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Voltage and Temperature Measurements 2-7

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 Section 5.

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 Section 3.

Keithley 2182 User Manual

Other manuals for Keithley 2182