User manual

Programmer’s Guide Page 48 of 64
Acqiris digitizers feature a Trigger Time Interpolator (TTI), which measures the time between the trigger event and
the next sampling clock to a fraction of the sampling interval. It permits very precise positioning of the acquired trace
in highly zoomed displays, particularly when multiple acquisitions of the same signal are used. In many other
applications, this value can be ignored.
The following drawing completes the description of a 'real-life' waveform:
• The value of delayTime positions exactly the left edge of the display (or the exact nominal beginning of the
waveform) with respect to the “stable” trigger time, which is the real reference point. We define the time 'trigger
time + delayTime' as the time origin for the waveform, which is equivalent to saying that the trigger always
occurs exactly at the time -delayTime.
• The first data point of the waveform is defined as the last acquired data point before the time origin. It is indexed
with i = 0 in the formula below.
NOTE: It is important to realize that if a single segment is read (e.g. with AcqrsD1_readData) the first data point
will be dataArray[readPar.indexFirstPoint] and that this is not necessarily the first point given.
• The exact position of the first data point with respect to the time origin is a negative number horPos. It is by
definition in the range [-sampInterval, 0].
• The time between the trigger and the first data point hOffset need not be recorded since it can always be
computed as delayTime + horPos. Note: delayTime + horPos < delayTime by definition.
• In order to obtain a very stable image, even in a highly zoomed display, the user only needs to position the
acquired data points with the aid of horPos, by using the following formula for the x-position of point i with
respect to the left edge of the display:
x[i] = horPos + i * sampInterval
3.14. Sequence Acquisitions
For digitizers in Sequence acquisition mode, multiple waveforms are acquired autonomously, with a single start
command AcqrsD1_acquire. Whenever a trigger is received, the current acquisition segment is normally terminated.
The digitizer then automatically initializes another acquisition into the next memory segment, until all requested
segments are filled.
3.15. Time stamps
The U1071A, 10-bit-FAMILY and U1084A digitizers implement a time stamp to measure the time of the trigger for
each acquisition segment. These time stamps can be used to calculate the time between any two triggers for any pair
of triggers over multiple acquisitions.
The other, “older” Acqiris digitizers feature a 'time stamp' in order to measure the time between the triggers of
consecutive segments in the same acquisition. In fact, the time stamp counter is started when the Sequence
acquisition is started, and keeps counting during the entire sequence. The difference between the time stamps of any
pair of (not necessarily adjacent) segments is the time between their respective triggers.
The resolution of the timestamp is identical to that of the trigger time interpolator for that model. For most models
the time stamp value is returned as a 64-bit integer, in units of picoseconds, but there are certain exceptions, for
example the timestamp resolution U1084A-DGS in Zero-suppression mode will depend on the model options and the
Trigger
delayTime
hOffset
horPos
sampInterval
First data point
nbrSamples
Time Origin