Putting a digital multimeter in series with a board changes the supply path. Its current-measurement circuitry, fuse, leads, and contacts introduce a voltage drop known as burden voltage. On a low-power board, a small drop during an active burst can change firmware behavior even if the displayed sleep current looks stable.
Measure the voltage delivered to the board
Keysight discusses burden and dynamic operating states in How to Measure Low-Power Device Current. Use the meter’s documented burden for the selected range as a starting point, then observe the supply at the board during sleep entry, wakeup, and transmission. A specification at one current does not describe every autorange transition or transient.
For a hypothetical example, an effective series resistance of 10 ohms drops only 10 microvolts at 1 microamp but drops 0.2 V during a 20-mA burst. If that burst triggers a brownout, the following low reading may describe a reset board rather than the intended sleep state. A long averaging aperture can hide the short burst and the associated supply collapse.
Compare measurement configurations
Repeat the state sequence with a lower-burden range or a suitable external shunt and differential measurement. A larger current range may reduce voltage disturbance while sacrificing resolution. An external shunt allows the tradeoff to be chosen explicitly, but its amplifier, offset, and bandwidth become part of the measurement budget. Avoid adding a second return path through a debugger or serial adapter.
| Evidence | Interpretation |
|---|---|
| Current changes with meter range | Check burden, resolution, and integration settings |
| Supply dip coincides with wakeup | Check transient delivery and reset behavior |
| Debugger changes sleep current | Check back-power and firmware state |
| Average looks correct but bursts disappear | Check acquisition bandwidth and timing |
Report the operating state with the number
Record source setting, measured board voltage, current range, aperture, firmware state, and attached cables. A GPIO marker or retained reset-cause log can corroborate the intended sequence. Do not simply increase the supply voltage to compensate without checking the maximum voltage during low current.
Use the back-power path audit to eliminate alternate feeds and pulse-voltage budgeting when the real product runs from a battery. The accepted result should describe current at a valid board voltage throughout the complete operating cycle.
Move from reading to a build-ready project
Use the fabrication and assembly checklists to prepare drawings, production files and test requirements.
Open the DFM checklist