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Noisest Opamp?



		

			

									
					    didymus7
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Feb 17, 2015 
				
			
			
		
    

				
		
    
		    		    
		    
		    	
			

    Now that I've got your attention...no, no, this is real.
    
 
    My company wants to sell into China without having to wait months for an export license, we can accomplish this by making the resolution of our systems worse, especially in the bandwidth below 100 Hz.  I know that FET amps tend to have the most noise down in that area, but what is the noisest one?  We have amps in the SOIC-8 package (duals).  I'm also going to sprinkle some carbon comp resistors in there to boost the noise. 
    
 
    I know that I can do a redesign to add a noise generator, but I'm trying to get around this without a redesign and re-layout.
    
 
    Thanks...Steve
    
								
    

				
				
		
    
			
						
									
									
						
						
			
			
			
			
		
    

        
        

					
    
			
    


    


	
    
		
	
    















    
	
    
	
    
					



			








	
	
	
	



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                on Mar 7, 2015 9:53 PM
            
            
					
        
	
        

		
		
        
					
        And regarding "For gain, I had planned to perhaps boost the gain at this stage and allow the later range adjustment stage work lower."  
        
 
        Putting all your gain up front generally produces lower overall noise; So if you are trying to make the overall system noisier, then use lower gain up front.  
        
 
        If there is a log-amp in between then adding gain after the log amp is fairly pointless,  consider a random log-amp example:
        
 
        Vin 1mV-> 600mV out and  Vin 10mV-> 800mV out  and 100mV -> 1000mV out and 1000mV in -> 1200mV out
        
 
        So the changing the first stage gain by a factor of 10 changes the output by ~ 20%.
        
 
        Note that the log amp will also produce intermodulation,  so for example if you have a 20kHz carrier with some signal modulation, then the extra noise you add will appear as noise on your signal (and on the carrier), but maybe that's the effect you were after in the first place. 
        
			
        

		
		
		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
				
						
				
				
		
		
        
			
		
        
	
        

        

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													0
							
			
				
									
							        
			
			
			
          
            
			                            
                        
                on Mar 7, 2015 9:53 PM
            
            
					
          
	
          

		
		
          
					
          And regarding "For gain, I had planned to perhaps boost the gain at this stage and allow the later range adjustment stage work lower."  
          
 
          Putting all your gain up front generally produces lower overall noise; So if you are trying to make the overall system noisier, then use lower gain up front.  
          
 
          If there is a log-amp in between then adding gain after the log amp is fairly pointless,  consider a random log-amp example:
          
 
          Vin 1mV-> 600mV out and  Vin 10mV-> 800mV out  and 100mV -> 1000mV out and 1000mV in -> 1200mV out
          
 
          So the changing the first stage gain by a factor of 10 changes the output by ~ 20%.
          
 
          Note that the log amp will also produce intermodulation,  so for example if you have a 20kHz carrier with some signal modulation, then the extra noise you add will appear as noise on your signal (and on the carrier), but maybe that's the effect you were after in the first place. 
          
			
          

		
		
		
		
	
          
	
          
		
          
			
		
          
	
          
	
          
		
						
						
				
						
				
				
		
		
          
			
		
          
	
          

          

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