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sensors - why 2's compliment mode ?

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neddie

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Hi all.
I'm currently using an accelerometer (LIS3DH).
It's output is given in 2's compliment format.
Why do they do this.
This is pretty common practice with lots of sensors and high res a2d's etc.
I know it makes subtraction easy , but is there any other reasons for outputting data
in 2's compliment form.
Cheers
Neddie
 

Hi,

It seems you complain about 2´s complement.
But don´t tell: what alternative do you see?

Binary addition and subtraction, 2´s complement or not, is always the same.

Klaus
 

I'm not complaining about it , I was just wondering why 2's compliment and not straight binary.
It's easy to convert 2's compliment , it's just an extra step.
 

Hi,

Straight binary is positive only: for an 8 bit code: 0....255 (= uint8. Zero is at the lower limit)
Two´s complement is positive and negative: -128 ... 0 ...127 (= int8. Zero is in the middle)

Examples:
Acceleration can be positive as well as negative. Therefore you can´t use straight binary.
Kelvin values or RMS values are positive only. Here you can use straight binary.

Klaus
 
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    Okada

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This is because signed arithmetic operations are much easier to do in 2's complement format (easier to design hardware for).

There're other methods to implement signed arithmetic - but 2's complement is the easiest to do in hardware (microcontroller / FPGA / etc...)

Watch this video:
https://www.youtube.com/watch?v=9W67I2zzAfo
 

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