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AD9208 Recommended for New Designs
The AD9208 is a dual, 14-bit, 3 GSPS analog-to-digital converter (ADC). The device has an on-chip buffer and a sample-and- hold circuit designed for low...
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AD9208 on Analog.com

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AD9208: Harmonic of Decimated FS mixes with NCO



		

			

									
					    Dwijen.Pandya
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Dec 19, 2017 
				
			
			
		
    

				
		
    
		    		    
		    
		    	
			
    Hi,
    
 
    I am using AD9208 for one of my application. I am using AD9208-3000EBZ+ADS7-V2EBZ for the evaluation
    
 
    In my setup Fs is 3GHz
    
 
    Decimation Rate is 8
    
 
    Decimated Fout (Decimated Fs) is 375MHz (3GHz/8)
    
 
    NCO is 702MHz
    
 
    RF In to the ADC is 800MHz
    
 
    I have observed that there is spur in the output spectrum that is arising from mixing of the harmonic of Fout with NCO.
    
 
    For the above setup there is spur at 48MHz.
    
 
    2*375MHz - 702MHz = 48MHz
    
 
    The snapshot of the ACE setup and the output spectrum is attached.
    
 
    Is this spur expected? What can be done to minimize this spur?
    
 
    Regards,
    
 
    Dwijen
    
									
			        
		
    

				
						
        

    
    Tags: 
                                            
                 ad9208
                
                                                    
                 High Speed A/D Converters >10 MSPS
                
                                                    
                 Standard High Speed A/D Converters
                
                                                    
                 fout nco mixing
                
                                
            
    
    
    

		
    
			
						
									
									
						
						
			
			
			
			
		
    

        
        

					
    
			
    


    
















    
	
    
	
    
					



			








	
	
	
	



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                on Dec 22, 2017 1:09 PM
            
            
					
        
	
        

		
		
        
					
        Hi Dwijen,
        
 
        Thanks for the explanation. We here do not see this spur.
        
 
        I am concerned that you are seeing this spur even when RF is turned off. What you see at 48MHz/3GSPS and 45.75MHz/2.991GSPS is the DC spur that has been rotated, and filtered. I have seen that in some cases this could be a possibility. To prove this, what we could do is to get the DDC setting to decimate by 16. And try the same thing (DDC spectrum without the RF).
        
 
        FYI, we set the AD9208 in our lab and didnt see the spur that high. See below the full bandwidth and DDC spectrums.
        
 
        Full Bandwidth
        
 
        
 
        DDC 
        
 
        
 
        From our spectrum I have to believe this spur at 48MHz is either a harmonic or the DC spur. The ACE program correctly identifies the H3 Image as falling on -100MHz at 65dBc. This power matches the H3 power in the FBW mode plot.
        
 
        We will continue to do more testing. However, we are going to be shutdown for the week and wont be back until after the New Year's.
        
 
        Thanks
        
 
        Umesh
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
				
				
						
				
				
		
		
        
			
		
        
	
        

        

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                on Dec 22, 2017 1:09 PM
            
            
					
          
	
          

		
		
          
					
          Hi Dwijen,
          
 
          Thanks for the explanation. We here do not see this spur.
          
 
          I am concerned that you are seeing this spur even when RF is turned off. What you see at 48MHz/3GSPS and 45.75MHz/2.991GSPS is the DC spur that has been rotated, and filtered. I have seen that in some cases this could be a possibility. To prove this, what we could do is to get the DDC setting to decimate by 16. And try the same thing (DDC spectrum without the RF).
          
 
          FYI, we set the AD9208 in our lab and didnt see the spur that high. See below the full bandwidth and DDC spectrums.
          
 
          Full Bandwidth
          
 
          
 
          DDC 
          
 
          
 
          From our spectrum I have to believe this spur at 48MHz is either a harmonic or the DC spur. The ACE program correctly identifies the H3 Image as falling on -100MHz at 65dBc. This power matches the H3 power in the FBW mode plot.
          
 
          We will continue to do more testing. However, we are going to be shutdown for the week and wont be back until after the New Year's.
          
 
          Thanks
          
 
          Umesh
          
			
          

		
			
	      

		
		
	
          
	
          
		
          
			
		
          
	
          
	
          
		
						
				
				
						
				
				
		
		
          
			
		
          
	
          

          

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