Problem with DAQmx Analog Input

[jLeschke]

The problem:

I have a photodiode connected to the NI PXI-6233 card in our RT PXI System (NI PXIe-8102RT) and regardless whether I measure it in MAX or in my VI, it give me a ~5V signal. When I cover the sensor, the signal decreases slowly to ~4.8V, when I uncover it it returns to its former value (extremely slow for a diode). This is the behaviour when the diode is "active", meaning bias is on. When its off, the signal is at about ~0.2V .

When I measure the diode with a Voltmeter I get 0V no matter if bias is on or off. So I'm sure the problem is caused by a wrong DAQmx configuration.

My configuration:

I created a task in the project explorer. Analog Input 0-10V, RSE, Sample on Demand.

Does anybody know what I'm doing wrong?

I've tested a bit more and I think we have a problem with our ground. I'm put the BNC Out from the diode to AI0 und AI Ground (Pin 1 &3). Am I doing something wrong? Sorry.. I'm sure the solution is really really easy, but I don't get it.. 😕

Another short question:

How can I set the aquistion mode to "Sample on Demand" using low-level API? I tried the DAQmx timing VI, but it's not possible to choose sample on demand.

[Henrik_Volkers]

See the photodiode as a small solar cell, more or less ligth more or less current, but a very small current.

So usually you use a TIA = transimpedance amplifier to read a photdiode ... , a TIA transforms the small current into a (low impedance) voltage yo can measure.

What I asume has happened: Your analog input outputs a BIAS current that is biasing the photdiode 🙂

[jLeschke]

Hallo Henrik,

dann schreib ich doch einfach mal auf deutsch 😉

Gibt es denn in LabVIEW eine Einstellung die den bias-Strom erzeugt?

For english readers: Is there a setting in LabVIEW that generates the bias current?

Grüße, Jan 😉

[reschri]

Hallo Jan,

Hierin findet man allgemein die Problematik mit Bias-Strömen und "floating signal source", also ungeerdeten Signalquellen.

http://www.ni.com/white-paper/3344/en/

(Hier besionders table 1 betrachten)

Prinzipiel war es eine gute Idee, eine RSE-Messung zu machen, aber leider ist 6233 ein isolierte Karte. Das Bezugspotential stellt der Anwender zur Verfügung. 

(Screenshott aus der bedienungsanleitung zur 6233)

Eine externe Erdverbindung sollte hoffentlich das Problem lösen,

Gruß

René