8588A/8558A
Operators Manual
40
Note
Data acquisition in Digitize starts from the front panel when you push
or by a remote command. You cannot start an acquisition
with . is normally used to toggle the Trigger
subsystem from the free-run state (Initiate Continuous ON) to the
idle state (Initiate Continuous OFF). Digitize does not have a free-
run trigger state. can be used to stop an acquisition if
desired.
Digitize uses a high-speed analog-to-digital converter to capture input signals.
The Digitize function has a tracking circuit that follows the analog input. When a
trigger occurs, the value on the tracking circuit is held, and converted to a digital
value. The conversion process takes about 85 ns. Once the conversion is
complete, tracking of the signal recommences. Another 115 ns tracking is
required before the analog-to-digital converter is ready for another trigger. See
Figure 11.
iei191.png
Figure 11. Digitize Track and Convert Timing
The Digitize Aperture is defined as the time difference between the occurrence of
the trigger and time when the tracking value is held. The default is 0 ns, which
means the analog value is held at 0 ns the time the trigger occurs. (In actuality,
there are latencies in the circuit, up to 10 ns.) The entire process for one reading
is 200 ns, which gives a maximum Digitize trigger rate of 5 MHz. Aperture
settings other than 0 ns uses an averaging algorithm. A 200 ns Aperture setting,
for example, averages two samples taken 200 ns apart. In this case, it takes an
additional 200 ns to process the data, giving an acquisition period of 200 ns +
200 ns, or 400 ns. Examples of different Aperture settings and sample values are
shown in Figure 12.
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