if you want to measure something that is in real world domain you have to convert it to electrical daomain by a transducer, transducer is an analog device and 51 is an digital device. again some a/d coverter is needed to convert it from continuous analog domain to descreate digital domain. here some loss occurs as analog domain can take infinite valus in certain range and mapping from the analog domain can not be onto. here you select the resolution so that no significant precision loss occurs. further more if output of the xducer is weak for convertion instrumentation amplifier comes into picture.

thtas how you connect a real world device.

abhishek