How to build a Floating Ground Buffer Box?
What you need is a differential amplifier with high common mode rejection ratio. A simple unity gain differential can be built by four identical resistors, one unity gain stable operational amplifier and a +/- 15V split power supply with proper filtering.
The construction is simple so I describe it here in words:
+15V of power supply connects to operational amplifier positive power pin;
0V (power supply ground) connects to DAQ module ground;
-15V connects to op-amp negative power pin;
1st resistor connects between motor controller output and op-amp "+" input;
2nd resistor connects between op-amp "+" input and DAQ moduile ground;
3rd resistor connects between motor controller ground and op-amp "-" input;
4th resistor connects between op-amp "-" input and op-amp output:
op-amp output connects to DAQ module input.
You may use op-amp like MC1458 and four 10k ohm 1% resistors to build the buffer for small signal frequency up to 10 kHz.