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Photodiode amplifiers for 24 bit ADCs



		

			

									
					    bharris
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Mar 4, 2013 
				
			
			
		
    

				
		
    
		    		    
		    
		    	
			
    I would like to interface a photodiode transimpedance amplifier to a 24bit ADC (AD7765, AD776, AD7767 or similar). The largest source of error appears to be the drift of the transimpedance amp (TIA), rather than noise. 
    
 
     We prefer to use a linear amplifier, rather than a logarithmic amplifier. The topology is thus a linear TIA followed by either by a single ended to differential amp stage or by grounding one side of the ADC input if it provides better performance.
    
 
    Assuming a 40 degree change in temperature, a 0.5uV/degree offset coefficient leads to a 20 uV input.  If both the TIA and the SE to DE stage have a 0.5uV/degree drift coefficient, total drift could be as large as 40uV, which is too big for our application.  For comparison a TIA gain of 5K in a 25kHz bandwidth has less than 2uV RMS noise, using a low noise TIA.
    
 
     I would very much appreciate recommendations for TIA amplifiers, single-ended to differential amp, and input topology for achieving the lowest possible total error, within a -5 to 65 degree temperature range.
    
 
    System parameters are:
    
 
    Photocurrent range: 20nA to 1mA
    
 
    Sampling rate: approximately 30 to 50 kilosamples/sec
    
 
    Photodiode capacitance: 10pF
    
 
    Thanks
    
 
    Bernard Harris
    
								
    

				
				
		
    
			
						
									
									
						
						
			
			
			
			
		
    

        
        

					
    
			
    


    


	
    
		
	
    















    
	
    
	
    
					



			


				
	









	
      
	

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      •  Analog Employees  
      
            
			                            
                        
                on Mar 4, 2013 9:24 PM
            
            
					
      
	
      

		
		
      
					
      Hi Bernhard, 
      
 
      I'll get some recommendations on the amplifier stages from my colleagues in the amplifier division of Analog Devices and we can get some cct diagrams together for you to look at.
      
 
      Regards,
      
 
      Michael.
      
			
      

		
		
		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
						
				
				
		
		
      
			
		
      
	
      

      

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      •  Analog Employees  
      
            
			                            
                        
                on Mar 7, 2013 2:19 AM
            
            
					
      
	
      

		
		
      
					
      Hi-
      
 
      I moved this discussion to the Amplifiers Community.  Someone here should be able to assist you.
      
 
      Regards,
      
 
      AndyR
      
 
      EngineerZone Community Manager
      
			
      

		
		
		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
						
				
				
		
		
      
			
		
      
	
      

      

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      •  Analog Employees  
      
            
			                            
                        
                on Mar 7, 2013 4:35 AM
            
            
					
      
	
      

		
		
      
					
      Hi Bernard Harris,
      
 
      ADA4528-X is a precision zero-drift operational amplifier. It as a maximum drift of 0.015uV/degC which could be used in a TIA configuration for your application. A dual ADA4528-2 can be configured to act as a single-ended to differential output converter at the same time. Please see picture below:
      
 
      
 
      R2 is calculated based on the minimum VOH of ADA4528-2 from Tmin to Tmax for dual supply operation. Vp and Vn outputs will have peak output voltages of almost 2.5V each, which is suitable to the AD776X ADC inputs. R3 and R4 must be equal and could be set to 1kohms each. The feedback capacitor C2 stabilizes the TIA but limits the bandwidth to less than the unity gain bandwidth of AD4528-2. I think a C2 of 40pF should be able to give enough bandwidth and stability to counter the effects of the diode capacitance, trace capacitance, and amplifier input capacitance. I have simulated this circuit using Multisim, but can't guarantee its actual performance in the real world.
      
 
      I hope this helps.
      
 
      Regards,
      
 
      Neil
      
			
      

		
		
		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
						
				
				
		
		
      
			
		
      
	
      

      

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      •  Analog Employees  
      
            
			                            
                        
                on Aug 2, 2018 11:46 PM
            
            
					
      
	
      

		
		
      
					
      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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