Nexys 4 DDR - Read signals from photodiode reciver

[ezadobrischi]

Hello! 

I made a reciver for procesing the signal front a light source and now I want to read the signals front the board with Nexys 4 DDR. After reading the signal I want to filter and process that signal, reduce the noise, applying various algorithms. How do you think it would be better to use Nexys 4DDR in this project? How can data be read using this Board?

[jpeyron]

Hi @ezadobrischi,

Welcome to the Digilent Forums.

Please be more specific on the Photo Diode output. Based on basic Photo Diode output I would:

1) Convert the photo diode receiver current output to voltage and use an ADC to read the voltage signal.

2) The Nexys 4 DDR has and on-board XADC (xilinx analog to digital converter)  7 Series FPGAs and Zynq-7000 SoC XADC Dual 12-Bit 1 MSPS Analog-to-Digital Converter User Guide. Here is our XADC demo for the Nexys 4 DDR done in Verilog.

3) The voltage input range is 0v to 1v in unipolar mode and -.5v to .5v for bipolar mode for the on-board XADC.

4) If the voltage is not within the 0v -1v  then I would either use a level shifter circuit to bring the analog signal into the 0-1v range or use something like the Pmod LVLSHFT.

5) If the voltage is in the 3.3v - 5v  range and do not want to use use a level shifter you can use other ADC's  like the Pmod AD1.

Once you have the signal in the Nexys 4 DDR you can filter the signal.  

cheers,

Jon

[ezadobrischi]

thanks for the information ?

for the first step will be using XADC   because i have external ADC ( EVAL-AD7400AEDZ but i don't have another board for processing and make connection with Nexys 4 DDR).

I will try to read directly with the Nexys 4 from the photodiode circuit.

 

I will return with results!!!!!
 

[xc6lx45]

Hi,

>> applying various algorithms

if you don't know the algorithms yet, it might be easier to get the ADC data into a softcore CPU, then prototype in C using floating point. The point is, experimental prototyping of algorithms in fixed point RTL is a slow and painful process.whereas reloading a .elf binary takes a second.

I'm guessing your intent but I suspect you'll find eventually that an FPGA is not the optimal platform choice for your application. Meaning, you can most likely get the same result cheaper and with less effort e.g. using a Raspberry PI + SPI converter. I'm guessing this simply because higher-rate ADCs where FPGA makes sense (hundreds of MHz) are hard to come by as OTS modules. Otherwise, if you design for, say, 1 MSPS, the FPGA fabric will do less than 1 % of the work it could do but you pay for 100 % so people usually don't use FPGA, if a CPU or DSP will do the job.

[xc6lx45]

... was just in the middle of rephrasing the above post somewhat more diplomatically. There are many good reasons to use FPGA, e.g. an FPGA engineer will probably earn a higher hourly rate than a Raspberry PI programmer so don't let me distract you ?

[zygot]

45 minutes ago, xc6lx45 said:

FPGA engineer will probably earn a higher hourly rate

Never make assumptions about how much your skills are worth... I know from experience. Engineers are, in most places, at-will employees. If you don't know what this means then I'll provide a hint: You can fly from across the globe for an interview and come to work a week later to find that you are unemployed before you log in an hour and that's just too bad. I know of such a case. The days when engineers were valued as assets are long gone. Engineers aren't professionals in any meaningful way and those salary surveys might not have anything to do with you. Companies want to cut costs where it's easiest and they get the best short term (often imaginary) benefits. Tha's not to say that a company that's stuck and manager's jobs are being threatened won't pay exorbitant prices in hope of extricating themselves from self-inflicted gunshot wounds to the foot.... but don't make a career choice hoping to make money. Because they are collectively pretty stupid they aren't likely to make good decisions about throwing money at digging themselves out of a hole either. There are books about how a group fairly bright people can pool their collective intellectual resources into an entity with the brain power of a nail....

Perhaps another way to make xclx45's argument is to say that an FPGA often not the proper solution to a great many problems. FPGA devices by their very nature will never compete with one chip commercial solutions for a large variety of product needs. FPGAs aren't the future as they say, but when you need them it's nice to know how to use them.