Section 2: General operation Model 2651A High Power System SourceMeter® Instrument Reference Manual
2-22 2651A-901-01 Rev. A / March 2011
5. Once the desired value displays, press the ENTER key (the EDIT indicator is not illuminated).
6. (Optional) Press the EXIT (LOCAL) key to cancel source editing.
To edit compliance values:
1. Press the LIMIT key.
2. Use the CURSOR keys or the navigation wheel
to move the cursor to the desired digit.
3. Press the navigation wheel or the ENTER key to enter edit mode. The EDIT indicator is
illuminated.
4. Do one of the following to modify the compliance limit value:
• Rotate the navigation wheel to adjust the value. The digit automatically increments or decrements
the next significant digit when changing from 9 to 0 or from 0 to 9.
• If the keypad feature is enabled, use the number keys (see 1. Special keys and power switch
(on page
2-2)) to enter the value.
5. Once the desired value displays, press the ENTER key (the EDIT indicator is not illuminated).
6. (Optional) Press the EXIT (LOCAL) key to back out of the compliance menu.
NOTE
The up and down range keys changes the format and range of the limit value.
Operation considerations
The following paragraphs discuss autozero and NPLC caching.
Autozero
The integrating ADC of the Model 2651A uses a ratiometric A/D conversion technique. To ensure
accuracy of readings, the instrument must periodically obtain fresh measurements of its internal
ground and voltage reference. The time interval between needing to update these reference
measurements is determined by the integration aperture being used for measurements. Separate
reference and zero measurements are used for each aperture. When using the fast ADC, autozero is
not needed.
There are three different settings for autozero as summarized in the table below. By default, the
instrument automatically checks these reference measurements whenever a signal measurement is
made (AUTO). If the reference measurements are out of date when a signal measurement is made,
the instrument will automatically take two more A/D conversions, one for the reference and one for
the zero, before returning the result. Thus, occasionally, a measurement takes longer than normal.
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