Creating VI for photodiode detector. Help me

[akhpal]

Hi,

I am student and doing research on liquid crystals for LCDs and smart windows. Recently, my Professor bought some NI Hardware and two photodiode detectors.

details of my setup is given below,

1. photodiode detectors (PDA36A-EC Si Amplified Detector, ThorLABS) here are detector specifications

2. NI cDAQ-9172 and NI9205 module which is analog input 10V.

3. 632.8nm HeNe Laser.

I want to measure the light intensity on photodiode detector. the response time of LIQUID CRYSTAL CELL to electric feild in milliseconds (ms). Now I want to know that how can I make such VI by which I can measure the light transmittance and sve the signals to a file? I am new one to this feild and no one knows, how to use this setup. plz help me out in wiring the photodiode detectors wiring to module NI 9205 and making VI.  Thanks alot.

[smercurio_fc]

My suggestion is to hold off on the code for right now. Your first order of business should be to connect the sensor to the DAQ module. The sensor has a BNC connector. Do you have the 9205 with the D-sub connector or the spring terminal connector? If you have the D-sub you'll need to make a cable, which isn't that difficult since you're only talking about one signal. You may want to consider getting the 9933. Either way don't forget the 50 terminating resistor at the DAQ module end if your cable is longer than 12 inches. If you can, keep this cable short. If you don't have the skills to make the cable, then just find someone - there must be someone there who knows a little about electronics, and it's not like you woud be asking them to rebuild Apollo's landing computer.

Once you've done this you can use Measurement and Automation Explorer (MAX) to test the connection and make sure the DAQ system is reading correct voltages. MAX provides test panels so you can see what the DAQ module is reading. Thus, you can vary the laser and see the voltage change on the test panel. MAX is installed automatically when you install LabVIEW or DAQmx (which you will need if you don't already have it).

Once you've gotten this far then you can look at what you need to do with the LabVIEW code. In your case it will probably be a simple loop with a DAQ Assistant Express VI to collect the data and a Write Measurement to File. Both are Express VIs which have dialogs for setting up how they work. But, like I said, get the hardware connected first, then worry about the sofware. 

P.S. By the way, is this in any way related to your query from a few months ago?

Message Edited by smercurio_fc on 05-03-2010 11:02 PM

[akhpal]

Thanks for fast reply.

I have done hardware setup. but the signals from the detector are not just as I was wanting. the detectors are giving me response in voltage. it has following drawbacks,

1) I am wanting signals (data rate) such that it could read thousand time in one sec. I mean that I want signals at a rate of 1millisecond. It is giving me the following messege when I try to get data at rate of 1000. (please solve this problem for me. It is very necessary.)

2) when I probes full laser intensity, it saturates and give maximum of 10 V as output. please help me

[akhpal]

here is my VI.

[GarPar]

Hi akhpal-

This error is occurring because you are not reading the data off the FIFO fast enough. The general rule of thumb is to set your samples to read to about 10% of your sampling rate. The samples to read will be your buffer size. You have set your samples to read at 1. Try setting it to 100 and see what happens. You might try setting it even higher, to about 300-400 and see how the VI performs. The DAQ assistant can be finnicky at times because it is a prepackaged set of VI's under the hood, which brings a lot of overhead along with it. The code, as you currently have it, may not run very deterministically. That's why you should increase your buffer size (number of samples to read) - so that there is more space to store samples as they come in while the DAQ assistant waits for the rest of the code in the while loop to iterate.

It looks like your probe has a maximum of 10V as an output.  If I understand you correctly, you are wondering why, at full intensity, the probe outputs 10V.  It seems like that should be the case, since the maximum is 10V from the specs document you supplied.

Best of luck with your application.  I hope this information helps!

[johnsold]

akhpal,

From the data sheet you linked the sensitivity at the lowest gain setting for the photodetector is 1.5e3 V/A into a high impedance load multiplied by the photodiode conversion of 0.65 A/W.  With a bit of calculation the total power to drive the output to the maximum value is less than 10 mW or about 78 mW/cm^2 if uniformly illuminated.  Does your laser put out more power than 10 mW?  Is it focused such that the intensity can exceed 78 mW/cm^2 on even part of the detector?  This would explain the saturation.  You may also need to consider the laser wavelength and the  photodetector's spectral response curve.

I know many students have not had much exposure to datasheets, but this is a good time to learn.  Read the datasheet, verify the calculations above, and compare to the capabilities of the devices in your lab (which may require reading a few more datasheets).

Lynn