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Photodiode signal units

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Luis posted on Fri, Sep 26 2014 1:54 PM

Hi, as they said in a previous posts (, the vertical deflection is ((A+B) - (C+D))/SUM, where A, B, C and D are the signals from the quadrants on the photodiode and SUM is A+B+C+D.

I was wondering about the units of this signals. It should have no units because it is normalized. In Nanoscope Software and in the Signal Access Module, they show this signals in Volts, but the SUM signal and (A+B) - (C+D) signal should both be in Volts too.

I know I'm missing something here, does anyone know how this works?


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Verified by Thomas Mueller

I believe that the units used just reflect that the signal is a voltage (i.e. the microscope software is reporting the voltage measured at the ADC). Strictly speaking, the deflection should be a dimensionless quantity (as your logic shows) but the signal is still a signal - if it wasn't represented by a physical quantity, the microscope wouldn't be able to measure it.

Also, deflection signal is not strictly normalized (if it was, it would be impossible to have a deflection with a value greater than 1). I assume that deflection values greater than 1 are the result of the normalized deflection being amplified after the difference-over-sum calculation, but this is just a guess (and wouldn't affect the units).

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