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ADXL375 Recommended for New Designs
The ADXL375 is a small, thin, 3-axis MEMS accelerometer that provides low power consumption and high resolution measurement up to ±200 g. The digital...
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ADXL375 on Analog.com

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Calibrating ADXL375

		
	
				

			

									
					    ricardogomes
					
							

            
            			
				
									
											
			
			
				
			
			
			    	
			                    
                
				                on Jul 2, 2015 
				
			
			
		

		
		                    	    

        	    
Thread Summary

                The user is experiencing large offsets and inconsistent readings with the ADXL375 accelerometer when configured at 800Hz bandwidth. The final answer suggests lowering the ODR to 100Hz and using burst read to improve noise. Additional advice includes checking the justify register bit in Reg 0x31 and ensuring stable power supply with decoupling capacitors.
                        	    

                
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Hi, I have a ADXL375 accelerometer evaluation board which I've connected to an atmel board. I'm now able to configure the accelerometer as I want and read the acceleration. I've defined the Bandwidth = 800Hz and I'm reading X0 to Z1 at the same time. With the offset = 0 and Z at -1g and X and Y at 0g, I'm getting the following:

 
X0 = 64
X1 = 0
Y0 = 80
Y1 = 0
Z0 = 32
Z1 = 255

 
my sensitivity is 400/8192 = 0.048828125 g/LSB (~=49mg like the datasheet says).

 

So:

 
Xaccel = 64 * 0.049 = 3.136g
Yaccel = 80 * 0.049 = 3.92g
Zaccel = -224 * 0.049 = 10.976g

 
Well, like it says in the datasheet, 1g equals 20.5 LSB so it is not reasonable that I'm getting values >>>>>> 1g.

 
In the datasheet also says that the typical offset is 400mg ( offset ~= 0x02 for each axis because 1 LSB corresponds to 196 mg, right??) and if we look to the values I'm getting, it makes sense that my sensitivity should be 4.9mg so my values would be:

 
Xaccel = 0.3136

 
Yaccel = 0.392

 
(which would be ~0g with the 400mg of offset

 
and Zaccel = 1.0976g

 
Do you know what I'm doing wrong? Are the values that I'm getting normal or do they seem weird for you to?

 
Thanks in advance.

			

			
									
		                             
                        

    

                                            
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							FormerMember
					
    
		
    
		    
		    		    
			
				
									FormerMember
							        
			
			
			
            
						
                            
            
            				
            
                on Jul 4, 2015 4:13 AM
            
            
						
			            		
    
		
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    Hi Ricardo,
    
 
    The typical sensivity for ADXL375 is 20.5LSB/g and typical scale factor is 49mg/LSB. There is no problem with these. I would suggest you to lower ODR to 100Hz to improve noise influence in test and retry. What's more, please make sure you use burst read to read 3-axes output.
    
 
    BR
    
 
    Neil
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
						
				
				
		
		
    
			
		
    
	
    

    

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                on Jul 6, 2015 9:32 AM
            
            
						
			            		
    
		
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    With 49 mg/LSB, offset(x,y,z) = 0, ODR = 100Hz and the device on the table in order to get z=1g, x=0g and y=0g, I'm getting:
    x= 64 -> 3.14g
    y = 72 -> 3.528g
    z = 120 -> 5.89g
    
 
    this is huge! Is this normal? Almost 5g offset in Z axis?
    
 
    In different acquisitions, I can get y=3g and y=4.5g. 1.5g is a very big difference! I wanted this to be more precise and reliable
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
						
				
				
		
		
    
			
		
    
	
    

    

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							FormerMember
					
    
		
    
		    
		    		    
			
				
									FormerMember
							        
			
			
			
            
						
                            
            
            				
            
                on Jul 6, 2015 10:22 AM
            
            
						
			            		
    
		
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    Please try to get 1000 samples and average them to improve the noise influence further. BTW, did you monitor the power supply for ADXL375, is it stable? You can try use 10uF//0.1uF decoupling capacitor to improve the power supply noise too. LDO is recommended to power ADXL375.
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
						
				
				
		
		
    
			
		
    
	
    

    

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                on Jul 6, 2015 11:04 AM
            
            
						
			            		
    
		
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    I'm using the ADXL375 Evaluation Board and it has the declouping capacitors. The supply is stable @ 3.3V because I'm powering with a voltage source. I will try to make an average but a difference so big between measurements may not fulfill my demands. I thought this would be more precise.
    
 
    Thanks
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
						
				
				
		
		
    
			
		
    
	
    

    

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    •  Analog Employees  
    
            
						
                            
            
            				
            
                on Jun 14, 2016 3:34 PM
            
            
						
			            		
    
		
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    Hi, 
    
 
    The ADXL375 does have a min/max range of +/-6g, but with that said it would be very unusual for you to see offsets as large as 4.9 g (as you can see in the datasheet Figures 4, 5 and 6). 
    
 
    Please check the setting of the justify register bit in Reg 0x31.  By default this is set to 0 (right justified), but if this is set to 1, the data will be left justified.  If the data is left justified the lsb is actually in bit location D3 of the lower byte.  Coincidentally, all the values you mention show that the 3 lowest bits are 0. Which could suggest the data is left justified, but is being read as
    though it is right justified.  
    
 
    If you are still having problems, please lay the board flat and readback 10 time, then flip the board 180 degrees and readback again 10 times and provide this raw data.
    
 
     
    
 
    Regards,
    
 
    Anthony  
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
						
				
				
		
		
    
			
		
    
	
    

    

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    •  Analog Employees  
    
            
						
                            
            
            				
            
                on Aug 2, 2018 11:06 AM
            
            
						
			            		
    
		
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    This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

    Thank you,
    EZ Admin
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
				
						
						
				
				
		
		
    
			
		
    
	
    

    

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