Chapter 5 Digitizing 139
uses whichever command was specified last. (When using the SWEEP
command, the sample event is automatically set to TIMER.)
• When direct-sampling an input signal with a frequency content ³ 1 MHz,
the first sample may be in error because of interpolator settling time. To
ensure the first sample is accurate, insert a 500ns delay before the first
sample (DELAY 500E-9 command).
Direct Sampling
Example
The following program is an example of DC-couple direct-sampled
digitizing. The SWEEP command specifies an interval of 30µs and 200
samples. Level triggering is set for 250% of the 10V range (250% of 10V =
25V). The samples are sent to reading memory in DINT format. The samples
are then sent to the controller, converted, and printed. By deleting line 110,
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 134k-bytes/second or the multimeter will generate the
TRIGGER TOO FAST error. Refer to "High-Speed Transfer Across the Bus"
in Chapter 4 for more information.
10OPTION BASE 1 !COMPUTER ARRAY NUMBERING STARTS AT 1
20INTEGER Num_samples,I,J,K !CREATE INTEGER VARIABLES
30Num_samples = 200 !200 SAMPLES
35ASSIGN @Dvm TO 722 !DESIGNATE MULTIMETER ADDRESS
40ASSIGN @Buffer TO BUFFER [4*Num_samples] !SETUP CONTROLLER BUFFER FOR
45 !SAMPLES, (4-BYTES/SAMPLE * 200 SAMPLES = 800 BYTES)
50ALLOCATE REAL Samp(l:Num_samples) !CREATE REAL ARRAY FOR SAMPLES
60OUTPUT @Dvm;"PRESET FAST" !DINT FORMATS, TARM SYN, TRIG AUTO
70OUTPUT @Dvm; "SWEEP 30E - 6,200" !30ms INTERVAL, 200 SAMPLES
80OUTPUT @Dvm; "DSDC 10" !DIRECT-SAMPLING, 10V RANGE
90OUTPUT @Dvm;"LEVEL 250, DC" !LEVEL TRIGGER AT 250% OF RANGE (25V)
100OUTPUT @Dvm;TRIG LEVEL" !LEVEL TRIGGER EVENT
110OUTPUT @Dvm; "MEM FIFO" !ENABLE READING MEMORY, FIFO MODE
120TRANSFER @Dvm TO @Buffer;WAIT !TRANSFER SAMPLES TO CONTROLLER
130OUTPUT @Dvm; "ISCALE?" !QUERY SCALE FACTOR FOR DINT FORMAT
140ENTER @Dvm;S !ENTER SCALE FACTOR
150FOR I=1 TO Num_samples
160 ENTER @Buffer USING "#,W,W";J,K !ENTER ONE 16-BIT 2’S COMPLEMENT
161 !WORD INTO EACH VARIABLE J AND K (# = STATEMENT TERMINATION NOT
165 !REQUIRED; W = ENTER DATA AS 16-BIT 2’S COMPLEMENT INTEGER)
170 Samp(I)=(J*65536.+K+65536.*(K<0)) !CONVERT TO REAL NUMBER
180 R=ABS(Samp(I)) !USE ABSOLUTE VALUE TO CHECK FOR OVLD
190 IF R>2147483647 THEN PRINT "OVLD" !IF OVERLOAD OCCURRED, PRINT MESSAGE
200 Samp(I)=Samp(I)*S !APPLY SCALE FACTOR
210 Samp(I)=DROUND(Samp(I),8) !ROUND CONVERTED READING
220 PRINT Samp(I) !PRINT READINGS
230NEXT I
240END
Sub-Sampling
In sub-sampling (also known as sequential-sampling), the multimeter takes
one or more samples on each period of the input signal. With each successive
period, the beginning sample point is delayed further-and more samples are
taken. After a number of periods have occurred and the specified number of
samples have been taken, the samples can be reconstructed to form a
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