Unable to read voltage correctly from photodetector.

[hamjam]

I am trying to do something very simple, but am new to Labview.  I simply want to read a voltage input from a photodetector.  I am trying to use the DAQ solution wizards' prefab VI to do this, but the voltage output from the detector isn't making it to the computer.  The interface consists of:

-photodetector

-CB-68LP I/O board

-PCI-6023E DAQ

-LabVIEW 6.1 on windows XP

After configuring a virtual channel (which I may be doing incorrectly) to use with the VI, the voltage just maxes out a 5V and stays there even though the input is <3V.

Any ideas would help.

[Jordan F]

Hi,

I am curious as to what pins you are using when connecting your photodector to the CB-68LP (6023E).  It seems to me as if there is a grounding problem with your signal source.  Here is an article that shows how to connect a grounded and ungrounded signal source to the analog input on your DAQ card.  There is a table toward the middle of the document that shows some different connections.

Field Wiring and Noise Considerations for Analog Signals

http://zone.ni.com/devzone/cda/tut/p/id/3344

If you have the proper connection, I would recommend to try out some of the Shipping Examples with Traditional DAQ.  Here is how to find those examples:

Where are the LabVIEW Traditional NI-DAQ Examples?

http://digital.ni.com/public.nsf/allkb/46D0C7360A10D25F862571B5007B4411?OpenDocument

 Once here, navigate to daq>>anlogin>>anlogin.llb and try one of the analog input examples.

[hamjam]

I was plugging in directly to pins 28 & 29, but after reading some of your first reference, I see there may be more to it.  I tried using the suggested configuration for a NRSE, but this time I see zero signal instead of saturation.  To be more specific here is what I was doing:

Channel configuration selections:

 -analog input

-single ended non-referenced

-AI -4

Use "Continuous Voltage Measurement" from the DAQ solutions wizard and select the channel I configured above.

Wiring:

-BNC from photodetector 

- + wire into AI4

- - wire into AI sense

- a line from each( with a 10K resistor)into AI GND

Any corrections/suggestions?

[AnalogKid2DigitalMan]

What is the expected voltage range of the output of the photodetector? Can you provide the manufacturer and model number?

-AK2DM

[hamjam]

I would say the expected output of the PD is from 0-5V, but that is completely variable as I am not interested in the absolute power.  I only need a relative measurement of the power fluctuation of the laser .  I am using a Thorlabs DET110, with a pulsed laser centered at 800 nm(~100fs pulse, 1kHz reprate, ~1mJ pulse).  After you brought this up, I noticed that Thorlabs recommends a load resistor between the BNC and measuring device.  This might have had something to do with the saturation. 

Any ideas on the setup would be welcome.  I just want it to be robust and consistent.  

[AnalogKid2DigitalMan]

Yes, a load resistor is required in order to convert the current to a voltage that you can read. Also make sure the battery in the unit is good- and the power switch is on too 🙂

-AK2DM

[hamjam]

I have a load resistor for the photodetector, and the voltage going into the I/O board is ~1.5V.  I'm still saturating at 5V though when I run the VI (even when the photodetector is turned off and blocked).  I'm not sure what the problem is. 

[Jordan F]

What is the load resistance value you are using?  How do you have everything connected now?  Also, since your photodetector is a floating source you can also use a single ended ground reference mode instead.  This connection will use the AI for the positive termial and AIGND for the negative terminal.

[hamjam]

I was using a 50 ohm resistor and 50 ohm BNC as Thorlabs recommended.  However, I have changed the setup to RSE (+ on AI6 and - on AI GND), with the same result:  saturation at 5V.  I ran a test on the channel I configured with MAX, and simultaneously checked the input voltage to the I/O board with a digital multimeter.  As soon as I touched the multimeter probes to the inputs, the level of the test dropped from 5V to the correct voltage around 2.5V.  I checked the same pins on the I/O board with a AA battery, and the readings were normal which makes me think there isn't necessarily a grounding issue with the board itself, but a grounding problem with the photodetector.

Any ideas on what this means, and how to fix it?

[hamjam]

I was using a 50 ohm resistor and 50 ohm BNC as Thorlabs recommended.  However, I have changed the setup to RSE (+ on AI6 and - on AI GND), with the same result:  saturation at 5V.  I ran a test on the channel I configured with MAX, and simultaneously checked the input voltage to the I/O board with a digital multimeter.  As soon as I touched the multimeter probes to the inputs, the level of the test dropped from 5V to the correct voltage around 2.5V.  I checked the same pins on the I/O board with a AA battery, and the readings were normal which makes me think there isn't necessarily a grounding issue with the board itself, but a grounding problem with the photodetector.

Any ideas on what this means, and how to fix it?