
    • Otii Arc
    • Power Profiler 2 kit
  • This is a different way of working and might feel weird to people that grew up with knobs and dials
  • Matt’s background is in consulting, mostly around firmware (though he does hardware and FPGAs)
  • A main task while consulting was working with the firmware to lower power, hence the desire to build the Joulescope
  • When hunting down current problems, Matt recommends “Divide and conquer”. Other things to look at:
    • Check voltages across resistors
    • Pullups to VCC and not turning off power rails properly
    • Backpowering pins via protection diodes
    • Odd problems he has seen
      • Flux residue causing more leakage current than expected
      • Capacitor leakage (through the series resistance)
  • Matt was on Embedded.fm a couple of years ago
  • Lower power modes in STM32 clock tree
  • Matt’s tactic for a simple low power system: “Turn everything on, do what you need, go back to sleep”
  • Matt has discussed struggles to get the product out in the world on the Consulting Forum.
  • Matt has been a solo consultant since 2011. He knows how to carve out consulting time, in this case his “client” was his own project.
  • But why develop a product?
  • Matt’s Contract Manufacturer (CM) is 15 mins up the road
  • Matt has set up a test station at his CM and trained the technician who watches the devices that fail testing.
  • Parts on allocation
  • Lot size is still 500
  • Matt was recently on the Hello Blink show talking about hiring subcontractors. He has managed employees in the past at Hillcrest.
  • 2 FPGAs internally, both of them Ice40 (but not using the open toolchain yet)
  • This is an isolated design, meaning you can safely plug it into your USB port and whatever is being tested is galvanically isolated.
  • Device side FPGA does math on the other side of the isolation barrier
  • The host side FPGA doesn’t do as much
  • Device FPGA is there to be really responsive and to handle both ADCs in lockstep
  • Open source FPGA toos
  • Current model is the JS110, Matt is not sure on other models yet. He would like to focus on two models that try to go lower cost / higher performance (2 separate things)
  • CMRR
  • Possibly going to make a module that acts like a high current sensor
  • Supporting multiple versions of hardware
  • Matt’s lab is all scripted using python
  • Buy a Joulescope for $799, more than 1/10th the price of a similar class of instrument.
  • Matt now has two distributors overseas, and is hoping to be part of the Digikey marketplace soon, to avoid needing to get on Approved Vendor Lists (AVLs) at large clients.
  • Twitter
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Comments

  1. Moien says

    January 25, 2021 at 10:06 am

    Hey guys,

    I have been listening for a while now but I reviewed the show notes
    for the first time today. Its a great reference and helps a lot in following the discussion. Thanks a lot.

    Best
    Moien

  2. Ido Gendel says

    January 25, 2021 at 1:36 pm

    Before even listening to this episode… someone contacted me not long ago, regarding an IoT board of his that mysteriously drained batteries too fast. It was such a breeze finding the issue using a Joulescope! (Yes, it was a software bug 🙂 )

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