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The AD8304 is a monolithic logarithmic detector optimized for the measurement of low frequency signal power in fiber optic systems. It uses an advanced...
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AD8304 on Analog.com

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about AD8304 VNEG current

		
	
				

			

									
					    nagashimatamotsu
					
							

            
            			
				
									
											
			
			
				
			
			
			    	
			                    
                
				                on Aug 21, 2025 
				
			
			
		

		
		                    	    

        	    
Thread Summary

                The user asked about the driving capability required for VNEG when using the AD8304 with ACOM=0V and VNEG=-0.5V, and an input current of 1mA. The final answer suggests that the VNEG supply must sink the photodiode current, buffer output current, and bias current, totaling up to 15mA for a 10mA photodiode current, given the part's quiescent current of 4.5mA.
                        	    

                
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Product Number:  AD8304  

    		        		    		    

		    		    
		    			

			    
Hi


When using the AD8304 with VNEG and ACOM separated (e.g., ACOM=0V, VNEG=-0.5V, VPS=5V), if the input current to INPT is 1mA, what kind of driving capability is required for VNEG? In other words, what is the maximum current for VNEG?

			

			
									
		                                 
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                 Datasheet/Specs
                
                                                    
                 Logarithmic Transimpedance Amplifiers
                
                                                    
                 Specialty Amplifiers
                
                                                    
                 ad8304
                
                                                    
                 Current Sense Amplifiers
                
                                
            

    
    
		


				
				        
        		

			
						
									
												
			
			
                        
                                    
                        
                                                    
                        
            
			            
						
						
			
			
			
			
		


        
        
        
    
					

			





















	

	

					



			


				
	









	
    
	

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    •  Analog Employees  
    
            
						
                            
            
            				
            
                on Aug 22, 2025 12:32 PM
            
            
						
			            		
    
		
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    The photodiode current will flow out of the VNEG pin and into your negative supply. So if you plan to have photodiode currents as high as 10mA, you the supply will have to sink that. The datasheet also says “The bias current, buffer output (and its load) current, and the full IPD all have to be absorbed by this negative supply, and its supply capacity must be ensured for the maximum current condition”.  It's unclear how much current all this adds up to. We know that the part has a total quiescent current of 4.5mA. All of this current is not going through VNEG. But without knowing more, I would suggest that you provide a VNEG that can sink 15mA. 
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
				
						
				
				
				
		
		
    
			
		
    
	
    

    

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