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P - 2 APPENDIX P
CHANNEL ACQUISITIONS
For three- or four-channel acquisition, the ADCs are used in conjunction with the
chopped mode or alternate mode.
In the chopped mode the first ADC takes a single sample on channel 1, then one
on channel 3, then one on channel 1 again and so on. The second ADC chops
between channels 2 and 4. The chopping rate is so high that the effective sample
rate is 5 MS/s on four channels in parallel. This is more than adequate for single
shot capture in most applications (power switching, audio, process control, etc.)
and certainly for electro/mechanical applications, which are typically in the kHz
range.
The advantage of a high speed chopper is that it allows single shot acquisitions
to be taken on four channels simultaneously. The chopper is also an advantage
for acquisitions at low timebase speeds.
ALTERNATING ACQUISITION
Some applications require the use of an alternating signal acquisition mode. In
this mode each ADC can be used to its highest speed. Channels are acquired in
the following sequence :
Channels 1 and 2 are acquired in parallel, with synchronized ADC acquisition,
followed by channels 3 and 4. Because each ADC can be used up to 100 MS/s,
glitch capture is now possible for glitches down to 10 ns.
RANDOM SAMPLING FOR FULL 200 MHz ACQUISITIONS AT HIGH TIME/DIV
AND FOR FOUR CHANNELS IN PARALLEL.
For high timebase settings (200 ns/div to 2 ns/div), real-time signal acquisition
would require a real-time sampling rate of 25 GigaSamples/second. Such sample
rate is not achievable.
The oscilloscope can be used with time base speeds exceeding the capabilities
of the ADCs. For such acquisitions repetitive signals are required. A sampling
method known as random sampling is used.
This sampling method is automatically selected at higher time base speeds. In the
random sampling mode, the chopper is used to acquire four signals
simultaneously without restriction. The glitch detection circuit is not needed at
these high timebase speeds because the sample distance is smaller than the
glitch capture capability of 10 ns would allow (40 picoseconds at 2 ns/div).