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trouble connecting photodetector to DAQ, input impedance

Hi Labview masters,


I would like to connect a photodetector to the analog input of DAQ to measure incoming light power in voltage. I am relatively new to this type of experiment. The hardware models are:

1. Photodetector: Thorlabs DET36A, here is manual

2. DAQ: NI 6343 X Series Data Acquisition, here is specs


According to manual of the photodetecoter, I need to use a 50 Ohm coax cable and 50 Ohm terminating resistor to connect between the output of the detector to a voltage measurement device. So here is what I did:

Connect one end of RG-58 U/C to the output of DET36A, then attach a 50 Ohm terminating resistor to the other end of RG-58 U/C which then is connected to the AI0 BNC termination of the NI 6343 (FS). I used NI MAX test panel to monitor the signal of ai0, continuous mode. But the votage  value is fluctuating around zero when I shone light on the detector, which is obviously too small, because the output of DET36A is 0-10V.


I also tried not including the 50 Ohm terminating resistor, but then the voltage was fluctuating very close to around 10.7 V when I shone light on the detector or shield the detector from any light. 


Could I have suggestions about how to get the correct voltage signal from the detector?

I suspect it has something to do with input impedance of the AI of NI6343. The detector manual says 'Note the input impedance of your measurement device since this will act as a terminating resistor'.  But The specs of 6343 says the input impedance >10 GΩ in parallel with 100 pF, which is confusing. What does it mean?  Many thanks!





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Message 1 of 3

Your detector is generating a relatively small current based on a bias voltage of ~12 V. The reponsivity is in the order of 0.5 Amp / Watt but the power of your light is not that high, so expect relatively small variations.


When you load with 50 ohm the bias voltage of your signal will drop to almost nothing and your voltage responsitivity will be very low, sounds like what you are seeing.


When you don't have a load resistance the input impedance of your DAQ board is very high (> 10 Gohm) and the signal bias will probably still be more than 10V, so likely overload your DAQ board (the 10.7 V you are observing is out of your board working range of +/- 10 V).


If the signal you see with a 50 ohm load is too small (have you checked it using the DAQ board +/- 200 mV range?) increase the load resistance, trying different load values like 1 kohm, 10 kohm ... 1 Mohm, until you find a good working point (say +150 mV) with highest responsivity. This will affect the bandwidth of your measurement but your achieved bandwidth is probably still higher than your DAQ device.

Message 2 of 3

Thanks a lot, it works!

Increasing the terminating resistor help increase the detector readout voltage, only we need to match it with very large load. So we also decided to purchase a detector with gain. 

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Message 3 of 3