PMOD and FPGA-based LIDAR

[Edgar Vallar]

Hi!

My professor loaned to me their Digilent BASYS2 FPGA board.

We wish to develop an FPGA-based Laser Remote Sensor (LIDAR) system.

The BASYS2 will be triggered by the pulse driver of our Laser Diode.  The Laser Diode pulses will be detected by a photomultiplier tube detector at selected rates and bin widths. 

We will connect the output (analog) of the photomultiplier tube to the BASYS2.  How can we do this?  Is it via the PMOD?  Do we need an A/D converter like the PMOD AD1 or PMOD AD2? 

The BASYS2 has 4 PMODs with 6 pins each.  Which PMOD (AD1 or AD2) is better for us?

The AD1 has 6 pins only but has 2 channels while the AD2 has 12 pins and 4 channels (and is also half the price).

If we buy the AD2, can we connect it to two PMODs in the BASYS2?  Will this work?

For another question, once the BASYS2 receives a trigger, the BASYS2 will read the PMOD to get the analog reading from the photomultiplier tube.  How can we get the reading? Is it via the register address for that PMOD?  Since this is digital, how can we know the voltage reading of the photomultiplier tube?

Sorry for asking too many questions.

Thanks in advance!

Sincerely,

 

Edgar Vallar

[JColvin]

Hi Edgar,

If you want to connect an analog output to the Basys 2 board, you would need to use a Pmod such as the PmodAD1 or the PmodAD2. Either one of these Pmods can connect to a Pmod port on the Basys 2. The PmodAD1 would connect to just one port, and you can actually connect the PmodAD2 to just one port by only using just the upper or lower set of four pins. (The reason this can be done is because the PmodAD2 communicates via I2C, so there is a redundant set of pins on the Pmod so that it could be placed in an I2C chain).

To determine if either one of those Pmods will suit your needs, we'll need to ask a few questions. The first is what voltage does your photomulitplier tube operate at? The Pmods are only to operate from approximately 2.5V to 5V for each Pmod (it varies between the two Pmods) and they are only able to accept voltage inputs ranging from 0V to whatever voltage they are being operated at.

It will also depend on how much data you want to receive within a set time frame. The PmodAD1 can provide up to 1 mega-sample per second of data, and the PmodAD2 by default will send out data at 25 ksps, but can get up to 62 ksps. If you need data slower than either of those rates, then either Pmod will work (as long as the voltage provided by the photomultiple is within the Pmods working range).

As for receiving the data, both Pmods will provide a stream of digital data with 12 of the bits representing the received analog data. You can think of these 12 bits representing the proportion of the measured analog voltage to the reference voltage of the Pmod (usually whatever voltage the Pmod is powered at). So, if you were operating the Pmod at 3.3V and you collected a data stream that gave you (010101010101) that represents a value of 1365 out of a maximum of 4096 (which is 2^12) so you would say that you measured a voltage equal to 33% (1365 / 4096), of the 3.3V or 1.1V. 

Note that each Pmod will not provide just 12 bits of data though; each Pmod communicates through it's own protocol (either SPI or I2C) so you would need to follow the communication protocols respective style.

Let me know if you have any more questions.

Thanks,
JColvin

 

[Edgar Vallar]

Hi!

Thank you very much for the information!

What will happen in case we exceed the voltage for the ADC?  Will we permanently damage it?

Thank you very much.

Sincerely,

Edgar

 

[JColvin]

Hi Edgar,

The two datasheets for the ADCs on the PmodAD1 and the PmodAD2 (here and here, respectively), indicated in their 'Absolute Maximum Ratings' section that neither ADC can withstand any voltage outside of the range from -0.3V to Vdd + 0.3V without sustaining permanent damage, but they also say that being outside of the recommended ranges but inside of the absolute maximums does not guarentee performance either, so I would guess that yes, it would probably damage the ADC unfortunately.

Thanks,
JColvin

[Edgar Vallar]

Hi!

Thanks a lot.

Edgar