Step 5: Specify whether you want to use Auto Zero. Autozero provides the most accurate measurements, but
requires additional time to perform the zero measurement. Note that there is no autozero setting for 4-wire resistance
measurements.
With autozero enabled, the DMM internally measures the offset following each measurement. It then subtracts that
measurement from the preceding reading. This prevents offset voltages present on the DMM’s input circuitry from
affecting measurement accuracy. With autozero disabled, the DMM measures the offset once and subtracts the offset
from all subsequent measurements. The DMM takes a new offset measurement each time you change the function,
range, or integration time.
Negative Resistance Measurements
Under certain conditions, the instrument may report negative resistance measurements. These may occur in 2-wire
and 4-wire resistance measurements or continuity measurements.
Conditions that may cause negative ohms values include:
l Changes in Front/Rear switch contact resistance
l Reversed Sense Hi and Lo leads
l Circuits with external bias or thermal voltages at circuit connections
l Changes in measurement connection after a NULL operation
Under the same conditions, the 34401A returns the measurement’s absolute value so as to prevent the confusion
associated with negative readings. The Agilent Truevolt Series DMMs will return negative values. This allows the most
accurate results after a NULL operation.
44 Agilent Truevolt Series DMM Operating and Service Guide
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