Sensor and Pi common ground MCC-172

[ToVi]

Hi,

I'm currently testing MCC-172 for industrial application and for now using traditional non-IEPE two wire piezoelectric sensor (most propably this will be replaced with some other sensor in the future). I'm having lot of noise and it seems to be that it's coming from the cable between sensor and MCC-172. When connecting sensor directly on inputs, the noise is gone. How I should improve my setup?

 

From datasheet I noticed the Note: For best results, connect the signal source and the Raspberry Pi to a common ground. If a floating source is required, connect the MCC 172 to earth ground via the DGND screw terminal to minimize common mode noise.

 

What is meant with that sentence and how I should connect the two wire sensor to achiece common ground between Pi and the sensor?

 

 

[JRys]

When using charge mode sensors, your system noise will be a function of the cable length. It is for this reason that IEPE sensors are more desirable. On the other hand, charge mode sensors can withstand higher temperatures because they lack internally sensitive electronics. There are charge mode amplifiers that convert the signal to IEPE. A charge mode amplifier can shorten the cable length if your application is at a high temperature. 

[JRys]

The note about grounding is because the Raspberry Pi power is essentially a floating measurement device. Many IEPE sensors are isolated from their case, making them floating sensors, too.  Because of this, they can be more susceptible to noise. Connecting to a common earth ground will help.

[ToVi]

Thank you for your reply. Most probably we will find suitable iepe sensor in the future hence the temperature is not the issue. But for now, I think we can proceed with the shorter cable.