Keithley 2750, User Manual

Page: 95 / 452

3-4 Basic DMM Operation Model 2750 Multimeter/Switch System User’s Manual
Performance considerations
Warm-up
After the Model 2750 is turned on, it must be allowed to warm up for at least two hours to
allow the internal temperature to stabilize. If the instrument has been exposed to extreme
temperatures, allow extra warm-up time.
Autozero
To help maintain stability and accuracy over time and changes in temperature, the
Model 2750 periodically measures internal voltages corresponding to offsets (zero) and
amplifier gains. For thermocouple temperature measurements using the internal reference
junction (i.e., Model 7700 switching module installed), the internal temperature is also
measured. These measurements are used in the algorithm to calculate the reading of the
input signal. This process is known as autozeroing.
When autozero is disabled, the offset, gain, and internal temperature measurements are not
performed. This increases the measurement speed. However, the zero, gain, and tempera-
ture reference points will eventually drift resulting in inaccurate readings of the input sig-
nal. 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 measure-
ments, especially if the ambient temperature has changed by several degrees.
NOTE
To force a rapid update of the internal reference points, set the integration rate
to 0.01 PLC, and then back to the desired rate (i.e., 1.0 PLC). The NPLC com-
mands to set the integration rate are covered in Section 4.
Remote programming can be used to enable or disable autozero ( Table 3-1 ). Autozero
cannot be disabled from the front panel; however, it can be enabled by restoring factory
default conditions.
2750-900-01.book Page 4 Wednesday, August 3, 2011 7:56 AM