Revision 1.3 Joulescope JS220 User’s Guide Page 32 / 51
www.joulescope.com
unique algorithm that runs in an FPGA. The FPGA selects the active shunt resistors using MOSFETs,
which were carefully qualified to meet Joulescope’s demanding performance characteristics.
The shunt resistor selection does take some time. How fast does the shunt resistor need to change to
have a minimal impact? The linearized equation is:
t = C * ΔV / ΔI
Let’s take a typical example. The target device takes 3.3V and can tolerate a temporary 3% (0.03)
voltage glitch on a 1 amp change. If the target device has 10 μF of capacitance, the required shunt
resistor switching time is approximately:
10 μF * 3.3 V * 0.03 / 1 A = 1 μs
Joulescope was designed to achieve 1 µs switching speed to meet this demanding case. Most target
systems have larger capacitance and less dramatic step current changes. In all cases, the shunt resistor
selection is always make-before-break to ensure uninterrupted connectivity to the target device.
10.2. Voltmeter
The voltmeter measures the voltage between Voltage + and Voltage -. The voltmeter scales the input
voltage, and uses an analog-to-digital converter (ADC) to digitize the value. The Joulescope instrument
sends the voltage samples, along with the current samples, to the host computer. The software applies
the calibration to convert from ADC values to actual voltage.
The voltmeter has two different ranges, and you can manually select the best range for your application.
The 2V range provides higher resolution for smaller voltages, while the 15V range allows for larger
voltage measurements.
10.3. Host Computer
The Joulescope instrument requires a host computer to operate. The Joulescope instrument sends 2
million current and voltage samples per second immediately to the host computer over USB. Unlike
traditional oscilloscopes, a Joulescope is always capturing data. The terms “trigger” and “sample buffer”
do not fully apply to the Joulescope device, since all sample data arrives at the host computer. The
Joulescope User Interface (UI) temporarily stores capture data in a circular buffer. The UI allows you to
adjust the buffer size to use your host computer’s RAM. Joulescope can also record to a file, and the
capture size is only limited by your computer’s available storage on your hard disk drive or solid-state
drive.
The Joulescope host software Is open-source. The pyjoulescope “driver” communicates with the
appropriate operating system USB driver. Joulescopes use Microsoft’s WinUSB on Windows, which is
included by default with the Windows operating system. On Linux and macOS, Joulescope uses the
popular, open-source libusb driver. The pyjoulescope driver provides an easy-to-use scripting
environment. You can also use the driver to build integrated applications, such as the Joulescope UI,
that use the Joulescope instrument data.
10.4. Power, Energy, Charge, and Statistics
Joulescopes only measure current and voltage. The Joulescope software uses math to compute the
power, energy, and charge. Power is the current multiplied by the voltage:
To Next Page