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AD8338 Recommended for New Designs
The AD8338 is a variable gain amplifier (VGA) for applications that require a fully differential signal path, low power, low noise, and a well-defined...
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AD8338 on Analog.com

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AD8338 AGC Oscillation

		
	
				

			

									
					    Egecan
					
							

            
            			
				
									
											
			
			
				
			
			
			    	
			                    
                
				                on Dec 3, 2025 
				
			
			
		

		
		                     
				
        		

		    
		    		    

		      		            		        
Category: Hardware

    		        		            		    
Product Number:  AD8338  

    		        		    		    

		    		    
		    			

			    
Hello! Below is the AGC circuit I designed for an AC coupled square wave input. I am using single-ended input thus connected INMR to the ground through C1. COMM and MODE are grounded. I have a bypass capacitor at VBAT and VAGC as well.


I have tested the circuit with VAGC = 2V and inputs of square wave 0.5-5MHz and the outputs OUTP and OUTM are oscillating. The output pins contain a 2.5MHz square wave for any given input. I have tried changing the VAGC values and the same output is observed for VAGC = 0.5-3V, below 0.5 and above 3V there is no output. Any idea what I am missing here? Thanks in advance!


 

			

			
									    

			        
Edit Notes

    				tested VAGC at 2V as the optimal operation

[edited by: Egecan at 10:50 AM (GMT -5) on 3 Dec 2025]
				

						
		                             
                        

    

                                            
                 Analog Control Variable Gain Amplifiers (VGAs)
                
                                                    
                 hardware
                
                                                    
                 ad8338 agc
                
                                                    
                 Variable Gain Amplifiers (VGA)
                
                                                    
                 AD8338 no output
                
                                        
                                            
                
                
                                
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    •  Analog Employees  
    
            
						
                            
            
            				
            
                on Dec 9, 2025 1:34 AM
            
            
												
						in reply to Egecan
					
										
			            		
    
		
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    Hello ,
    

    Apologies for the delayed response. Could you please provide the following details:
    

    1. The square wave input signal level at the INPR pin.
    2. Do you have any capacitive load connected to the OUTP and OUTM pins? What components or circuitry follow these pins?
    3. Could you share the oscilloscope shots of the output signal at the OUTP and OUTM pins that you have obtained?
    4. Is this behavior repeatable across multiple setups, ICs, or boards?

    Having the data above will surely be helpful. Thank you.

    Best regards,
    Paul
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
						
				
				
		
		
    
			
		
    
	
    

    

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