Isolating PLC analog Inputs/Outputs

Gday,

I'm having trouble with the analog input of my PLC (S7-314IFM).

I am currently trying to run a DC VSD using voltage following, connected to the PLC's analog output with +-10V, and then a tachogenerator is used to measure the speed. The voltage from the tacho is stepped down from 105V to 10V (voltage divider) and connected to the analog input of the PLC.

The PLC outputs the correct voltage and the motor follows it exactly, the problem is the input value fluctuates dramatically. Example if the speed is set to 1750RPM it would chnage from 300 to 550 to 287 to 1300 to whatever.

The problem was eventually found to be caused by the connection of the analog output of the PLC as the VSD terminals were not isolated from the AC source and in the VSD user manual it states that a Isolated voltage source is needed and that it should not be connected to ground (which it is in my case,when connected to the PLC).

So by disconnecting the voltage following terminals of the VSD and connecting them to a DC power supply, the readings of the analog input of the PLC was steady and accurate.

So now I have the background situation explained, my question is... How do I go about isolating either the analog input or output of the PLC, so that I can use both to control and monitor the DC motor?

I hope I have given enough detail, any help would be greatly appreciated. I dont know if this is theright place to ask this question.