Section 4
Theory of operation
In this section:
Analog to digital converter ........................................................ 4-1
Source-measure concepts ....................................................... 4-2
Reduction in gain-bandwidth .................................................. 4-20
Pulse rise times ...................................................................... 4-21
Pulse width ............................................................................. 4-23
Analog to digital converter
The Model 2651A has two analog-to-digital converters (ADC): an integrating ADC and a fast ADC.
The integrating ADC uses a ratiometric analog-to-digital conversion technique. Depending on the
configuration of the integrating ADC, periodic fresh reference measurements are required to minimize
drift. The measurement aperture is used to determine the time interval between these measurement
updates. For additional information, see
(on page 2-22). To help optimize operation of this
ADC, the instrument caches the reference and zero values for up to ten of the most recent number of
power line cycles. For additional information, see
(on page 2-24).
The fast ADC can acquire measurements at speeds up to 1 million samples per second. The fast
ADC does not take reference measurements, nor does it have any other of the ratiometric concerns.
A reading measurement acquisition buffer allows up to 5,000 readings to be made at the maximum
acquisition rate of the fast ADCs. If this buffer is filled, the instrument slows its acquisition rate to the
rate at which the instrument can process the data.
Data acquisition takes priority over both source operation and display operation. Sustained high data
acquisition rates will cause the display to stop updating. A sustained high data acquisition rate during
a sweep or pulse train may slow source operations. This can cause erratic sweep or pulse timing and
may lead to triggers being missed (trigger overruns). Use the status model to monitor for trigger
overruns. If a high sustained data acquisition rate causes undesirable sweep or pulse timing, reduce
the data acquisition rate or reduce the total number of measurements until the desired sweep or pulse
timing is achieved.
