02-02-2016 09:16 AM
I try to use NI USB 5133 to read the DC output voltage of a photodetector. But the output voltage is smaller than I expect.
First, I mesure output voltage with a 50Mohm multimeter. The voltage is 5V. Then I use Ni USB 5133 (1Mohm) to measure it, but the output voltage is 1V. Since both multimeter and DAQ are high impedance, I think the output voltage should be the same, but they are not. Can any tell me how to solve this problem? Thank you.
The photodetector I use is ThorLabs DET10C.
02-02-2016 09:32 AM
If you apply a battery you will find that both units will read 1.5( or so) V 🙂
The output of the Detector is a current! and since you use a 5133 , I assume you want high bandwidth , so why not read the detector manual and use a 50Ohm termination and read that with both intruments ? 😉
02-02-2016 09:48 AM
Thanks for the quick reply. I am reading a DC voltage, so I don't really care about the bandwidth.
I am treating the detector as Thevenin's model, an output voltage and a 50 ohm series resistance. Since in both cases, the load resistance (Mohm) is much larer than the source resistance (ohm), I think I should read similar output voltage. Can you explain why? Thank you.
02-09-2016 11:50 AM
So you are treating the output of your detector as Voltage or current?
In Thevenin model, your detector will be the Voltage Source?
If you want the output of your detector to be Voltage, you have to wire a resistance (50 Ohm) as the load resistance.
https://www.thorlabs.com/thorcat/13100/DET10C-Manual.pdf