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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 Autozero

(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 NPLC caching

(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.

Keithley 2651A Reference Manual

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