Offset Limits in RIGOL Oscilloscope

[Markus Hoeg]

Hello,

I have a very specific problem and didn't find any way to solve it.
My signal is very slow changing at 5V +-100mV
When I want to display this signal on my RIGOL Oscilloscope, the problem is, that with AC-mode I don't see those slow changes and in DC-mode the offset range is limited to 2V when displaying <500mV/DIV.
My changes are around 100mV so my display settings need to be 100mv/DIV.
Is there any way to display this kind of signals?

Thanks in advance
Markus

 

[berkeman]

Welcome to the PF, Markus.

Do you have a non-changing 5V reference signal? If so, use two 'scope probes and do a math subtraction (like A = Ch1-Ch2) to be able to get rid of the offset...

 

[Markus Hoeg]

No I don't have any reference signal. The signal is created by a photodetector after light passes through a fibre. So the voltage depends on the light passing through the fibre, which is slow-changing.

 

[berkeman]

Then you could make one, with a very-lowpass-filter. Or use a power supply that you set to the average voltage. Are you digitizing the signal and doing any post-processing?

 

[Markus Hoeg]

Okay I will give that one a try, Thanks!
Yes I need to send the data via USB to MATLAB afterwards.

 

[Tom.G]

You can probably use the power supply of the photodetector as a reference voltage, perhaps through a voltage divider if needed.

 

[Markus Hoeg]

The power supply of the photodetector is constant, while the output DC component is changing for every measurement depending on the light through the fiber. That would mean that for every measurement I have to adjust the voltage divider.

 

[Svein]

Insert a difference amplifier after the photodetector:

Connect V₂ to the +5V supply and V₁ to the photodetector output. The value of R depends on the choice of Op-Amp, but I would recommend something around 0.1 -1MΩ.

 

[Markus Hoeg]

Thank you for your useful instructions

 

[Tom.G]

How about this? Depending in the DC level from the detector, you may want the Zener to be 3.3V. The 2.2k resistor value is just a starting suggestion, the actual value depends on the maximum available output current of the detector and the leakage current of the Zener.

Cheers,
Tom