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Use AD8295 for Loadcell Amplifier Board



		

			

									
						NgocAnh
					
							

			
				
									
											
							 		
		        	            							
			
			    	
			                    
                
				                on Jan 14, 2015 
				
			
			
		


				
		

		    		    
		    
		    	
			
Hello, I'm making a loadcell amplifier board. Input from -18mVDC to +18mVDC. Output +-2.5V. Input signal frequency from 0 to 14Hz. I have made a design as the attach. The total gain changes from about 140 to 50 according to real condition. 

 
- First, I don't know why the output is very fluctuated and gaining is wrong.

 
- Second, I intend changing the low-pass filter to pre-amp + filter. The IA will became to post-amp.

 
Anyone help me what is the wrong in my design and how to solve it.

 
Thanks,

 
Anhcao

								


							

				
				
			

					
				
		

			
						
									
									
						
						
			
			
			
			
		


					

			





	

		
	











	

	

					



			








	
	
	
	



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                                 Analog Employees 
                                
                            
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                on Feb 10, 2015 2:01 AM
            
            
					
    
	
    

		
		
    
					
    Hello Anh,
    
 
    I am sorry once again for the delay.
    
 
    Assuming that the AD8295 in-amp inputs and outputs are within a working range, the Rg pin will be a small diode drop ~0.5V below the corresponding input. So with both inputs at a voltage near 0V, both Rg pins will be at about –0.5V. You can observe this with a DVM if you disconnect the digipot from the Rg pins. The internal feedback conditions for the in-amp ensure that this voltage drop is constant, which imposes your differential input voltage across Rg. See the AD8221 data sheet for a more detailed drawing of a similar instrumentation amplifier.
    
 
    The AD8231 input voltage range is relative to the supply, no matter whether it is a single supply or a dual supply. Looking at the +5V single supply table, you could rewrite the input range as -Vs + 0.05V to +Vs - 0.05V. ±2.5V is the same voltage span as +5V, so it will translate directly as -2.45V to +2.45V.
    
 
    To check all of the in-amp headroom limits for the AD8231, including input voltage range, output voltage range, and internal nodes, you can have a look at our in amp tool: Instrumentation Amplifier Tool - Common Mode vs. Output Voltage (Diamond Plot)
    
 
    I hope this helps.
    
 
    Best regards,
    
 
    Scott
    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
						
				
				
		
		
    
			
		
    
	
    

    

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    • 
                                 Analog Employees 
                                
                            
      • 
                        
     
                                                           			
			                            
                        
                on Feb 10, 2015 2:01 AM
            
            
					
    
	
    

		
		
    
					
    Hello Anh,
    
 
    I am sorry once again for the delay.
    
 
    Assuming that the AD8295 in-amp inputs and outputs are within a working range, the Rg pin will be a small diode drop ~0.5V below the corresponding input. So with both inputs at a voltage near 0V, both Rg pins will be at about –0.5V. You can observe this with a DVM if you disconnect the digipot from the Rg pins. The internal feedback conditions for the in-amp ensure that this voltage drop is constant, which imposes your differential input voltage across Rg. See the AD8221 data sheet for a more detailed drawing of a similar instrumentation amplifier.
    
 
    The AD8231 input voltage range is relative to the supply, no matter whether it is a single supply or a dual supply. Looking at the +5V single supply table, you could rewrite the input range as -Vs + 0.05V to +Vs - 0.05V. ±2.5V is the same voltage span as +5V, so it will translate directly as -2.45V to +2.45V.
    
 
    To check all of the in-amp headroom limits for the AD8231, including input voltage range, output voltage range, and internal nodes, you can have a look at our in amp tool: Instrumentation Amplifier Tool - Common Mode vs. Output Voltage (Diamond Plot)
    
 
    I hope this helps.
    
 
    Best regards,
    
 
    Scott
    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
						
				
				
		
		
    
			
		
    
	
    

    

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