# How to distinguish between voltage resolution and bit resolution

One might way that an ADC has a resolution of 12-bits. One could also say that it has a resolution of 1.22mV (5v/4096).

Is one of these uses of the word 'resolution' incorrect?

Is there a word to replace one of these uses of the word 'resolution'?

Context: I am trying to write a datasheet. In the electrical properties table, I wish to describe the properties of the DAC outputs. However, I seem to have used the word 'resolution' twice. This seems wrong to me. Is there a better word I could use for one of these?

• The term "resolution" is correctly used in both cases. It's used twice because you have expressed it in two different ways - once in terms of max analog range and the other in terms of digital bits. (But, you knew that :-) ). You could use Resolution once in the left hand column and then have two inset labels - "Digital" and "Analog". If he can be bothered Olin will comment on resolution, accuracy and more. – Russell McMahon Sep 27 '12 at 16:29
• You should not give these parameters the same name, RAO. RAO cannot be 16 and 8 at the same time. Basically your table is potentially confusing to someone, who might read the wrong row and think that the resolution is 16 bits. It is not necessary to give the resolution in two different ways when one can be calculated from the other using a single arithmetic operation. – Kaz Sep 27 '12 at 20:13
• @Kaz - That's correct. What I'm asking is what I should replace one of them with. – Rocketmagnet Sep 27 '12 at 21:56

The 1.22mV is the step size, sometimes also called LSB. Take a look at this google search.

Step size:

Step size is the minimum change in input voltage which can be resolved by the ADC. The concept of step size is closely associated with the resolution of ADC.

Resolution:

The resolution of an ADC refers to the number of bits in the digital output code of the ADC.

The relation between step size, resolution, and input range can be given by: You're right, mentioning it twice isn't a good idea: one parameter will be dependent on the other one, and therefore be redundant. The risk is inconsistent data.

You would usually only give resolution in number of bits. Your 16 mV will follow from the reference voltage, which seems to be 4.096 V here. Instead of giving the 16 mV you would have to specify the 4.096 V reference. Is this an internal reference? If yes, mention the limits, like 4.096 V typical, 4.014 V minimum, 4.178 V maximum. The user will then be able to deduce the tolerance in step size from that. Also, if you (can) use an external reference the 16 mV is basically meaningless; if the user would apply a 2.000 V reference the step size would be 7.8 mV.

Also note that ENOB (Effective Number Of Bits) and SINAD (Signal-to-Noise and Distortion) are referenced to the resolution in bits.