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3v3 Output level DC1766A/LTC6957-3



		

			

									
						electrobirdy
					
							

			
				
									
											
							 		
		        	            							
			
			    	
			                    
                
				                on Dec 8, 2021 
				
			
			
		


				
		

		    		    
		    
		    	
			
All,

Trying to buffer a 10Mhz sine wave to a 10Mhz 3V3 square wave. The LTC6957-3 seemed ideal to me.
I have been trying to use the EVK board "DC1766A" to generate the required output. But so far I am only getting a pk-pk of 600mV, half of it can be accounted for because of the EVK boards output.
"AC coupled and attenuated through two 100Ω resistors as shown in the DC1766A schematic diagram to present a 50Ω source impedance appropriate to connect to a 50Ω input impedance instrument."
But I would expect 1.65V to appear at my output in this case and not the 600mV im seeing at the moment.

Input single-ended 10Mhz 1v pk-pk sine
V+ = Vdd = 3v3

								


				
				
		

			
						
									
									
						
						
			
			
			
			
		


					

			





	










	

	

					



			


				
	









	
    
	

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                                 Analog Employees 
                                
                            
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                on Dec 8, 2021 4:47 PM
            
            
					
    
	
    

		
		
    
					
    Hi,
    

    When terminating with a 50 ohm instrument the 50 ohm(instrument) and the 100 shunt are in parallel.  50||100=33 ohms.
    

    the eval board has a 100 ohm series that creates a voltage divider with the 33 ohms.  3.3Vpp *33/(100+33) = 0.8Vpp.  The other thing to consider is the LTC6957 is also driving 133 to GND which pulls some current out (~12ma) of the CMOS output. These plots show how the max output decreases level varies with load current.
    

    CMOS outputs drivers are typically not made to drive 50 ohms directly.  The LTC6957 abs max output current listed in the datasheet (+/-30mA), 3.3V/50ohms > this abs spec.
    

     
    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
						
						
				
				
		
		
    
			
		
    
	
    

    

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