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The ADuC7060/ADuC7061 are fully integrated, 8 kSPS, 24-bit data acquisition systems incorporating high performance multi-channel sigma-delta (Σ-Δ) analog...
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The AD7794/AD7795 are low power, low noise, complete analog front ends for high precision measurement applications. They contain a low noise, 24-/16-bit...
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ADuC706x - Full Scale & Zero Scale Ranges with External Reference

		
	
				

			

									
					    jbuckf50
					
							

            
            			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Jul 8, 2016 
				
			
			
		
    
		
		                     
				
        		
    
		    
		    		    
		    			
    
			    
    Hello.  I'm using an ADuC7061 with an external reference.  
    
 
    External reference:  460mV differential on top of approximately 1.25V common mode.  
    
 
    Signal:  Up to ~125mV on top of ~1.5V common mode.
    
 
    My gain = 4, AMP_CM = 0.
    
 
    The noise performance of this is great but I'm having trouble getting rid of a ~0.1% offset.  I thought implementing zero & full scale cal would do the trick but I'm seeing even more issues there.  
    
 
    For zero scale, I replace my signal with about 1mV; common mode is unchanged.
    
 
    For full scale, I jumper over (via analog switches) the reference to the analog inputs.   
    
 
    I'm picking up a ~4% error running zero scale cal, then a big error (25%) running full scale.
    
 
    Looking at the data sheet, I see that my nominal signal ranges fall within the specs, but the full scale cal violates the max input specs.  However, the datasheet indicates one should replace the input with 'an external full scale voltage' for FS cal.  
    
 
    Does the external full scale voltage have to equal the reference voltage, or can it be any level I want (that is less than the reference)?
    
 
    Above, I mentioned my signal is ~125mV; the real range of interest I care about is about 80mV to 125mV.  Can I just use 80mV for zero scale cal and 125mV for full scale?  The datasheet is somewhat vague in this regard.  I consulted the AD7794 datasheet as well but it didn't fully address my question.
    
 
    Thank you for your time.
    
			
    
			
									
		                             
                        

    
    
                                            
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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Jul 26, 2016 4:00 AM
            
            
						
			            		
      
		
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      Hello，
      
 
      You are right. An correct calibration can get rid of the system offset.
      
 
      Some comments as below.
      
 
      1. In your application, the Gain = 4, so ADC system zero-scale and full-scale calibration are recommendable. With this way, the system level offset and gain error can be calibrated, including in ADC itself. When you run calibration progress, please configure gain = 4.
      
 
      2. For zero-scale, an external zero voltage should be added at the input pins. An easy way is short the input pins directly, but also adding the common voltage. Be sure to run zero-scale calibration firstly, then full scale calibration.
      
 
      3. Full system full -scale calibration, because gain= 4 is configured, so the external full-scale voltage should be equal to external reference / 4. As to your case the external full scale signal is 460/4=115mV. Here I am a little concerned about your input signal range, which is ~.up to 125mV. If you use 460mV external reference with Gain = 4,ADC should be over range when input signal over 115mV. So an external reference with 600mV is preferred.
      
 
      4. For the 80mV ~ 125mV signal range, the way you mentioned cannot improve the noise performance. A better way is to add an additional circuit to minus the 80mV. Then the actual input range is 0~45mV, Gain = 8 is recommendable when external reference is 460mV.
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
				
						
						
				
				
		
		
      
			
		
      
	
      

      

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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Jul 26, 2016 4:00 AM
            
            
						
			            		
      
		
										0
					
	
      

		
		
      
		
      
                                
                                                            
      
					
      Hello，
      
 
      You are right. An correct calibration can get rid of the system offset.
      
 
      Some comments as below.
      
 
      1. In your application, the Gain = 4, so ADC system zero-scale and full-scale calibration are recommendable. With this way, the system level offset and gain error can be calibrated, including in ADC itself. When you run calibration progress, please configure gain = 4.
      
 
      2. For zero-scale, an external zero voltage should be added at the input pins. An easy way is short the input pins directly, but also adding the common voltage. Be sure to run zero-scale calibration firstly, then full scale calibration.
      
 
      3. Full system full -scale calibration, because gain= 4 is configured, so the external full-scale voltage should be equal to external reference / 4. As to your case the external full scale signal is 460/4=115mV. Here I am a little concerned about your input signal range, which is ~.up to 125mV. If you use 460mV external reference with Gain = 4,ADC should be over range when input signal over 115mV. So an external reference with 600mV is preferred.
      
 
      4. For the 80mV ~ 125mV signal range, the way you mentioned cannot improve the noise performance. A better way is to add an additional circuit to minus the 80mV. Then the actual input range is 0~45mV, Gain = 8 is recommendable when external reference is 460mV.
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
				
						
						
				
				
		
		
      
			
		
      
	
      

      

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