AN8008 Current Measurement Error

The most annoying problem with ANENG AN8008 multimeter is the poor accuracy of high current measurements.  Significant error is seen for the measurements above 1A.  The error drifts high quickly at higher current.

It is apparent looking that the circuit board the current sense circuitry is laid out incorrectly.  The voltage measurement across the current sensing shunt includes a section of PCB trace.     The copper trace width is about 150mil, and the length is about 700mil; so the copper trace could be as much as 2-3 mOhms.  The shunt wire is labelled 0.01 Ohm.  Assume the shunt wire is Manganin with temp co 1.5e-5 /C  (vs Copper 3.9e-3 /C).  The size is about 14 AWG, so the resistance is about 2 mOhms/cm; the length appears to be 4-5cm, so 8-10 mOhms is reasonable.  We could bypass this section, but the meter may be calibrated this way. (The EEPROM may be updated for new calibration.)

The section of the PCB is already different from the earlier version based on other teardown pictures online.  The copper trace is exposed and vias are added; the trace is coated with solder.  It appears that attempts are made to improve thermal dissipation.

When I actually measured the voltage drop across the shunt wire and the copper trace, I did not see the drift that I expected.  The shunt wire has about 8 mOhms, and the trace less than 1.5 mOhms.  When 7A is applied to the meter, the area near the fuse gets pretty hot.

The burden on the current measurement is also high because of the fuse and traces.  I measured as much as 180 mOhms at the terminals.