# What does “LSB per degree per second” mean?

I'm currently working with a digital output gyroscope. Its sensitivity is given as 120 least-significant bit per degree per second.

While every single component of that unit is easy to understand I don't quite get what it means altogether.

Can you explain it to me?

• Which digital output gyroscope? – Nick Gammon Aug 24 '15 at 7:31

Let us say we have a number, "4025". The "most significant digit" of the number is the 4 in the thousands position, since it tells us the most about the magnitude of the number ("in the four thousands"). Consequently, the "least significant digit" is the 5 in the units position, since it tells us almost nothing about the magnitude of the number. "Most/least significant bit" is the same, except it applies to binary numbers (which the MCU cares about, of course; we can still use any radix we like to talk about them). So the device's output count can change by up to 120 per degree per second.

• Isn't the LSB in a binary number always either incrementing the number or not? I mean the value of the LSB has to be always 1 or 0. I understand what the LSB is but not its roll in the unit. – Hedge Aug 24 '15 at 2:03
• @Hedge: 125 is 2 LSBs greater than 123. – Ignacio Vazquez-Abrams Aug 24 '15 at 2:06
• I don't really understand the question, nor the answer. What does "120 least-significant bit per degree per second" mean? Let's say the gyroscope returns some units. Without knowing the model they could be anything, but let's say it's 100th of a degree. Isn't that just saying it is accurate to 120 units per degree per second? I mean, a least-significant bit is 1, right? So that is 120 x 1 (of whatever units) per degree per second? – Nick Gammon Aug 24 '15 at 7:36
• @NickGammon: The gyro returns a count. 1 LSB is 1 count. – Ignacio Vazquez-Abrams Aug 24 '15 at 7:38
• OK I will give you +1 LSB of rep for that. :) – Nick Gammon Aug 24 '15 at 8:56

With further elaboration on Ignacia's answer:

A gyroscope itself is generally an analog device in the sense that in reality, it will have a continous output. This output is digitized using some form of Analog to Digital A/D converter where the analog output is converted to a digital number. The output from the gyroscope is a measurement of angular speed, generally millidegrees/sec.

The number read by your Arduino from the gyroscope will be a digital version of your current angular speed at the time of measurement. For example, if you're getting an 8bit number back equal to 121, this in binary is equivalent to 01111001, where the '1' furthest to the right is the least significant bit.

The term "lsb per degree per second" means how many bits change with your angular speed. For example, with a gain of 1 (meaning 1 degree/second = 1 bit), an angular speed of 90 degrees/sec will produce a read value of 90 or 01011010. Therefore, 1 bit changes per degree/second. Increasing the essential "gain" (which will most likely be referred to as "dps" will change how many lsb's change per degree/second.