I'm not familiar with that particular DMM, but here are some comments anywa=
y!

Years ago I worked with a DMM chip. The range switch was, I thought, very
clever. There was a 10M external series resistor to the probe. The chip
then switched in various external shunt resistors to form a voltage
divider for the appropriate range. The result was the accuracy was
determined by the accuracy of the external resistors and the ADC reference
voltage. The simplest method of building something with the chip was to
just use precision resistors and reference. No calibration was then
required.

I have seen some DVMs read high as the battery voltage went down. The
reference voltage generator was falling out of regulation, causing the ADC
reference voltage to be low. Since ADCs are ratiometric, a low reference
caused a high reading.

You mention the use of NiMH batteries. Is the meter designed to use NiMH
batteries? The lower cell voltage compared to alkaline or similar
batteries could also cause the reference voltage regulator to drop out. If
the reference has dropped out and then you calibrate the meter, the meter
will be very unstable with varying battery voltage.

Good luck!

Harold



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