Re: [EE] measuring the magnetic field from a small trace carrying a < 50uA load?

At 08:02 PM 2025-01-02, you wrote:
>When developing battery powered devices, sometimes it can be difficult to
>track down where the last few dozen microamps of load are being drawn.
>
>One can resort to systematically cutting traces and desoldering components.
>But that is destructive, labor intensive, and error prone.
>
>I would like to know if an active probe exists that can measure and
>discriminate the current/DC magnetic field of small < 50uA loads on small
>but loosely packed PCB traces.
>
>Are there other viable (hopefully nondestructive) ways to track down small
>loads? (A low noise thermal camera in a vacuum chamber?)

Hi, Jason:-

There are oscilloscope DC probes that claim to go down to fairly low currents,
for example this one says it can measure low currents with high accuracy and
mentions 1% of reading and 1mA. If one was optimistic one might assume a
resolution of 10uA (and confirm with an FAE). However, it's also close to $6k,
kind of pricey for an occasional-use tool.

As an alternative - it won't work on things like ground planes but a 1" (25mm)
0.25 mm (10 mils) trace  in 1oz copper would have 2 or 3uV across it.

My ancient daily driver bench DMM has as good as 100nV resolution, so that
would give you  2uA resolution (well, if you had the same meter or 
better) without
cutting any traces, just adding jumpers here and there.

Another approach is to make a prototype with a bunch of resistors
(and sometimes parallel diodes for when the current is not close to zero)
in the power paths and replace them with zero ohm resistors when the device
has been thoroughly probed. Same as those 'unused' inputs and outputs
that are best not (for example) tied directly to the GND EP underneath a
tiny QFN package-- at least in the first spin of the PCB. But, sadly, that's
not much help if the PCB is already spun.

Best regards,
Spehro Pefhany