Measuring sensor voltage with NI 9215 DAQ, (Please help!)

[LTBALAZS]

Hello,

There is an equipment which has a phototransistor. Phototransistor has voltage 0 to 5.2 V roughly.

I should do DC voltage data acquisition on the 1MOhm resistor of phototransistor.

For some reason i cannot do measurements with LeCroy oscilloscope with differential probe. (Equipment turns off when connecting the differential probe at startup)

Else i measure noise e.g. around 92V AC, 50Hz on phototransistor when one connector of the diff.probe is used.

Can i do safe measurement with NI DAQ connections between e.g. AI0+ and AI0-  or, AI0+ and COM is safer in this case?

Kind regards,

Balázs

[Tom_Powers]

Can you describe your photo transistor circuit a little better?  Maybe post an image of it.

[LTBALAZS]

[santo_13]

You need some sort of buffer or amplifier because the input impedance of the DAQ loads the signal.

If you've the BNC version, then the load is 200k ohm between AI+ and AI-

This 200k ohm impedance will load the signal heavily compared to the 1M ohm sense resistor you're measuring across.