#527 – Measuring Current with Matt Liberty

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
• Linkedin