Direct-Sampling - Agilent Technologies 3458A User Manual

10OPTION BASE 1
20Num_samples=256
30INTEGER Int_samp(l:256) BUFFER
40ALLOCATE REAL Samp(l:Num_samples)
50ASSIGN @Dvm TO 722
60ASSIGN @Int_samp TO BUFFER Int_samp(*)
70OUTPUT @Dvm;"PRESET DIG"
71 !PER TRIGGER, TIMER SAMPLE EVENT, TIMER INTERVAL = 20µs, TRIG
75 !LEVEL (0%, AC-COUPLED), 3µs INTEGRATION TIME, SINT FORMATS
80OUTPUT @Dvm;"TIMER 10E-6"
90OUTPUT @Dvm;"APER 1.4E-6"
100OUTPUT @Dvm;"MEM FIFO"
110OUTPUT @Dvm; "TARM SYN"
120TRANSFER @Dvm TO @Int_samp;WAIT
121!READING MEMORY AND THEN INTO AN INTEGER ARRAY IN THE COMPUTER;
122!SINCE THE COMPUTER'S INTEGER FORMAT IS THE SAME AS SINT, NO DATA
123!CONVERSION IS NECESSARY HERE (INTEGER ARRAY REQUIRED)
130OUTPUT @Dvm; "ISCALE?"
140ENTER @Dvm;S
150FOR I=1 TO Num_samples
160 Samp(I)=Int_samp(I)
165 !FORMAT (NECESSARY TO PREVENT POSSIBLE INTEGER OVERFLOW ON NEXT LINE)
170 R=ABS(Samp(I))
180 IF R>=32767 THEN PRINT "OVLD"
190 Samp(I)=Samp(I)*S
200 Samp(I)=DROUND(Samp(I),4)
210NEXT I
220END

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 Specifications
in Appendix A).
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 idea1 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 30 shows 20 samples made using direct sampling on a sine wave input
(the numbers indicate the order in which the samples were taken). With direct
!COMPUTER ARRAY NUMBERING STARTS AT 1
!SPECIFY NUMBER OF SAMPLES
!CREATE INTEGER BUFFER
!CREATE REAL ARRAY FOR SAMPLES
!ASSIGN MULTIMETER ADDRESS
!ASSIGN I/O PATH NAME TO BUFFER
!TARM HOLD, DCV, 10V RANGE, 256 SAMPLES
!10µs INTERVAL BETWEEN SAMPLES
!MAXIMUM APERTURE FOR 100kHZ SAMP. RATE
!ENABLE READING MEMORY, FIFO MODE
!SYNCHRONOUS TRIGGER ARM EVENT
!SYN EVENT,TRANSFER READINGS INTO
!QUERY SCALE FACTOR FOR SINT FORMAT
!ENTER SCALE FACTOR
!CONVERT EACH INTEGER READING TO REAL
!USE ABSOLUTE VALUE TO CHECK FOR OVLD
!IF OVLD, PRINT OVERLOAD MESSAGE
!MULTIPLY READING TIMES SCALE FACTOR
!ROUND TO 4 DIGITS
Chapter 5 Digitizing 137