Keithley 2182 User Manual page 47

Front Autozero
With Front Autozero for the front-end amplifier enabled (which is the default setting), the
Model 2182 performs two A/D measurement cycles for each reading. The first one is a normal
measurement cycle, and the second one is performed with the polarity of the amplifier reversed.
This two-cycle, polarity-reversal measurement technique is used to cancel internal offsets in the
amplifier. With Front Autozero disabled, the second A/D measurement cycle is not performed.
Benefits of Front Autozero disabled:
• Twice as fast
• Lower Pumpout Current noise
Drawbacks of Front Autozero disabled:
• High drift (20µV/°C) in normal voltage mode
• High offset voltage (±500µV) in normal voltage mode
NOTE To increase the speed of Delta measurements, disable Front Autozero. The two-
measurement cycle, polarity-reversal technique used by Front Autozero is *not
required for Delta. Delta uses its own polarity-reversal technique to cancel offsets.
Delta measurements are covered in
Autozero
When Autozero for the second amplifier is disabled, the offset, gain, and internal reference
temperature measurements are not performed. This increases measurement speed (a few % at
1PLC). However, the zero, gain, and temperature reference points will eventually drift resulting
in inaccurate readings for the input signal. It is recommended that Autozero only be disabled for
short periods of time.
When Autozero is enabled after being off for a long period of time, the internal reference
points will not be updated immediately. This will initially result in inaccurate measurements,
especially if the ambient temperature has changed by several degrees. A faster update of
reference points can be forced by setting a faster integration rate.
Rate — With Autozero disabled, pressing the front panel RATE key will change the speed
setting, and will also enable Autozero. Rate changes using remote programming have no effect
on the state of Autozero.
To force a single rapid update of the internal reference points when Autozero is enabled, set
the integration rate to FAST (or 0.01 PLC for remote programming), and then back to the desired
rate (i.e., MED; 1.0 PLC). Details on Rate are covered in
Voltage and Temperature Measurements
Section 5.
Section 3.
2-7