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.

$20 Multimeter

I wrote about $4 multimeter (also see here), which is fine for crude measurements.  For $20, we can have a meter with the following enhancements: 10Meg-Ohm input impedance, auto range, auto power off,  higher resolution (9999 vs 1999 counts or 4 digits vs 3.5 digits), capacitance measurement, true RMS for AC voltage and current, frequency and duty measurement, continuity beep.  Those are the features of ANENG AN8008, which has received favorable reviews online.  There are even modifications (adding capacitors to the reference and supply) to improve its performance.  The meter also comes with an extra set of probes with changeable tips.

The voltage measurements are generally very good.  The current measurements are poor at high current range.  The measurement drifts a lot.  For example, at 7A current input, the initial measurement is 7.03A (well within spec), but very quickly it drifts 7.15A and keeps climbing.  For current 1A or below, the measurements seem stable.  (It appears that the poor layout of the current sensing shunt resistor contributes to the poor performance.)  The $4 meter actual does better: measuring 7.05A and steady.  The shunt resistance in the uA range is 100 Ohms.  The resistance measurement excitation voltage appears to be 1V.  The diode voltage measurement goes up to 3V; the open circuit voltage is 3.3V .  It has a square wave output (+/-1.37V) at selected frequencies, which I don't know a good use for.  The current draw in the voltage mode is about 1.5mA from two AAA batteries; that gives about 600 hrs operating time.

The specifications are fairly conservative and easily met except for the high current measurements.

Is it worth $20?  Maybe.  It is not for professional use.  The poor high current measurement accuracy is a disappointment.  There is no 10mA and 100mA current range.  The input protection is pretty minimal: 10A/250V and 200mA/250V fuses for current protection, a bidirectional TVS diode and a 1.5K-Ohm PTC thermistor.  The 600V CAT III and 1000V CAT II ratings should not be relied on.

The Fluke 107 has comparable features and specs (the only enhanced feature is 40M-Ohm range); the cost is $80.   The extra cost buys guaranteed spec and verified 600V CAT III rating.