User manual

Programmer’s Guide Page 53 of 66
xStampLo = tStampLo
End If
timeStamp = CDec(tStampHi * Two32) + xStampLo
...
timeDiff = timeStamp – previousStamp
Arithmetic operations between such decimal variants can be done as with other integer variables.
The manipulation of tStampLo is somewhat complicated because this variable is a signed 32-bit integer, but
must be added as an unsigned integer to the (shifted) tStampHi.
In LabVIEW, convert the time stamp to an extended floating point number, and do arithmetic operations as with
other variables.
In LabWindows/CVI, the easiest way to manipulate time stamps is to convert them first to doubles:
ViReal64 dlow, dhigh, tstamp;
dlow = (ViReal64)low;
dhigh = (ViReal64)high;
tsamp = dlow + 4294967296.0 * dhigh;
3.17. External Clock and Reference
The external reference mode replaces the internal 10 MHz reference clock with an external one at the same or a
similar frequency, from which the actual sampling clock is derived.
In the external clock mode, a waveform is sampled according to a clock derived from transitions of the external clock
signal through the user-defined threshold. We distinguish between continuous external clock operation and start/stop
external clock operation.
All external clock/reference modes are configured with the function AcqrsD1_configExtClock.
The external clock/reference signal should have a peak-peak amplitude of at least
0.5 V for the DC135/DC140/DC211A/DC241A/DC271A/DC271AR and 10-bit-FAMILY,
1 V for the other DC271-FAMILY digitizers, the U1071A-FAMILY, the 12-bit-FAMILY, the AC/SC
Analyzers, and the AP Averagers and Analyzers,
2 V peak to peak for all other models.
. The inputThreshold value should be set to the center of the signal.
3.17.1. External Reference
This external reference mode (clockType = 2) simply replaces the internal 10 MHz reference clock with an external
one at the same or a similar frequency. Alternatively, for the DC135/DC140/DC211/DC211A/DC241/
DC241A/DC271/DC271A/DC271AR, the AC/SC and the 10-bit-FAMILY, the PXI 10 MHz System Clock can be
used as the reference.
If you need a more precise timebase, or want to ensure that the timebases of several modules are at exactly the same
frequency, you should use clockType = 2 in the function, and apply an external 10 MHz signal. All other settings of
the digitizer are exactly the same as with an internal clock.
If you need to sample at a rate that deviates from the nominal values, you may apply an external reference signal
with a constant frequency in the range of
[9.97, 10.03] MHz for the U1071A-FAMILY and the 10-bit-FAMILY
[9.0, 11.0] MHz for the 12-bit-FAMILY
[9.0, 10.2] MHz for all other modules.
You need to correct for the reference frequency difference in your application since the digitizer and the driver do not
take the deviations into account.
NOTE: A square wave with better than 5 ns risetime should be used. This is needed to avoid false or multiple
transitions on a slower risetime signal. Alternatively, a >2 V amplitude signal could be used.
NOTE: When using this capability please make sure that the module is correctly synchronized on the signal. This
might require adjusting the threshold. If this is not the case the data will be useless and calibration can fail in rather
obscure ways.