Operation considerations for the ADC
The following paragraphs discuss autozero and NPLC caching with the analog-to-digital converter
(ADC) (on page 5-24).
Autozero
The ADC of the 2600B uses a ratiometric analog to digital (A/D) conversion technique. To ensure
reading accuracy, the instrument must periodically obtain fresh measurements of its internal ground
and voltage reference. Separate reference and zero measurements are used for each aperture.
As summarized in the table below, there are different settings for autozero. By default, the instrument
is set to AUTO, which automatically checks these reference measurements whenever a signal
measurement is made. If the reference measurements are out of date when a signal measurement is
made, the instrument automatically makes 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.
This extra time can cause problems in sweeps and other test sequences in which measurement
timing is critical. To avoid the extra time for the reference measurements in these situations, you can
select OFF. This setting disables the automatic reference measurements. Note that with automatic
reference measurements disabled, the instrument may gradually drift out of specification.
To minimize the drift, make a reference and zero measurement immediately before a critical test
sequence. You can use the ONCE setting to force a refresh of the reference and zero measurements
used for the current aperture setting.
Setting autozero from the front panel
To change autozero from the front panel:
1. Press the CONFIG key.
2. Press the MEAS key.
3. Turn the navigation wheel to select AUTO-ZERO, and then press the ENTER key or the
navigation wheel.
4. Turn the navigation wheel to select the mode (OFF, ONCE, or AUTO), and then press the
ENTER key or the navigation wheel.
5. Press the EXIT (LOCAL) key to return to the previous display.
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