low voltage with NI 9215 BNC and Thorlabs DET02AFC

[Agnea]

Hello everyone,

I am using a NI 9215 with BNC connector to acquire a voltage signal from Thorlabs DET02AFC photodetector, with a T4119 terminator connected between the NI card and the coax cable. Other ports on the 9215 are not in use. With this, I would like to measure light intensity from a M700F3 LED with 1.7 mW power, which theoretically should give me a signal of about 30 mV. But with the current setup, I get only about 3-4 mV at full LED power, measured via NI MAX. 

To exclude hardware problems, I have tried the following:

- put a flashlight in front of the detector - I do not see any response at all, so I'm assuming that the LED is working correctly.

- varying LED power - I do see a response in the signal - a step increase or decrease when the LED brightness changes. 

Could this be due to impedance matching between the LabView card and the detector? In the 9215 specs, I only found a cryptic (at least for me) information on the impedance: 

NI 9215 with BNC (Between
any two AI- terminals)
200 kΩ

Any idea what could cause a 10x loss in measured voltage? I would be grateful for any suggestions.

[GerdW]

Hi agnef,

the datasheet says this:

due to the 100kOhm pulldown resistors connected to each AI- terminal (see page 5 of the datasheet).

Did you check the sensor signal using a simple handheld DMM?

[Agnea]

I just did, with the recommended T4119 terminator I also read 0.003 V. So I guess it cannot be due to NI card or LabView settings. If I remove it, I get a 13 V reading at maximum LED brightness..

[GerdW]

Hi agnef,

---

@Agnea wrote:

I just did, with the recommended T4119 terminator I also read 0.003 V.

---

Are you sure this T4119 still provides that 50Ohm resistance?

Did you verify this value with your handheld DMM, too?

[AnalogKid2DigitalMan]

Check the 12V battery in the DET02AFC, if it is dead or out of spec that may explain you lack of output voltage.

-AK2DM

[Agnea]

So yes, the T4119 gives me 50 Ohms, I just double-checked.

And yes, I have checked the battery with the multimeter as well, it reads 12.45 V so I guess that is also ok.