Non-Linear Voltage Gain NI-6009

[flyer2o11]

I have here a circuit consisting of a one mega ohm resistor connected to a photo diode in reverse bias.

I have measured the voltage across the resistor with the NI-6009. Simultaneously, I measured the voltage by means of an oscilloscope but both voltage readings do not correlate.

I am aware that there is an enormous impedance between the circuit and the daq device so I implemented a unity gain
buffer circuit consisting of the LM324N op amp. However, this results in a non-linear voltage gain.

Previously I had a DAQ700 and this deemed very straight foward with great results.

Can any of you guys suggest an alternative buffering circuit for the NI-6009 that will result in a linear voltage gain.

[David-A]

Hello flyer2o11,

What kind of difference in voltage are you seeing between the 6009 and the oscilloscope?  Would you also be able to describe how your circuit is physcailly wired to the daq card?

Thanks,

David A.

[flyer2o11]

Hi David, thank you for the reply.

Lets forget about the Non-Linear Voltage Gain NI-6009 as that is what I was getting with a suggested amplifier LM241N from a colleague. If you could suggest an amplifier that would be great.

Each sensor is connected to a 1MOhm resistor, all of which are in parallel with one another. Each resistor is then connected to ground. A wire from each resistor, on the same side as the sensors provide the outputs for the NI6009.

The configuration of the sensor outputs to NI6009:

Sensor 1 - AI0
Sensor 2 - AI4
Sensor 3 - AI1
Sensor 4 - AI5
Sensor 5 - AI2

Signal ground is connected to GND
+5V is connected to terminal 31 while the corresponding ground is connected to the terminal 32.

So the impedance of the circuit is 1MOhm while the impedance of the NI6009 is about 150MOhm correct?

What amplifier can I employ to overcome this difference in impedance?

[flyer2o11]

Sorry that should be LM324N.

[kellam]

If you are looking to build your own amplifier, I'd suggest you look at this document.  If you are less cost sensitive, then you could either purchase an amplifier, or change to a different DAQ device that has a larger input impedance, such as the USB-6210.