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AD8352 Recommended for New Designs
The AD8352 is a high performance differential amplifier optimized for RF and IF applications. It achieves better than 80 dB SFDR performance at frequencies...
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AD8352 on Analog.com

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AD 8352 Common mode voltage - allowable range - not obvious in datasheet

		
	
				

			

									
					    mikep-j
					
							

            
            			
				
									
											
			
			
				
			
			
			    	
			                    
                
				                on Jun 1, 2025 
				
			
			
		

		
		                     
				
        		

		    
		    		    

		      		            		        
Category: Datasheet/Specs

    		        		            		    
Product Number:  AD8352  

    		        		    		    

		    		    
		    			

			    
To get a  low frequency response on the AD8352  (< 1kHz) we are currently not using the  in-built half rail generator, and our common mode voltage is about 2V, and Vcc 5.0-V  - but   I can see nothing in the datasheet to indicate the effect of doing this but we are seeing greatly reduced gain even at the top end 100+ MHz.


Is this a credible cause of reduced gain ? Does anyone know how far the common mode voltages can be pulled about  and what the symptoms are if you run it  a bit lop sided.


We need about 15dB gain BTW. and seem to have about 1.8V on the VCM pin.


Mlke.

			

			
									    

			        
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    				changed title to better describe Query

[edited by: mikep-j at 10:45 AM (GMT -4) on 30 May 2025]
				

						
		                                 
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                 Datasheet/Specs
                
                                                    
                 low frequency
                
                                                    
                 Fully Differential Amplifiers
                
                                                    
                 Differential Amplifiers and ADC Drivers
                
                                                    
                 ad8352
                
                                        
                                            
                
                
                                
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    •  Analog Employees  
    
            
						
                            
            
            				
            
                on Jun 3, 2025 4:04 PM
            
            
						
			            		
    
		
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    Hi Michael,
    I would not have immediately expected that adjusting the common mode voltage would have a significant effect on the gain. The datasheet says that the Vcm voltage can be adjusted from 1.2V to 3.8V. As you move to the top or bottom of this range, I would imagine that the output swing would be affected because the instantaneous signal level is coming closer to ground or supply (if you have a large output signal). Can I suggest a few experiments to get a sense for what is going on.
    1. Set the externally applied VCM to the default value of  VCC/2 (2.5V for a 5V supply). Vary the input signal swing and verify that you are getting the gain and output signal levels that you would expect. 
    2. Set the externally applied VCM to your target level (1.8V). Dial down the amplitude of your input signal so that you can measure small signal gain (you want your input signal to be small enough so that you are certain that the output signal swing is not coming anywhere near the supply rails.  If you getting the expected gain, start increasing the input signal swing. It might also be good to look at the output signal on an oscilloscope to see if there are signs of supply-related clamping. 
    Best Regards
    Eamon Nash
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
				
				
				
		
		
    
			
		
    
	
    

    

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