Analig Input with DAQ - inpedance problem

[notimperial]

Hello,

I am trying to collect a signal from a photodetector (http://www.thorlabs.com/thorProduct.cfm?partNumber=DET10A) and feed it through a 50ohm coax cable to a NI-DAQ 6009 card connected to my PC. 

No matter how high the illumination is , the signal I am getting oscillates from 1e-4 to 1e+4 V which don't to match those I obtained previously using an oscilloscope. I have checked that the photodetector is on and batteries are fine.

My guess is that there may be a problem with the impedance matching in the connection of the coaxial cable and the DAQ card but I'm no expert in this kind of thing. What do you guys think?

Thanks in advance 🙂

[K C]

Hi,

The output is 0-10V with a 50 Ohm load. You should add a resistor across the DAQ input.

How did you connect the DAQ ? Which input mode ?

Kees

[notimperial]

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@K C wrote:

Hi,

The output is 0-10V with a 50 Ohm load. You should add a resistor across the DAQ input.

How did you connect the DAQ ? Which input mode ?

Kees

---

The core conductor of the cable is connected to A0+ and the outer wiring is connected to A0-.

I am a beginner in this so I'm sorry if my answer is over-specific 😉

[K C]

You can also connect it as an SE device.

Add a 500Ohm resistor (Not 50) accross the input. With a 50Ohm resistor there will flow too much current.

Maybe you can try this.

[notimperial]

Thanks for your reply.

Can you explain what you mean by an SE device?

[Henrik_Volkers]

Take a look into the spec sheet. You will need a defined (low)  input resistance to make a measurement. 500 Ohm or 50 Ohm between A+ and A- at the input terminal. (if you have a BNC adapter at your input, add a BNC T and a BNC 50 Ohm termination.

If you stay with differential mode, add a 1M to 100k from A- (shielding of your coax) to AGND.

AND

Have a look at aliasing! That sensor can send signals up to 500MHz ( 50MHz with 500Ohm). If exposed to room light these fluorescent lightsources with electronic ballast will/can create AC content higher than the 6009 can read!

Since you have a scope use it with a 50 Ohm termination and have a look at the signal. By using the 1 (10) Meg input of the scope you decrease the max output frequency (see spec).

[notimperial]

---

@K C wrote:

You can also connect it as an SE device.

Add a 500Ohm resistor (Not 50) accross the input. With a 50Ohm resistor there will flow too much current.

Maybe you can try this.

---

Can you tell me what you mean by an SE device? 🙂

[notimperial]

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@Henrik Volkers wrote:

Take a look into the spec sheet. You will need a defined (low)  input resistance to make a measurement. 500 Ohm or 50 Ohm between A+ and A- at the input terminal. (if you have a BNC adapter at your input, add a BNC T and a BNC 50 Ohm termination.

If you stay with differential mode, add a 1M to 100k from A- (shielding of your coax) to AGND.

AND

Have a look at aliasing! That sensor can send signals up to 500MHz ( 50MHz with 500Ohm). If exposed to room light these fluorescent lightsources with electronic ballast will/can create AC content higher than the 6009 can read!

Since you have a scope use it with a 50 Ohm termination and have a look at the signal. By using the 1 (10) Meg input of the scope you decrease the max output frequency (see spec).

---

So just to make sure I understood it right, what you suggest is that I place a 50 ohm resistor between the A+ and A- terminals in the DAQ card or add a BNC T to connect the photodiode and a BNC 50 ohm termination to the coax that goes to the DAQ. Is this right?

specs:

DAQ 6009 card http://sine.ni.com/nips/cds/print/p/lang/en/nid/201987

Thorlabs detector http://www.thorlabs.com/NewGroupPage9_PF.cfm?Guide=10&Category_ID=77&ObjectGroup_ID=1295

[notimperial]

I tried connection either a 47ohm or 470ohm resistor through the the A0+ and A0- terminals of the card but it didn't help as I could not reproduce the 1V constant signal I saw in the scope.

Any suggestions?

[SnowMule]

The input impedance of the 6008/6009 is about 140kΩ.  That low impedance just soaks all the current the little photodector can pump out. 

I built a voltage follower out of an LM358 and piped that into the DAQ's input... works real well.  Opamp has a real high input impedance and provides enough current that the daq's low input won't cause it to crowbar.