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The ADAU1772 is a codec with four inputs and two outputs that incorporates a digital processing engine to perform filtering, level control, signal level...
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ADAU1772 on Analog.com

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ADAU1772 AINx Input Protection and Bias

		
	
				

			

									
					    Neitekiller
					
							

            
            			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Oct 5, 2025 
				
			
			
		
    
		
		                    	    
    
        	    
    Thread Summary
    
                The user inquires about the AINx pins of the ADAU1772 and their protection using external clamping diodes. The final answer confirms that a 5Vpp signal with a 2V offset will be heavily clipped by the diodes, which forward bias at ±0.43V, and that the ADC clips at 2.54Vpp with AVDD=3.3V. The built-in over-voltage protection of the ADAU1772 kicks in around 1V beyond the power rails, so the external diodes help protect against large signal swings but do not prevent ADC clipping.
                        	    
    
                
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    Category: Hardware
    
    		        		            		    
    Product Number:  ADAU1772  
    
    		        		    		    
    
		    		    
		    			
    
			    
    Hello,
    

    I have a question regarding the AINx Pins of the ADAU1772.
    

    The Datasheet as well as this thread leads me to the decision to implement external protection to the AINx Pins with low Vf Clamping Diodes as shown in the screenshot below.
    

     
    

    Then the datasheet and this thread mentions that the AINx Pin will be biased around AVDD/2 after AC Coupling (in my case: 1.65 V).
    

    Am I understanding the following three scenarios correctly?
    

      

    • Lets look at an example of a Analog_IN Signal with 5Vpp and 2V offset. After AC-Coupling and biasing of the ADAU1772 the Signal (without clamping) would be 5Vpp with offset of 1.65V (from -0.85V to 4.15V). The clamping clips the Signal at AIN0 to 0V-Vf to 3.3V+Vf.
      • 

    • Analog_IN Signal with Amplitudes <3.3Vpp won’t be clipped by the external Clampig Diodes.
      • 

    • The full resolution of the ADC is up to Analog_IN Amplitudes of 2.54Vpp with AVDD=3.3V

        

      1. This can be achieved by either having a strong enough input Signals or..
        2. 

      2. Using the PGA and amplifying the signal to a suitable amplitude
        3. 

      

      • 

    

    Am I understanding this correctly?
    
			
    
			
									
		                             
                        

    
    
                                            
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      •  Analog Employees  
      •  Super User  
      
            
						
                            
            
            				
            
                on Oct 13, 2025 12:20 PM
            
            
						
			            		
      
		
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      Hello Neitekiller,
      

      

      Here is the spec in the datasheet and you did reference this:
      

       
      

      

      Neitekiller said:
      The full resolution of the ADC is up to Analog_IN Amplitudes of 2.54Vpp with AVDD=3.3V

        

      1. This can be achieved by either having a strong enough input Signals or..
        2. 

      2. Using the PGA and amplifying the signal to a suitable amplitude
      

      Yes,  A full scale input signal at the pins would be 2.54Vpp. (using 3.3V AVDD). 
      

      Yes, if the signal at the pins was really small, like the output of a mic, then the PGA would be enabled and add more gain. 
      

      

      Neitekiller said:
      • Lets look at an example of a Analog_IN Signal with 5Vpp and 2V offset. After AC-Coupling and biasing of the ADAU1772 the Signal (without clamping) would be 5Vpp with offset of 1.65V (from -0.85V to 4.15V). The clamping clips the Signal at AIN0 to 0V-Vf to 3.3V+Vf.
        • 

      • Analog_IN Signal with Amplitudes <3.3Vpp won’t be clipped by the external Clampig Diodes.
      

      These two statements I do not fully understand. 
      

      If you put in a 5Vpp signal it would be clipped. Heavily clipped.
      

      

      I had to look up the diodes you are specifying. They are Schottky diodes and the forward voltage is very low, 430mV. See the datasheet for more details. 
      

      So to forward bias these diodes the voltage on the pin would have to be greater than 3.3V + 0.43V = 3.73V in the positive direction.
      

      In the negative direction it would need to be -0.43V of course. 
      

      These numbers are respective to ground. 
      

      

      A signal will be riding on the CM voltage of 1.65V on the pin. 
      

      

      So looking at a signal voltage before the series cap we can see how far positive will it need to go to forward bias the "upper" diode and how far negative to forward bias the "lower" diode. Since the CM voltage is exactly half of the 3.3V this makes the math easy. 
      

      For the positive, 1.65 + 0.43 = +2.08V
      

      For the negative, -1.65 - 0.43 = -2.08V
      

      This is a total peak to peak signal of 4.16V to start clipping the signal from the diodes. 
      

       Now we do not publish the specifications of the over-voltage protection we have built into the part. It will probably be kicking in with a signal over 0.2V above or below the voltage rails. 
      

      

      

      

      This level of signal would have clipped the ADC long before it reached a p-p voltage of 4.16V. 
      

      The circuit would protect against really large signal swings but there is also over voltage protection built into the pin of the chip. That kicks in but it is when the voltage on the pin exceeds the power rails on the part by around 1V. 
      

      So I guess it would serve to help protect the chip but it would not be clipping the signal before the ADC itself clips, so you are safe there,
      

      

      Dave T
      

      

      

      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
						
						
				
				
		
		
      
			
		
      
	
      

      

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