Self-oscillation PCI6229 and SCB-68 measurement

[LemonOE]

hello,

I am using PCI-6229 card to aquisite voltage from a photo diode.

I use differential mode to connect plus node on ai21 and nagetive on ai29 of PCI-6229. 

this corresponding with terminal 60 & 26 on SCB-68 connector.

But after several seconds the voltage will climb on to the maximum 10V even I leave the terminal of ai21 & ai29 open.

Is it normal of PCI-6229 ?

How to solve this problem?

Thanks a lot!

[LemonOE]

I tried with USB 6211 under the same condition, it works well.

It seems that is the problem of PCI6229 or SCB-68?

How can I solve this?

[LemonOE]

Here is the graph

1st is self-excited oscillation

2nd is signal using USB 6211

[johnsold]

You probably need bias resistors to AI Gnd.  Check the manual for connections information.

Lynn

[LemonOE]

Thank you for helping. I used before but this will introduce noise on GND and the noise is almost 10 times larger than only use differential input.

I just wonder why there is difference between using USB 6211 and PCI 6229. Because the connection is the same.

Is is because PCI 6229 use longer cables than USB 6211 before the signal to be ADC?

or PCI 6229 has a higher sensitivity than USB 6211 and this cause an oscillation more easily?

[AnalogKid2DigitalMan]

Do you have any signal conditioning between the photodiode and the DAQ?

I have a hunch that without any the diode is acting as a very high impedance source and the DAQ input circuity is essentially open circuit.

You can see such behavior sometimes on analog inputs that have nothing connected to them.

Thus you may want to look over the specs and architecture of the two cards to see if there are any subtle differences between them.

-AK2DM

[LemonOE]

Thank you for replying.

Yes you are right, there is no other conditioning circuit only Photodiode.

Maybe there are differences between two cards.

[johnsold]

A photodiode or any other self-generating sensor only produces a meaningful signal when connected in a circuit which provides a suitable load for the device.  Photodiodes are often treated as current sources.  A current needs a complete loop to flow. Unless you know what the input circuit of the DAQ device is, you do not know how suitable it is for your application.

Try simply putting a resistor in parallel with the diode. Make the value <= 0.1*(Input resistance of DAQ device). Then put a resistor from the AI- input to AI Gnd. Make this value as large as you can while still satisfying the bias current requirement.  Try it. The voltage you will get will be the photocurrent multiplied by the parallel resistance. You have a slight imbalance in the input circuit but no current loop to pick up noise.

Lynn