Multifunction DAQ

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usb-6009 noise

Hi all!

I want to measure the voltage on the LED using the USB-6009. Scheme in attached file. If I measure voltage on R2 I see pic 1. But when I have connected USB-6009 I saw pic 2. And USB-6009 was measured this signal. Why the waveform is changing?  

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Tried using a buffer inbetween the DAQ and the measurement point?  Unity-gain op-amp? 

The 600x's impedance is really low, which can cause problems like what you're seeing.  Use the high input impedance of an op-amp, and let the op-amp's output drive the DAQ.

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