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Design and Implementation of a 2-Channel High Precision and High Speed Digitizing system

Llimós Muntal, Albert LU (2016) EITM02 20161
Department of Electrical and Information Technology
Abstract
Recently, new spectroscopic techniques have been developed such as photocurrent detected two-dimensional spectroscopy [1], which measure the response from a sample that is excited by a high intensity laser pulses. The responses are linear and nonlinear signals that have very different amplitudes; the linear signal can be from 103 to 105 times larger than the nonlinear signal. These two signals have to be digitized and transferred to a computer to be able to isolate the photocurrent nonlinear signal. This requires analog to digital converters (ADC) with very high dynamic range.
Some ADCs in the market meet these requirements but they can only sample one channel and this application requires two. Using two separate converters is not... (More)
Recently, new spectroscopic techniques have been developed such as photocurrent detected two-dimensional spectroscopy [1], which measure the response from a sample that is excited by a high intensity laser pulses. The responses are linear and nonlinear signals that have very different amplitudes; the linear signal can be from 103 to 105 times larger than the nonlinear signal. These two signals have to be digitized and transferred to a computer to be able to isolate the photocurrent nonlinear signal. This requires analog to digital converters (ADC) with very high dynamic range.
Some ADCs in the market meet these requirements but they can only sample one channel and this application requires two. Using two separate converters is not optimal due to phase delays in the sampling. Therefore, in this project, a two-channel high-speed and high-precision digitizing system is built. It is a complete system that includes both analog and digital hardware as well as software. It consists of two ADC boards that are controlled using a Field Programmable Gate Array (FPGA) included in an FPGA board. In order to communicate this modules, an interfacing board has been designed. The code elaborated in this project for the FPGA programming has been written in VSIC Hardware Description Language (VHDL). In order to control the data communication with the PC, a software interface application is developed.
The digitizing system built can function at a sample rate up to 4MS/s and has a resolution of 23 bits. This system only uses one programmable module, which reduces its cost drastically compared to two separate one channel digitizing systems.
The tests performed with the system demonstrate that even with just one programmable block, a high speed digitization and data transfer can be achieved. The data acquisition system is able to sample one million samples per channel per acquisition event in 780 ms. Moreover, it can sample up to 4MS per channel per acquisition event. It is worth mentioning that the phase delay between the two digitized signals is low, averaging 0,383°, with a standard deviation of 0,247°, which is below the maximum 1° allowed for this application.
This high-speed and high-resolution system is not only suited for photocurrent spectroscopic applications but it can also result on the improvement of other systems like high end sound cards or ultrasound imaging. (Less)
Popular Abstract
In the Chemical Physics field, new techniques have been developed where the photocurrent generated has to be measured when a source of instantaneous energy is provided (pulsed laser). Photodiode’s response to this stimulus has a linear component as well as a nonlinear one depending on the energy absorption characteristics. For those techniques it is essential to process and study the nonlinear response. For this purpose, it is necessary to isolate the two signals and transfer them to a computer. This requires the digitization of the signals, which means describing the signals generating a series of number (bits) that describe a discrete set of its samples. This is done using an analog to digital converter (ADC). Given that the linear... (More)
In the Chemical Physics field, new techniques have been developed where the photocurrent generated has to be measured when a source of instantaneous energy is provided (pulsed laser). Photodiode’s response to this stimulus has a linear component as well as a nonlinear one depending on the energy absorption characteristics. For those techniques it is essential to process and study the nonlinear response. For this purpose, it is necessary to isolate the two signals and transfer them to a computer. This requires the digitization of the signals, which means describing the signals generating a series of number (bits) that describe a discrete set of its samples. This is done using an analog to digital converter (ADC). Given that the linear signal is a lot larger than the nonlinear, ADCs with very high dynamic range are needed, which is the ratio between the maximum and the minimum input signal that can be digitized. The digitized data has to be stored to avoid data loss and then transferred to the computer.
In the current market there are no two-channel digitizing systems that meet the requirements for this application, either its sampling rate is not high enough or its dynamic range does not meet the specifications. Therefore, the goal of this project is to build a two-channel high-speed and high-precision digitizing system suited for this application. In order to do that, two ADC boards are selected to implement the two-channel digitizer. For the storage and transfer of data a logic programmable module as well as a memory is needed. To fulfill this requirement, another board containing a field programmable gate array (FPGA) and a memory is included in the system. This board also contains a USB microcontroller that enables the data transferring from the system to the computer via USB. Finally, connecting these boards is done through an interfacing board, which is designed in this project.
The logic in the FPGA is programmed using VHDL code. The data shifted from the ADC is in a serial bus and, since its storage in the memory has to be done through a parallel bus, the first step is to deserialize it. Then, another block is implemented to manage the reading and writing from the memory since it cannot be done simultaneously. Finally, a software interface is developed based on an application programming interface provided by Opal Kelly, the manufacturer of the FPGA board. This interface contains a set of protocols and tools to build an application that enables the communication and data transfer between the system and the computer.
The digitizing system built in this work meets all the speed and precision requirements and can sample the signals up to the frequency that is needed. The storage, management and transfer of data is also done efficiently since the time required for the system to acquire all the data is significantly lower than other systems. Moreover, the phase delay between the two digitized signals has successfully been minimized as well as its variance, which was a critical aspect for this work. Finally, the cost of this system is lower than two separate one-channel digitizing systems.
The implementation of this digitizing system is not only limited to spectroscopic techniques but it can also be used in many other applications that require a high-speed and high-precision digitizing system. This can include, for example, sound cards systems or even ultrasound applications. Moreover, this project can be a good base for improvements as its performance is limited by the memory capacity as well as the transfer rate of the USB bus, modules that can potentially be changed. Finally, its proven functionality and operation method can be a base for building an integrated one-board system containing all the modules described in this project. This also opens the possibility for increasing the number of channels of the system. (Less)
Please use this url to cite or link to this publication:
author
Llimós Muntal, Albert LU
supervisor
organization
course
EITM02 20161
year
type
H2 - Master's Degree (Two Years)
subject
keywords
Analog, Digital, ADC, Digitizing, data acquisition, two channel, precision, resolution, speed
report number
LU/LHT-EIT 2016-545
language
English
id
8893694
date added to LUP
2016-10-19 10:52:50
date last changed
2016-10-25 09:09:33
@misc{8893694,
  abstract     = {Recently, new spectroscopic techniques have been developed such as photocurrent detected two-dimensional spectroscopy [1], which measure the response from a sample that is excited by a high intensity laser pulses. The responses are linear and nonlinear signals that have very different amplitudes; the linear signal can be from 103 to 105 times larger than the nonlinear signal. These two signals have to be digitized and transferred to a computer to be able to isolate the photocurrent nonlinear signal. This requires analog to digital converters (ADC) with very high dynamic range. 
Some ADCs in the market meet these requirements but they can only sample one channel and this application requires two. Using two separate converters is not optimal due to phase delays in the sampling. Therefore, in this project, a two-channel high-speed and high-precision digitizing system is built. It is a complete system that includes both analog and digital hardware as well as software. It consists of two ADC boards that are controlled using a Field Programmable Gate Array (FPGA) included in an FPGA board. In order to communicate this modules, an interfacing board has been designed. The code elaborated in this project for the FPGA programming has been written in VSIC Hardware Description Language (VHDL). In order to control the data communication with the PC, a software interface application is developed.
The digitizing system built can function at a sample rate up to 4MS/s and has a resolution of 23 bits. This system only uses one programmable module, which reduces its cost drastically compared to two separate one channel digitizing systems.
The tests performed with the system demonstrate that even with just one programmable block, a high speed digitization and data transfer can be achieved. The data acquisition system is able to sample one million samples per channel per acquisition event in 780 ms. Moreover, it can sample up to 4MS per channel per acquisition event. It is worth mentioning that the phase delay between the two digitized signals is low, averaging 0,383°, with a standard deviation of 0,247°, which is below the maximum 1° allowed for this application.
 This high-speed and high-resolution system is not only suited for photocurrent spectroscopic applications but it can also result on the improvement of other systems like high end sound cards or ultrasound imaging.},
  author       = {Llimós Muntal, Albert},
  keyword      = {Analog,Digital,ADC,Digitizing,data acquisition,two channel,precision,resolution,speed},
  language     = {eng},
  note         = {Student Paper},
  title        = {Design and Implementation of a 2-Channel High Precision and High Speed Digitizing system},
  year         = {2016},
}