136 Chapter 5 Digitizing
• The multimeter’s triggering hierarchy (trigger arm event, trigger event, and
sample event) applies to DCV digitizing. Refer to Chapter 4 for more
information on the triggering hierarchy. For DCV digitizing, you can use
either the TIMER sample event and the NRDGS n,TIMER command: or
the SWEEP command. The NRDGS and SWEEP commands are
interchangeable, the multimeter uses whichever command was specified
last. (When using the SWEEP command, the sample event is automatically
set to TIMER.)
• Aperture time is the time when the multimeter is actually sampling the
input signal. For direct- and sub-sampling using the track-and-hold, the
aperture time is fixed at 2ns and cannot be changed. For DCV digitizing,
the aperture time is equal to the A/D converter’s integration time and can
be varied from 500ns to 1s. The multimeter effectively averages the input
signal during its aperture time. An amplitude error is introduced when the
signal is changing during the aperture time. Table 27 shows the input signal
frequencies where 3dB of amplitude error occurs for selected aperture
times and the bits of resolution produced for these aperture times.
DCV Example The following program takes 256 DC voltage samples at a rate of 100,000
samples per second and places them in reading memory using SINT format.
The samples are then transferred to the controller using the SINT output
format. The controller converts the samples from SINT format and stores the
samples. By deleting line 100, samples will be transferred directly to the
controller instead of using reading memory. However, the controller and
GPIB must be able to transfer samples at a rate of at least
200k-bytes/second or the multimeter will generate the TRIGGER TOO
FAST error. Refer to "High-Speed Transfer Across GPIB" in Chapter 4 for
more information.
max._input Parameter Selects Range Full Scale
0 to .12 100mV 120mV
>.12 to 1.2 1V 1.2V
>l.2 to 12 10V 12V
>12 to 120 100V 120V
>120 to 1E3 1000V 1050V
Table 27: Amplitude Error and Resolution vs. Aperture
Aperture Time Bits of Resolution Frequency For 3dB
Error
2ns 16 100MHz
500ns 15 400kHz
1µs 16 206kHz
3µs 17 69kHz
6µs 18 35kHz
100µs 21 2kHz
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