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What does 12- or 16-bit resolution mean?

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
2.5 to 5
5 to 7.5
7.5 to 10

00
01
10
11

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 new U6-Pro provides some of the best performance around from a 24-bit ADC, and it does 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:

http://www.maxim-ic.com/appnotes.cfm/appnote_number/1870/

Hardware with a 24-bit ADC

LabJack U6-Pro USB 24Bit Analog Input Output Device
22-bit Effective Res
LabJack T7-Pro USB Ethernet Wireless 802.11b/g WiFi Analog Input Output Device
22-bit Effective Res
LabJack UE9-Pro USB Ethernet 24Bit Analog Input Output Device
20-bit Effective Res

Hardware with a 16-bit ADC or less

LabJack U6 USB 16Bit Analog Input Output Device
19-bit Effective Res
LabJack T7 USB Ethernet Wireless 802.11b/g WiFi 16Bit Analog Input Output Device
19-bit Effective Res
LabJack UE9 USB Ethernet 16Bit Analog Input Output Device
16-bit Effective Res
LabJack U3 USB 12Bit Analog Input Output Device
12-bit Effective Res
LabJack U12 USB 12Bit Analog Input Output Device
12-bit Effective Res
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Comments

#1

Excellent! Thank you

#2

Well explained!

I wish all companies would list the effective resolution of their ADC though experimental data.

I feel confident in using LabJack for many more of my research projects in the future.

 

A++ 

#3

it's an explanation easy to understand

#4

Very useful. Thank you!