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DXP Introduction
The Digital X-ray Processor (DXP) core is a
patented circuit topology that efficiently detects randomly occuring
analog voltage pulses and measures their amplitudes at optimal
signal-to-noise ratio. The analog input directly accepts solid-state
detector preamplifier signals of either pulsed-reset or resistive-feedback
type (ie. 'staircase' and 'exponential decay' pulses, respectively)
and uses a novel tracking circuit to compact the signal's dynamic
range into that of the high-speed sampling ADC; the raw preamplifier
pulses are thus digitized without the nonlinear error term typically
introduced by a pole-zero differentiator. Noise optimization is
achieved through a very efficient digital FIR algorithm that reduces
the dead time per event by nearly 50% compared to an equivalent
shaping amplifier with the same energy resolution performance.
Efficient baseline handling, combined with a tight pile-up inspection
algorithm produce a measure of energy with very little dependence
on count rate. Detector and preamplifier allowing, the DXP will
happily run at 95% dead time and beyond!
DXP modules are completely digitally controlled
and execute all functions under computer control, including setting
gains, peaking times, pileup inspection criteria, and MCA conversion
gain. With
an appropriately matched detector, these instruments are superb
performers. For example, the DXP-XMAP achieve 120-125 eV Fe55
energy resolution at 20 microseconds, and can output over 700
kcps at its shortest peaking time of 100 nanoseconds.
Typically, DXP throughputs are twice those
of conventional analog amplifier/SCA systems, while providing
full MCA analysis. XIA's MESA software allows convenient control
of large detector arrays, even those consisting of 100 elements
or more. Our Xerxes software library allows DXP module control
to be integrated within existing customer data collection and
analysis programs. The original DXP-4C achieves output counting
rates exceeding 250 kcps at an energy resolution of better than
275 eV, using an 0.5 microsecond peaking time. With a 20 microsecond
peaking time the same module can achieve better than 150 eV resolution
with a maximum output count rate of almost 10 kcps.
The DXP family is digitally based with an
internal clock, and thus can readily implement time based collection
modes not normally associated with X-ray spectrometers. The XMAP, Mercury
and Mercury-4 provide several continuous (zero dead-time) mapping modes:
- Continuous XAFS or X-ray Mapping in full spectrum mode (XMAP, Mercury
and Mercury-4)
- Continuous XAFS or X-ray Mapping in SCA mode (XMAP, Mercury
and Mercury-4)
- Continuous XAFS or X-ray Mapping in list mode (XMAP, Mercury
and Mercury-4)
Specialized collection modes can be also created
on a custom basis, since these are programmable logic algorithms
which are downloaded at startup time. These modes are proving to
be particularly popular in scanning x-ray and electron beam microprobe
applications.
Being entirely digitally controlled, the DXP
series is particularly effective in both complex experimental setups
and in situations requiring repetitive, standardized measurements.
Once a DXP module is configured for a particular experiment or measurement,
its setup can be saved to a configuration file. Whenever the
identical setup is required again, downloading the saved configuration
file immediately restores the required setup. Since everything
is under computer control, both complex and tedious measurements
can be automated and repeated reliably as often as required
DXP modules are easily configured to operate
with a wide range of common detector/preamplifier systems of either
polarity, including pulsed optical reset, transistor reset, and
resistive feedback preamplifiers. With proper parameter setting
the DXP can also work with proportional counters, surface barrier
detectors, and scintillator/PMT combinations. Multi-channel analysis
for each channel not only allows optimal use of data to separate
fluorescence signals from backgrounds or monitor other spectral
information, but also enables automated gain setting and calibration,
which greatly facilitates tuning and calibrating multi-element detector
systems. An external Gate allows data acquisition to be synchronized
on all channels.
Originally designed to instrument array detectors
in very high count rate synchrotron radiation applications, the
4 channel DXP-4C was introduced
in 1995. Thanks to a novel conceptual design and patented hardware
implementation, the DXP-4C proved itself, matching the energy resolution
of contemporary analog instruments at roughly twice the output rate.
It is currently employed in experiments at national laboratories
and other facilites worldwide.
The second-generation of DXP instruments,
including the DXP Saturn and DXP-2X,
improves upon the original DXP concept. While retaining the basic
topology of the original DXP-4C, the front end has been redesigned
to significantly reduce noise, its clock speed is doubled, from
20 MHz to 40 MHz, an ADC with improved differential non-linearity
is used, and a newer DSP is employed which is twice as fast, has
8K channels of internal spectral memory, and allows addressing
of larger external memory, up to 1MB. A new approach to
ICR and livetime measurement has further improved the accuracy
of the deadtime correction. These features couple nicely to the
most recent advances in HPGe detector arrays.
The third-generation saw a bifurcation in
development on the one hand for low-power, low-cost embedded systems,
and on the other hand for high-speed, high-resolution multi-element
systems. The MicroDXP is a credit-card
sized circuit board that is highly customizable and consumes less
than 500mW. The DXP-XMAP, Mercuryand Mercury-4
have an improved memory architecture, a 50MHz ADC, and fast
digital interfaces to provide unprecedented performarnce and functionality.
These features couple nicely to the most
recent advances in HPGe detector arrays, allowing the following
functional gains:
- Up to 16K channels per spectrum
- Maximum throughput up to 1,000,000 counts/sec/channel
- Peaking times from 80 nanoseconds to 160 microseconds
- Low noise front end: DXP-XMAP modules have achieved 120 eV
FWHM Fe55 energy resolution with HPGe detectors at 20 microseconds
peaking times and resolved the Boron K line using thin window
Si(Li) detectors.
- Operates with preamplifiers of either polarity, gains of
0.1 - 5 mV/keV, and up to +/- 10V output range.
XIA is committed to producing the highest quality
instruments for our customers. Due to the programmable nature of
the DXP, this ongoing pursuit is not limited to new hardware releases,
but also takes the form of periodically updated FPGA algorithms
and DSP code. These files can be downloaded
online and uploaded to your existing DXP hardware, resulting
in performance improvements at no additional cost.
The latest DXP algorithms, released in January
2006, provide significant performance improvements over previous
releases. A new approach to baseline handling yields better energy
resolution and allows lower thresholds. Combined with the minimal
dead time and excellent pileup rejection of the DXP, this latest
development opens up the soft x-ray region (100eV-1keV) to a new
level of scrutiny and is of particular interest to our analytic
microscopy and XRF users. Of course, nearly all of our customers
will benefit from:
- Better energy resolution, especially at high rate
- Reduced peak shift with rate - under 0.05% full range
- Improved pileup inspection - especially for soft x-rays
- Better ICR and livetime measurement resulting in very accurate
dead time correction
- Enhanced all-around soft x-ray performance
Please check our News
page for future updates...
Please take a look at DXP
Sample Plots and Data. Datasheets, manuals, journal articles
and application notes can be found on the DXP
Resources page. Of particular interest are the following application
notes:
- Pileup Inspection and Deadtime
in the DXP-4C
(208,717 bytes)
Abstract:This Note answers some commonly asked question about
how the pileup inspection in the DXP-4C works, and how should
the deadtime corrections be implemented by experimenters using
this and similar instruments from XIA.
- Answers to Common Questions about
the DXP-4C
(15,935 bytes)
Abstract: This Note answers many general commonly asked question
about the DXP-4C and similar instruments from XIA.
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