Here is a design for an ADC front-end that accepts inputs of +/- 6V and outputs a voltage in the ADC-compatible 0 - 1.8V range, while offering some protection against over-driving the ADC inputs. Sorry for the poor image quality. The resistors R1. R2 and R3 can easily be scaled to different input voltage ranges, and the circuit can be simplified if only positive input voltages are anticipated. It might be convenient to replace the 1.5V sources with 1.8V reference provided by the BeagleBone, buffered by one section of the TL084 op amp. Another section of the '084 will produce the negative rails shown.
I’d welcome comments on how this design can be improved. My simulations look good, but I’ve not built any actual hardware yet.