What is resolution?
Resolution in this context refers to the conversion of an analog voltage to a digital value in a computer (and vice versa). A computer is a digital machine and thus stores a number as a series of ones and zeroes. If you are storing a digital 2-bit number you can store 4 different values: 00, 01, 10, or 11. Now, say you have a device which converts an analog voltage between 0 and 10 volts into a 2-bit digital value for storage in a computer. This device will give digital values as follows:
|Voltage||2-Bit Digital Representation|
0 to 2.5
So in this example, the 2-bit digital value can represent 4 different numbers, and the voltage input range of 0 to 10 volts is divided into 4 pieces giving a voltage resolution of 2.5 volts per bit. A 3-bit digital value can represent 8 (23) different numbers. A 12-bit digital value can represent 4096 (212) different numbers. A 16-bit digital value can represent 65536 (216) different numbers. It might occur to you at this point that a digital input could be thought of as a 1-bit analog to digital converter. Low voltages give a 0 and high voltages give a 1.
In the case of the LabJack U12, a single-ended analog input has a voltage range of -10 volts to +10 volts (20 volt total span) and returns a 12-bit value. This gives a voltage resolution of 20/4096 or 0.00488 volts per bit (4.88 mV/bit).
What does it mean to say a device is 12-bit, 16-bit, or 24-bit?
When you see analog input DAQ devices from various manufacturers called 12-bit, 16-bit, or 24-bit, it generally just means they have an ADC (analog to digital converter) that returns that many bits. When an ADC chip returns 16 bits, it is probably better than a 12-bit converter, but not always. The simple fact that a converter returns 16-bits says little about the quality of those bits.
It is hard to simply state "the resolution" of a given device. What we like to do, is provide actual measured data that tells you the resolution of a device including typical inherent noise.
If you look at a device called "24-bit" just because it has a converter that returns 24-bits of data per sample, you will find that it typically provides 20 bits effective or 18 bits noise-free (like the UE9-Pro). The U6-Pro and T7-Pro provide some of the best performance around from a 24-bit ADC, and they do about 22 bits effective or 20 bits noise-free. You will see with these devices we might mention they have a 24-bit ADC (as that is what people look and search for), but we try not to call them "24-bit" and try to stick with the effective resolution.
Another interesting thing about your typical 24-bit sigma-delta converter, is that you can look at them as only having a 1-bit ADC inside, but with timing and math they can produce 24-bit readings: