Hi all, I have this question which I need assistance on an assignment:

A PLC analogue input module is described as: 8-bit, 0 - 10 v dc
What is the smallest change in voltage that this input can detect (i.e. the resolution)?
What changes would need to be made to enable the input module to detect a change of at least 3mV for a 0 – 10v dc input?

 

So, how many steps can be represented by 8 bits? What happens if you divide the highest voltage this device handles by this number of steps? What are your units?

How many steps does it require before you get a step of =< 3mV. How many bits are required to represent that number of states?

 

This has good example of what you need: http://en.wikipedia.org/wiki/Analog-to-digital_converter#Resolution

 

8 bits represents a resolution of 255.
So you can calculate the smallest change that 10v will represent.
Max.

 

8 bits represents a resolution of 255.
So you can calculate the smallest change that 10v will represent.
Max.

256, not 255.
"Resolution limits the precision of a measurement. The higher the resolution (number of bits), the more precise the measurement. An 8-bit ADC divides the vertical range of the input amplifier into 256 discrete levels. With a vertical range of 10 V, the 8-bit ADC cannot ideally resolve voltage differences smaller than 39 mV."
From NI: http://www.ni.com/white-paper/4806/en/