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Electronic Instrumentation

Guide GD2080Introduction to Digitizers

The CAEN Digitizers are Platform independentinstruments housing high speed (up to 5 GS/s)multichannel ADC, with local memory and FPGA forreal time data processing.

Available in different form factors: VME, NIM, Desktopand PCI Express.

• Up to 14 bit resolution• VME64X, Optical Link, USB 2.0, PCI Express Interfacesavailable• Memory buffer: up to 58MS/ch (max. 1024 events)• Multi board synchronization and trigger distribution• FPGA fimware for Digital Pulse Processing• Software Tools for Windows and Linux

CAEN Waveform Digitizers feature Digital Pulse Processing(DPP) firmware for physics applications.

DPP algorithms are implemented in FPGA and can bereprogrammed at any time. In one single module you havethe complete information and the capability to extract allthe quantities of interest.

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2 GD2080 - Introduction to Digitizer

CAEN has developed a complete family of digitizers that consists of several models differing in sampling frequency, resolution, number of channel, form factor, memory size and other parameters. The following table lists all models currently available. In parallel with the hardware development, CAEN have made a big effort in developing algorithms for the Digital Pulse Processing (DPP); you can install a DPP algorithm on the FPGA of the digitizer (firmware upgrade), run it on-line and implement new acquisition methods that go beyond the simple waveform recording. A digitizer with DPP becomes a new instrument that represents a fully digital replacement of most traditional modules such as Multi and Single Channel Analyzers, QDCs, TDCs, Discriminators and many others.

The purpose of this document is to provide an overview on the digitizers, explain the main characteristics and the different operating modes and present the available options in terms of hardware, firmware and software in order to guide you in your choice.

Digitizers Selection Table

MODEL(1) Form Factor Number of channels

Max. Sampling

Frequency (MS/s)

Number of Bits

Input Dynamic

Range (Vpp)

Single Ended / Differential

Input

Bandwidth (MHz)

Memory (Msample/ch)(4)

DPP firmware(5)

x724 VME 8

100 14 0,5; 2.25; 10 SE, D

40 0.5; 4 TF, Desktop/NIM 4 SE PCIe 2 SE

x720 VME 8

250 12 2 SE, D

125 1.25; 10 CI, NG Desktop/NIM 4 SE PCIe 2 SE

x721 VME 8 500 8 2 SE, D 250 2 no

x731 VME 8/4

500/1000 8 2 SE, D

250/500

no Desktop/NIM 4/2 SE 2/4 PCIe 2 SE

x751 VME 8

1000/2000 10 1 SE, D

500 1.8; 14.4 / 3.6;

28.8 NG

Desktop/NIM 4 SE

x761 VME 2

4000 10 1 SE, D

t.b.d. 7.2; 57.6 no Desktop/NIM 1 SE

x740 VME 64

65 12 2 SE 30 0.19; 1.5 no Desktop/NIM 32

x742 VME 32+2

5000(2) 12 1 SE 600 0.128 (3) no Desktop/NIM 16+1

All

mod

els

Analog Input Single Ended: MCX 50 - DC offset adjust in the full range (±FSR/2) Positive, negative and bipolar inputs

Trigger

External TRG-IN, Software or channel self-trigger Common (all models/firmware) or Individual (DPP only) TRG-OUT for trigger propagation Digital timing and trigger filters for pulse triggering (DPP only) 32 bit Time Stamp

Synchronization

Daisy Chain (VME only) or one-to-many clock distribution Clock Cable delay compensation Sync start/stop through S-IN or GPI input or Trigger In/Out External Time Stamp reset

Memory up to 1024 acquisition buffers (2) Independent read/write access

FPGA Ability to do on-line Digital Pulse Processing (DPP) for pulse shaping and height analysis, digital charge integration, gamma-

Readout USB (30MB/s), CONET (80MB/S), VME (60MB/s MBLT, 120MB/s 2eSST) Independent read/write access

Other features 16 programmable LVDS GPIOs (VME only) Analog Outp

(1) The x in the model name is V1 for VME, VX1 for VME64X, DT5 for Desktop and N6 for NIM (2) Sampling frequency of the analog memory (switched capacitor array); A/D conversion takes place at lower speed (dead-time) (3) The memory size for the x742 is 128 events of 1024 samples each

denotes different options (5) DPP-TF: Pulse Height analysis (Trapezoidal Filters), DPP-CI: Charge Integration (digital QDC); DPP-NG: -n Discrimination

VME Desktop

NIM PCI Express

Introduction

3GD2080 Introduction to Digitizer

CAEN Digitizer block diagram.- The mother board defines the form factor; it contains one

FPGA for the readout interfaces and the services- The daughter board defines the type of digitizer; it contains

the input amplifiers, the ADCs, the FPGA for the dataprocessing and the memories

Principle of OperationThe basic operating mode of a digitizer is essentially thesame as a digital oscilloscope: the analog signal, after aninput stage of signal conditioning mainly used to adaptthe dynamic range, is sampled by a flash ADC, whoseoutput, i.e. the stream of digital samples, is continuouslyread by an FPGA and stored in a circular memory bufferof a programmable size. At the arrival of the trigger, thebuffer is frozen and made available for the readout whilethe acquisition can continue in a new buffer.

However, there are few important differences between adigitizer and a commercial digital oscilloscope:

The digitizers allow for dead timeless acquisition:In fact, unlike most oscilloscopes, they have theability to accept two consecutive triggers very close

to each other(1) thanks to the multi buffer memorymanagement, there is no dead time between anacquisition window and the next one. It is even possibleto accept two trigger for which the acquisition windowsoverlap. The dead timeless acquisition is a veryimportant feature, especially in the case of eventsrandomly distributed in time (in nuclear physics this istypically a Poissonian distribution), so that two of them,even at low rate, can occur at very short distance but youstill want to capture both.(1) Except for 742 Series.

In the digitizers, all the channels can generate a trigger at any time: the individual trigger can be used locally bythe channel that have generated it (independent triggering) or can participate to the assertion of a global trigger

for all the channels in the board, as well as to generate a pulse on the TRG OUT.

The digitizers are designed for scalability: it is possible to synchronize several boards to make an acquisitionsystem with a theoretically unlimited number of channels. Board synchronization consists in distributing a

common clock reference on which all the ADC sampling clock are locked, aligning the acquisition time base of eachboard in order to have events with correlated time stamps, distributing the channel auto triggers and the globaltriggers according to a programmable trigger logic in order to implement coincidences, neighbour triggering and otheracquisition criteria and topologies.

The High bandwidth data readout links. Usually the oscilloscopes do not have any communication channel (datais only displayed) or, if they have, most probably it has a fairly low bandwidth (GPIB, Ethernet, etc…); conversely,

the digitizers are designed to allow high rate data transfer to a computer or an external data processing unit. CAENdigitizers have a minimum bandwidth of ~30MB/s in the case of the USB port up to more than 120MB/s for the VMEwith 2eSST.

On Line data processing. The acquisition in thedigitizers is based on FPGAs. These are programmabledevices with the ability to manage the ADC sample

stream and implement on line digital algorithms for signalprocessing. This feature is of fundamental importance forthe implementation of systems that are not simply basedon the acquisition, storage and readout of waveforms (rawdata) but rather on the calculation of certain quantities ofinterest (e.g. the charge associated with a pulse, the pulseheight, the leading edge, the baseline, the arrival time andother parameters) and the storage and transfer of just thefinal results, with clear advantages in terms of readoutbandwidth.

1

2

3

4

5

Dead timeless acquisition

Individual pulse self trigger

Multi board synchronization forsystem scalability

High bandwidth data readout links

On line data processing (FPGA orDSP)

TIME STAMPS[0]

S[n-1]

S[1]S[2]S[3]

Memory Buffer

POST TRIGGERPRE

ACQUISITION WINDOW

Time

Sampling Clock

ADC

FIXED GAINAMPLIFIER

+

DAC

FPGA(AMC)

SRAMMEMORY

DAUGTHER BOARDS

FPGA(VME)

LOCAL BUS

PLL

CONET

CLK-IN

INT. OSCILL.

MOTHER BOARD

TRG-INSYNC-INTRG-OUT

GLOBAL TRGSYNC

SELF TRG

I/Os

CLK-OUT

SAMPLING CLOCK

DAC MONITOR

ANALOGINPUTs

VME/USB

n CHANNELS

4 GD2080 Introduction to Digitizer

DETECTORENERGY

SHAPE

TIMING

COUNTINGDPPIN

SAMPLESA/D INTERF

DIGITIZER COMPUTER

VERY HIGH DATA THROUGHPUT

60Co gamma spectrum acquired with a HPGe and DPP TF at University ofPalermo. In the box, a least squares fit of the 1.33 MeV photopeak; a FWHM <2 keV was obtained.

Digital Pulse processing (DPP) for physics and biomedical applicationsIn recent years, thanks to the availability on the market of flashADC more and more fast and precise, applications in the field ofnuclear physics (or related fields) have started to make large useof data acquisition systems based on digitizers. Compared to thetraditional and almost analog acquisition systems, in which theA/D conversion was performed only at the end of the chain, thenew systems have reversed the approach: the conversion todigital is performed as close as possible to the source of thesignal (output of the detector or preamplifier).

However, systems based on digitizers have a basic fundamentalproblem: the extremely huge amount of data to manage. As saidabove, high trigger rate, multitude of channels and high readoutbandwidth are the features that characterize the digitizers whencompared to the digital oscilloscope; at the same time, thesefeatures lead to large data streams. For this reason it is soimportant to have an FPGA with the ability to do on line dataprocessing and to extract from the raw samples sequence onlythe specific parameters necessary to the acquisition.

For this purpose, special algorithms can be applied to the digital samples coming from the ADC for the extraction of thequantities of interest and the reduction of the data; the relevant digital filters are, in most cases, very similar (at leastfrom a functional point of view) to the old and familiar analog circuits, such as timing filters, shapers, CFDs, baselinerestorers, etc..

The benefits of the digital approach are great stability and reproducibility, ability to reprogram and tailor thealgorithms to the application, ability to preserve the information of the signal along the entire acquisition chain,flexibility, better correction of unwanted effect such as baseline fluctuation, pile up, ballistic deficit, etc… All this inone board.

To understand better the principle of operation of the DPP, let's take a practical example: suppose you have a detectorfor spectroscopy (scintillator and phototube) which generates pulses with a duration of about 100 ns. We can realize analgorithm that permits to extract the pulse charge on line in the FPGA of the digitizer (i.e. continuously and in realtime); to do that, we need the calculate the baseline by means of a moving average filter, detect the pulses when the

signal exceeds a certain threshold respect to thebaseline (trigger), implement a digital delay linein order to compensate the trigger latency andfinally sum a certain number of samples withinthe integration gate after having subtracted thebaseline from them. The charge thus obtained,together with a time stamp that identifies theposition of the pulse, is the only informationstored in memory for that event, resulting in a

significant reduction of the data, such as 8 bytes per event, instead of saving the portion of the waveform that containsthe pulse and a piece of baseline, which could consist of tens or hundreds of samples.

Energy [keV]0 200 400 600 800 1000 1200 1400 1600

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One single board can do the job ofseveral analog modules

Full information preserved

Reduction in size, cabling, powerconsumption and cost per channel

High reliability and reproducibility

Flexibility (different digitalalgorithms can be designed andloaded at any time into the samehardware)

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Energy [keV]1324 1326 1328 1330 1332 1334 1336 1338 1340

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5GD2080 Introduction to Digitizer

2D plot of Energy versus PSD using an AmBe source at 2 kcounts/s; the two lobes ofthe neutrons and gammas are well separated. The data were acquired at DukeUniversity (TUNL).

In this example the algorithm realizes in digital form the same acquisition chain which traditionally consisted of adiscriminator, a QDC and an analog delay line to fit the pulse within the gate coming from the discriminator. Other examplesof algorithms implemented on CAEN’s digitizer (generically called DPP, or Digital Pulse Processing) are those for Pulse HeightAnalysis using trapezoidal filters, algorithms for the gamma neutron discrimination realized by means of Pulse ShapeAnalysis, zero suppression (removal of parts of signal that do not contain pertinent information), implementation of digitaltiming filter or digital constant fraction discriminator to trigger pulses also in the presence of baseline fluctuation or pile up

and to determine precise timing information,mixed acquisition modes in which thequantities of interest calculated by the DPP(charge, height, etc ...) are saved together witha short but significant chunk of waveform (e.g.the rising edge) to allow for off line analysis,etc..

Furthermore, CAEN is willing to collaboratewith customers to create other types offirmware and algorithms for specificapplications.

From the commercial point of view, thedigitizers are sold with the standard version ofthe firmware that implement only thewaveform acquisition mode (oscilloscopemode) or, in certain cases, some techniques ofzero suppression. The firmware that

implements a certain DPP algorithm is sold separately. The user can download any of this DPP from the our Web site, install iton the digitizer that supports it and try it for free, with the only limitation that the acquisition may not last more than 30minutes, after which you must perform a power cycle (Time Bomb). Only after buying that firmware and its relevant license(that is installed on the card in the form of electronic key) the time bomb will be eliminated. The table below shows all typesof DPP and the relevant digitizers that support them. For more detailed information on the DPP algorithms and firmware,refer to the White PaperWP2081.

Name Model Status Detectors (typ.) Notes

DPP TF 724 Ready Hi res. Si, Ge Pulse Height analysis (Trapezoidal Filters)

DPP CI 720 Ready PMT, SiPM Charge Integration (digital QDC)

DPP NG 720, 751 Q2 2011 Organic liquid n Discrimination

Trigger and synchronization in multi board systemsIn most cases, the applications that require the use of a multitude of channels, need to synchronize the acquisitionacross different digitizers: this is done in three points:

Distribution of a common clock reference in order to have the same sampling clock on all the ADC channels. CAEN’sdigitizers feature a programmable PLL able to generate the sampling clocks locked to an external clock input (usually at

lower frequency) whose distribution can be done in parallel from a common source, using a fan out as well as through an inout daisy chain with the ability to use the first board as a clock master and to compensate for delays in the cables by means ofa programmable phase shift. The daisy chain mode is available only in the VME models.

Alignment of the time stamp associated with the triggers to allow off line reconstruction of the events read fromdifferent boards. This can be done by using a dedicated SYNC input as well as through the TRG IN, TRG OUT daisy

chain by issuing a first pulse acting as a start of acquisition; this pulse doesn’t trigger the channels. After that, the TRGIN and TRG OUT start their normal operating mode that is to receive and send triggers.

Distribution of the triggers from channel to channel and from board to board, according to a certain trigger logic. Eachcard has different sources of trigger: external TRG IN from the front panel, software trigger and channel self triggers. All

these triggers can be combined in order to make coincidences, majorities, global triggers and other functions. It is worthnoticing that the DPP firmware, compared to the standard version, gives important advantages also in terms of triggermanaging: first of all, the digital filters are able to detect the input pulses and generate triggers even in the presence of noise,baseline fluctuation and pile up that make inefficient the simple trigger mechanism based on a fixed threshold. Furthermore,with the DPP, the channels operate independently and it is possible to implement the trigger propagation on channel tochannel basis, also including channels on different boards. This option is particularly useful in the segmented detectors wherethe detection of one pulse in one segment requires the acquisition of the signal also in the neighbour segments in order tocalculate the exact position in which the particle hit.

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6 GD2080 Introduction to Digitizer

SoftwareCAEN provides the drivers for all the different type of physical communication channel (namely USB, the proprietaryCONET Optical Link managed by the A2818 PCI card or A3818 PCIe cards and the VME bus accessed by the V1718 andV2718 bridges), a set of C and LabView libraries, some demo applications and utilities. Windows and Linux are bothsupported.

More specifically, the available software is the following:

CAENComm library

It contains the basic functions for the access tothe hardware; the aim of this library is toprovide an unique interface to the higher layersregardless the type of physical communicationchannel. Note: for the VME access, CAENcommis based on CAEN’s VME bridges V1718 (USB toVME) and V2718 (PCI to VME). In the case ofthird part bridges or SBCs, the user mustprovide the functions contained in theCAENcomm library for to the relevant platform.

CAENDigitizer library

It contains the functions to program thedigitizers, manage the acquisition, executethe readout, unpack the data, send triggers,etc...

WaveDump

It is a C console application that lets you to program thedigitizer (according to a text configuration file thatcontains a list of parameters and instructions), to startthe acquisition, read the data, display the readout andtrigger rate, apply some post processing (such as FFTand amplitude histogram), save data to a file and alsoplot the waveforms using the external plotting toolgnuplot that is available on internet for free. Theprogram is quite basic and has no graphics but it is anexcellent example of C code that demonstrates the useof libraries and methods for an efficient readout anddata analysis. The user who intends to write thesoftware on its own is suggested to start with thisdemo and modify it according to his needs.For more details please see the WaveDump User Manul and Quick Start Guide (Doc nr.: UM2091, GD2084 ).

CAENScope

It is a fully graphical program thatimplements a simple oscilloscope: you cansee the waveforms, set the triggerthresholds, change the scales of time andamplitude, perform simple mathematicaloperations between the channels, save datato file and other operations. CAENscope isprovided as an executable file; the sourcecodes are not distributed.

NOTE: CAENScope does not work withdigitizers running DPP firmware.For more details please see the CAENScope Quick StartGuide GD2484.

WaveDump

VME Digitizers

VME

CONET2 (Optical Link)

Desktop Digitizers

A2818 A3818

NIM Digitizers

PCIe Digitizers

DPPRunner Other Application

CAENDigitizer Library

CAENComm Library

A2818 driver A3818 driver V1718 driver USB driver P72XX driver

PCI

PCIe

PCIe

USB 2.0 USB 2.0USB 2.0

V1718

V2718

7GD2080 Introduction to Digitizer

cvUpgrade

This is a tool that allows the user to update thefirmware of the digitizers, change the PLL settings(i.e. set the ADC sampling frequency, enable theclock output, etc...), load, when requested, thelicense for the pay firmware (for example, the DPPCI and DPP TF) and other utilities.For more details please see the cvUpgrade Quick Start GuideGD2084.

DPPrunner (TF or CI)

It is an application that managesthe acquisition in the digitizersthat have a DPP firmwareinstalled on it. The program ismade of different parts: there isa GUI whose purpose is to set allthe parameters for the DPP andfor the acquisition; the GUIgenerates a textualconfiguration file that containsall the parameters. This file isread by the Acquisition Engine,which is a C console application

that programs the digitizer according to the parameters, starts theacquisition and manage the data readout. The data, that can bewaveforms, time stamps, energies or other quantities of interest,can be saved to output files or plotted using gnuplot as an externalplotting tool, exactly like in WaveDump.

NOTE: DPPRunner TF works only with Mod. 724 and DPP TF whileDPPRunner CI works only with Mod. 720 and DPP CI

CAEN SpA is acknowledged as the only company in the world providing a complete range of High/Low Voltage PowerSupply systems and Front End/Data Acquisition modules which meet IEEE Standards for Nuclear and Particle Physics.Extensive Research and Development capabilities have allowed CAEN SpA to play an important, long term role in this field.Our activities have always been at the forefront of technology, thanks to years of intensive collaborations with the mostimportant Research Centres of the world. Our products appeal to a wide range of customers including engineers, scientistsand technical professionals who all trust them to help achieve their goals faster and more effectively.

CAEN S.p.A. CAEN GmbH CAEN Technologies, Inc.

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Electronic Instrumentation

Guide GD2080 Introduction to digitizer rev. 0 15 March 2011

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Copyright © CAEN SpA. All rights reserved. Information in this publication supersedes all earlier versions. Specifications subject to change without notice.