Keysight Technologies 3458A Guide D'utilisation page 205

Direct-Sampling
Direct-sampling is similar to DCV digitizing in that samples are taken in real-time
with each successive sample spaced a specified time interval from the preceding
sample. The difference between the two is that direct sampling uses the
multimeter's track-and-hold circuit and has a wider bandwidth input path
(12 MHz bandwidth). In addition, direct sampling has less trigger jitter but greater
measurement noise than DCV digitizing (see the
page 409).
The track-and-hold circuit takes a very fast sample of the input signal and then
holds the value while the A/D converter integrates it. By using the track and hold
circuit, the width of each sample is reduced from a minimum of 500 nanoseconds
for DCV to 2 nanoseconds for direct-sampling. This makes direct sampling ideal
for applications such as capturing the peak amplitude of a narrow pulse. The
disadvantage of direct-sampling is a slower maximum sampling rate of
50,000 samples per second versus 100,000 for DC voltage.
You specify direct sampling using the DSAC or DSDC command. The DSAC
command selects AC-coupling, which measures only the AC component of the
input signal. The DSDC command selects DC-coupling, which measures the
combined AC and DC components of the input signal.
Figure 5-9
(the numbers indicate the order in which the samples were taken). With direct
sampling, the minimum possible interval between samples is 20 µs.
Figure 5-9
Keysight 3458A User's Guide
shows 20 samples made using direct sampling on a sine wave input
Direct sampling
Digitizing
"Appendix A: Specifications"
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