Issue with Raspberry MCC 128 (ADC) and optical detector ground

[Saha]

I have a circuit connection like this:

The optical detector is powered by a 12V battery, the ground wire of the detector is left open.

The raspberry pi computer with MCC 128 data acquisition board (ADC) is powered by 5V from a step-down transformer.

When trying to connect the BNC cable of the detector to the ADC (computer), the detector shuts down. If either the detector or the computer is connected to the grid (other one is connected to the battery) then everything works fine. Is it a grounding issue? I’ve tried to connect all grounds together to the ground from the grid to have a common ground with no success. Can anyone please explain the mechanism here, or provide me some relevant links?

Let me know if you have any confusion with the circuit design.

Thanks.   

[JRys]

Could you send me the optical detector datasheet and model number, and let me know the voltage measured from the input ground to the output ground of the 12V to 5V Transformer?

[Saha]

This is a silicon detector (PDA36A2): https://www.thorlabs.com/thorProduct.cfm?partNumber=PDA36A

The input voltage is 12V and transformed to 5V for the computer.

 

Edited April 12 by Saha

[JRys]

The manual says to connect a 50-ohm resistor to the ground on the detector output. The output is current, and the resistor generates the output voltage. Connect the MCC 128 input across the resistor. I recommend placing the inputs in differential mode for the measurement. If the signal is too low with the 50 ohm resistor, use a 1000-ohm load resistor. 

[Saha]

It works! Thanks a lot JRys.