Resolution refers to the number of divisions that make up the entire measurement range of the A/D. The higher the resolution, the more precise the measurement. Resolution is usually indicated in bits. The number of divisions in the measurement range is equal to 2 ^ (bits). For example, a 16-bit A/D converter can resolve its input range into 2^16, or 65536, discrete values. A 12-bit A/D converter can resolve to 4096 values.
Input Range effect on Resolution
The A/D resolution is spread over the entire input range of the A/D, so the fineness of the measurement depends not only on the resolution but also on the input range. An A/D with a 10V input range (0-10V or +/-5V) has half the fineness in its measurement range of an A/D with a 5V input range (0-5V or +/-2.5V). This fineness is usually referred to as 1 LSB or 1 count, since a change in input voltage of this amount will result in a change in the A/D reading of 1 count:
0-10V A/D: 1 LSB = 10V / 65536 = 153uV
0-5V A/D: 1 LSB = 5V / 65536 = 76uV
Programmable Input Ranges = Better Performance
As you can see, the higher the resolution, the higher the precision of the measurement. Conversely, the larger the input range, the lower the precision of the measurement. Therefore, to get the most accurate measurement, select the smallest input range that covers the expected range of your input signal. Since most applications involve inputs with different voltage ranges, it makes sense to use an A/D board that has programmable input ranges. This lets you change the input range whenever you change input channels, so you can always match the A/D to your input signal. For example:
Voltage range for channel 1 is 0-4V: Select 0-5V input range.
Voltage range for channel 2 is -1V to +3V: Select +/-5V input range.
If you select 0-5V for both channels, you will miss the lower 1V of your input signal on channel 2. If you select +/-5V for both channels, you will get half the measurement accuracy on channel 1. A programmable input range board gives you the best overall measurement accuracy.