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VFB as a voltage reference



		

			

									
					    groger
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Jul 5, 2018 
				
			
			
		
    

				
		
    
		    		    
		    
		    	
			
    Hi -
    
 
    I am using an ADA4505 VFB as a voltage reference to obtain Vdd/2 (Vdd = 3.3V) as a midpoint reference in an active filter circuit, using a pair of 1% 10K resistors at the NI input. the 4505 is being supplied by an LDO, which is preceded by a switcher supply.
    
 
    My question is, how stable would this VFB output be for a comparator? The comparator requires a threshold of around 200mV, so I would need to take the VFB output and divide it down to feed Vref of the non-inverting comparator.
    
 
    My thinking is, perhaps the instability of the 3 different devices (switcher, LDO, VFB) plus the resistor divider may contribute a lot of instability so my Vref at comparator could vary by some conjugate of those numbers that could end up too low or high. I can tolerate around 50mV on the threshold.
    
 
    So...bad idea? Should I use a dedicated voltage reference?
    
 
    Thanks for your help!
    
								
    

				
				
		
    
			
						
									
									
						
						
			
			
			
			
		
    

        
        

					
    
			
    


    


	















    
	
    
	
    
					



			








	
	
	
	



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                on Jul 5, 2018 5:25 AM
            
            
					
        
	
        

		
		
        
					
        harry -
        
 
        Filters are centered around 100Hz - an audio application. For comparators, I've been looking at the ADCMP370, as it's got a very low Iq. (no bad pun here) For opamps, your suggestion of duals vs quads makes a lot of sense, in terms of space efficiency.
        
 
        So, please clarify - does it make sense to use a voltage reference for the active filter (at Vdd/2), and also use that same reference and divide that down for the comparator? Or, should I use the wizard's recommended VFB (substituted with a 4505 for power saving) and divide that down for the comparator? It's a 4 amp filter design. The ADA4505 has 40mA drive, that should be more than adequate. It does have a better Iq than the 8031.
        
 
        also - great read on the link, re: VFB and current drive and stability.
        
 
        thanks again!
        
			
        

		
		
		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
				
						
				
				
		
		
        
			
		
        
	
        

        

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													0
							
			
				
									
							        
			
			
			
          
            
			                            
                        
                on Jul 5, 2018 5:25 AM
            
            
					
          
	
          

		
		
          
					
          harry -
          
 
          Filters are centered around 100Hz - an audio application. For comparators, I've been looking at the ADCMP370, as it's got a very low Iq. (no bad pun here) For opamps, your suggestion of duals vs quads makes a lot of sense, in terms of space efficiency.
          
 
          So, please clarify - does it make sense to use a voltage reference for the active filter (at Vdd/2), and also use that same reference and divide that down for the comparator? Or, should I use the wizard's recommended VFB (substituted with a 4505 for power saving) and divide that down for the comparator? It's a 4 amp filter design. The ADA4505 has 40mA drive, that should be more than adequate. It does have a better Iq than the 8031.
          
 
          also - great read on the link, re: VFB and current drive and stability.
          
 
          thanks again!
          
			
          

		
		
		
		
	
          
	
          
		
          
			
		
          
	
          
	
          
		
						
						
				
						
				
				
		
		
          
			
		
          
	
          

          

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