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HMC856 Recommended for New Designs
The HMC856 is a wideband time delay device with a 5-bit digital control designed for timing compensation or clock skew management applications. The time...
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HMC856 on Analog.com

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HMC856 input interfacing

		
	
				

			

									
					    AlbertoF
					
							

            
            			
				
									
											
			
			
				
			
			
			    	
			                    
                
				                on Dec 5, 2024 
				
			
			
		

		
		                     
				
        		

		    
		    		    

		      		            		        
Category: Hardware

    		        		            		    
Product Number:  HMC856 

    		        		    		    

		    		    
		    			

			    
Hello


we are interfacing the output of a CML fanout buffer (NB7L1008MMNR4G) with the input of HMC856 , powered between GND and -3.3V


Here below the architectures of the 2 devices 


https://www.onsemi.com/download/data-sheet/pdf/nb7l1008m-d.pdf


 




As suggested by ONSemi app note and from one of Forum posts ( About connection of HMC 856) we initially used a DC coupled connection, relying on the 50ohm internal terminations to GND of HMC856, but the Signal Integrity at receiver was completely bad


 


 




Then we moved to AC coupling as suggested by OnSemi app note, but also in this case signal quality at receives is bad


https://www.onsemi.com/pub/Collateral/AND8173-D.PDF 


 




We then added 50 ohm terminations at transmitter , followed by AC coupling caps at receiver, and this is at the moment the best of the configurations, but still vith very poor signal integrity at receiver


 




Could you please suggest the optimal termination and coupling for interfacing this driver?


Also, using positive supplies for HMC856 would simplify the power architecture for our already limited space: reading other forum's post it seems possible to power it from between +3.3V and GND, but there are no clear guidelines for that:
Can you please detail the pin connections in case of powering from positive supply?


Thanks


Alberto 









			

			
									
		                             
                        

    

                                            
                 Clock Generation and Distribution
                
                                                    
                 input
                
                                                    
                 hardware
                
                                                    
                 interface
                
                                                    
                 hmc856
                
                                        
                                            
                
                
                                        
                                            
                
                
                                
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    •  Analog Employees  
    
            
						
                            
            
            				
            
                on Dec 16, 2024 3:38 AM
            
            
						
			            		
    
		
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    Hi Alberto,
    

    

    Sorry for the delayed response. For HMC856, as long as a good 50ohm interface is present, DC or AC coupling are not expected to make any difference. You have to make sure your inter-connect PCB traces are 50ohm single ended or 100ohm diff.
    

    

    Do you know the output return loss of the OnSemi driver?
    

    Up to which frequency or Gbps are you driving the part?
    

    Any eye-diagram screenshots? What is your test signal(PRBS?)
    

    

    Lastly, it is possible to shift all supply and control pin voltages by 3.3V so that the part could be used with GND and +3.3V. The part would operate when there is a 3.3V difference between high and low supply rails. The AMR table of the datasheet was generated with for -3.3V supply but you may still refer to that table by shifting all limits by +3.3V.
    

     
    

    HMC856’s high speed input output pins are referred to the high supply rail with an internal 50ohms. If the interfacing component’s high supply rail is the same with HMC856, no DC block would be required. Otherwise, high speed pins would require DC block.
    

    

    High speed I/O pins have adjacent ground pins for proper transition from PCB to the IC. When supplies are shifted, you would need bypass capacitors to the PCB’s RF ground for pins 1,4,21,24 because they are not connected to PCB ground (0V DC) anymore. These additional capacitors prevent signal integrity issues and helps the part to work as close as possible to the datasheet performance.
    



    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
				
						
				
				
				
		
		
    
			
		
    
	
    

    

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