The USB Test Card is a fairly simple build, but I have not seen it used and combined in that way. Even the USB.org Over-current Test Fixture only uses a standard Ampere Meter to measure the current, completely ignoring the voltage drop, while the USB.org Voltage droop/drop test fixture seems to be targeted for USB hubs and does not provide a way to measure the current. This might be sufficient to pass USB testing, but ignores the needs for device development and testing.
My current section is made of a 10mΩ Shunt resistor. This gives a maximum voltage drop of 9mV @ 0.9A (max USB 2.0 spec), very acceptable compared to most multimeter current path burden voltages. If ultra low power or USB3.0/ USB charging current capability with up to 5A is needed, it is easy to replace the shunt with a different value.
An INA214 Current Shunt Monitor boosts the shunt voltage a 100 times. If needed a different gain version could be used. The INA21x also acts as buffer so it does not matter if a meter/scope is connected or not. It also avoids the nasty auto-ranging current interrupts of some Ampere meters.
The INA210, INA211, INA212, INA213, and INA214 are voltage output current shunt monitors that can sense drops across shunts at common-mode voltages from –0.3V to 26V, independent of the supply voltage. Five fixed gains are available: 50V/V,100V/V, 200V/V, 500V/V, or 1000V/V. The low offset of the Zerø-Drift architecture enables current sensing with maximum drops across the shunt as low as 10mV full-scale. These devices operate from a single +2.7V to +26V power supply, drawing a maximum of 100μA of supply current. [www.ti.com, ina214.pdf]
In the VBUS voltage output section a resistive voltage divider was added so the 5V could be scaled down (I have more plans with it – stay tuned!).
I also kept the power supply easy but flexible. The system can be powered directly from VBUS (in front of the sense resistor) or from a VBUS derived voltage with the help of a TLV700xxDSE (1.8V, 2.5V, 2.8V, 3.0V, 3.3V, 3.6V), it might come handy one day to limit the output swing of the shunt monitor and provide a steady supply voltage.
Since this is a single supply scenario, a 1.25V reference module REF1112 is used to offset the output signal so that current in both directions can be measured. The reference voltage is provided on the differential output V-.
Schematic, Layout and Gerbers are available on https://github.com/ucDude/UsbTestCard.