By default, each encoder and DAC is set to the same index as the axis to which it is attached (for example, ATTACH AXIS0 ENC0 DAC0 ENC0). It is good programming practice to use the same index for the feedback or signal output as the axis to which you are attaching.
For example, to attach ADC 4 as position feedback, and DAC 4 as the command signal to axis 4, send the following:
ATTACH AXIS4 ADC4 DAC4 ADC4
There is one exception: dual-loop feedback. With dual-loop feedback, you can attach a second feedback source to an axis. In this case, you must indicate which additional encoder is being used.
For example, to attach encoder 4 as position feedback, DAC 4 as the command signal, and encoder 9 as the velocity feedback, send the following:
ATTACH AXIS4 ENC4 DAC4 ENC9
So for dual loop, you could use the analog sensor feedback for position and use the encoder feedback for the velocity:
ATTACH AXIS4 ADC4 DAC4 ENC4
You can use the same ADC and ENC on the same axis so you don't have to use an extra axis just as ADC input. Only thing with this last dual loop code, you can only monitor the ADC position, not the velocity, and only monitor the encoder velocity but that should be fine for most applications.
One example of analog position feedback sensor's includes MTS Sensor Corp's Linear Sensors:
http://www.mtssensors.com/
For an even better designed system, the ACR9000 axis connector supports SSI feedback which is a digital transmission instead of analog. This is a standard option from MTS Sensor and most other manufacturers and is configured the same as the analog above. The SSI feedback is better for industrial installations and it's feedback cannot be corrupted by EMI noise.