Teledyne LeCroy PXD222 - Manual (Page 21)

Operation
PXD222-OM-E Rev A ISSUED: November 2001
17
A
Operation
INTRODUCTION
This section is intended to give you an understanding of the standard acquisition, triggering, and
measurement features of the PXD 222 Digitizer. Detailed descriptions of how to operate the
PXD222 Digitizer can be found in the Driver Help file.
A. Acquisition Modes
The PXD 222 Digitizer has two modes of operation NORMAL and MIN/MAX. These are defined as
follows:
NORMAL: In Normal mode, the PXD 222 Digitizer operates as a typical digitizer. The module will
digitize the input signals during a specified time window and create a data array with the digitized
data that can be read out by the controller. The user programs the number of points (maximum of
1000) as well as the time window to be digitized. The maximum sample rate is 2.5 GS/s, and will
vary depending on the time window setting. Table 1 gives the sample rates that are achieved.
MIN/MAX: There are two MIN/MAX modes, which are referred to as Glitch Capture and Scope
Record. In these modes, the PXD 222 Digitizer outputs two arrays; a MIN array with the minimum
values detected during each sample interval, and a MAX array with the maximum value detected
during each sample interval. Here’s how it works. In MIN/MAX mode, the time window is divided
into sampling intervals. During each interval, the digitizer oversamples the input at high speed (see
tables 2 and 3 on the following pages) and stores the MIN and MAX values in data arrays. This
technique allows you to detect and record glitches in the waveform that would not be found if only
1 sample were taken during each sampling interval. The Acquisition type is programmed by
means of the function LcPXD222_ConfigureAcquisitionType. (Note: This mode is sometimes
also referred to as a Peak Detect mode.)
When in Min/Max mode, and for time windows of 10 seconds or less, the digitizer is in Glitch Cap-
ture Mode, and will record MIN/MAX arrays of roughly 255 points. For time ranges of 11 seconds
and higher, the digitizer is in Scope Record Mode, and will store min-max arrays of roughly 27kpts.
Understanding the difference between Normal and MIN/MAX modes
Take an example where the time window is set to 1 ms. In Normal mode, the time between sam-
ples would be 1 µs (1 ms/1000 = 1 µs), corresponding to a sample rate of 1 MS/s because exactly
one sample is acquired per sample interval in this mode.
When setting the time window to 1 ms in MIN/MAX mode, the sampling interval is approximately
4 µs. This is roughly 4X longer than the corresponding value in Normal mode because 255 points
are stored instead of 1000. The PXD 222 Digitizer, however does not take only 1 sample in this 4
µs time interval, but instead samples at the higher speed of 20 MS/s. Of the 80 samples that are
taken (20MS/s * 4 µs = 80), the min and max points are retained. This is done for each of the 255
time intervals to create the Min and Max data arrays.