7. Theory of operation#
Joulescope simultaneously measures current and voltage, and then computes power and energy. Joulescope combines a voltmeter and a fast auto-range current shunt ammeter.
7.1. Ammeter#
The shunt ammeter consists of six (6) current ranges with six (6) different shunt resistor values ranging from 0.01 Ω to 1111 Ω. Joulescope actively selects the best shunt resistor to keep the maximum voltage across the shunt resistor to 20 mV. This low voltage drop minimizes the impact on the device under test. Other equipment, including standard multimeters, have a much larger burden voltage that can cause the target device to behave unusually.
The Joulescope amplifies the voltage across the shunt resistor and then uses an analog-to-digital converter (ADC) to digitize the value. The ADC digitizes the value 2 million times per second to create 2 million samples per second (SPS). The Joulescope instrument processes these ADC values on the instrument using a sensor-side FPGA.
Given a known resistor value and the measured voltage, the Joulescope uses Ohm’s Law to compute current. The FPGA applies calibration coefficients to convert the measured voltage into the measured current. Joulescopes are individually calibrated and tested in the factory to ensure that every Joulescope performs to specification.
The instrument then downsamples by 2 from 2 Msps to 1 Msps. This downsampling saves USB bandwidth without affecting the 300 kHz measurement bandwidth.
7.1.1. Autoranging#
As the current changes, the ammeter selects the optimal shunt resistor value. Joulescope uses dedicated hardware to quickly select the appropriate shunt resistor to avoid impacting the device under test. The shunt value selection uses the ADC output, dedicated over-range analog hardware, and a 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.
7.1.2. Optimizing for Accuracy#
For many applications, the JS220 performs sufficiently without any additional configuration. However, additional configuration can improve accuracy for measuring lower-current systems, such as Bluetooth devices. Any system that never consumes more than 180 mA can benefit from additional configuration.
When the JS220 switches current ranges, the JS220 activates a MOSFET to select the new shunt resistor and deactivates the MOSFET for the previous shunt resistor. All MOSFETs are effectively capacitors between their gate and the source/drain. Activating and deactivating a MOSFET results in charge injection. While we have carefully selected these MOSFETs to minimize charge injection, it is not zero. You may observe a brief spike on the order of 300 ns. The JS220 may also measure this spike, but it will be spread in time due to the JS220’s 300 kHz measurement bandwidth.
The second effect is that the impedance from Current + to Current - changes very quickly. Many targets have large bypass capacitors. Since capacitors resist instantaneous voltage changes, the result is that a real current may flow over the shunt resistor to charge or discharge the capacitor to account for the new JS220 impedance. This current is real, and it cancels out over time. However, it’s often not what we want to measure, so it can be distracting for some systems.
When the JS220 autoranges on over-current, it always autoranges up to the maximum configured range, which is 10 A by default. It will then downrange as needed to the optimal range. While this behavior guarantees the 1 µs transition time for all support loads, it may result in increased error due to the 10 A current range error rating. You can limit this maximum range for lower current targets to improve performance:
When setting the current limit, ensure that the waveform never exceeds the current range value when viewed at the full 1 MHz sample rate. Otherwise, the JS220’s measurement will saturate, causing significantly increased inaccuracies. The JS220 does contain a soft-fuse feature for each current range to help prevent damage to the instrument if you select too low of a maximum current range.
You can also increase the minimum current range that may be affected by the higher input impedance. The most common reason to use this feature is when connected to a DC-DC switching converter with inadequate upstream bulk capacitance.
7.2. 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.
The voltmeter intentionally omits the pull resistors to ground, typically 1 MΩ to 10 MΩ for most multimeters and oscilloscopes. However, Voltage + and Voltage - will float when unconnected. The voltmeter measurement is only valid once Voltage + and Voltage - are both connected to within ±15V of the Current + terminal. See Section 7.5 for more detail.
7.3. 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.
7.4. 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:
P = V * I = V² / R = I² * R
Energy is the integral of power over time.
When we talk about energy, we often mean the energy consumed over a specific time interval, which is the definite integral:
The Joulescope host software performs the discrete-time Euler integration shown to the right. The Joulescope software carefully manages the numerical precision to ensure that it computes energy correctly. IEEE double-precision floating-point numbers do not provide sufficient precision.
Charge is the integral of current over time.
Charge and energy are mathematically similar. Both voltage and current contribute to energy, but only current contributes to charge.
7.5. High impedance / low leakage#
Joulescope measures currents at nanoamp resolutions and accuracies. Many contemporary devices have input impedances of 1 MΩ to 10 MΩ, which is not good enough. At 3.3V, a 10 MΩ load draws:
3.3V / 10 MΩ = 330 nA
Joulescope features a combined input impedance and leakage current of approximately 1 GΩ. This high input impedance allows Joulescope to make accurate and precise low current measurements.
7.6. Electrical isolation#
The sensor side of Joulescope is electrically isolated from the host computer. This isolation allows you to insert Joulescope into your test system without worrying about ground issues, including ground loops and ground noise. The JS220 sensor-side isolated ground is referenced to Current +.