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Programmer’s Guide Page 49 of 64

sampling rate being used (see the product brochure/datasheet to confirm the resolution). The waveform readout

function AcqrsD1_readData returns the time stamp value as 2 32-bit values. In order to do time differences, you

should transform them into a 64-bit integer:

• In Visual C/C++, use the 64-integer __int64 as follows:

__int64 timeStamp = timeStampHi;

timeStamp = timeStamp<<32 + (unsigned long)timeStampLo;

Arithmetic operations between such integers can be done as with shorter integers.

You also can convert a time stamp difference to an extended floating point number, and do arithmetic operations

as with other variables:

double deltaTime = (double)(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.

3.16. 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 U1084A-FAMILY, U1071A-FAMILY, 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.

3.16.1. 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 U1084A-FAMILY, 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.

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