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FastFETs are an entirely new class of amplifiers from Analog Devices, of which the AD8034 and AD8065 are the first two parts to be released. FastFETs combine...
AD8034 on Analog.com

LT1720 Production
The LT1720/LT1721 are UltraFast™ dual/quad comparators optimized for single supply operation, with a supply voltage range of 2.7V to 6V. The input voltage...
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LT1720 on Analog.com

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AD8034 as voltage follower, heat generation

		
	
				

			

									
					    NotBenoit
					
							

            
            			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Nov 12, 2024 
				
			
			
		
    
		
		                     
				
        		
    
		    
		    		    
    
		      		            		        
    Category: Hardware
    
    		        		        		    		    
    
		    		    
		    			
    
			    
    I'm designing a frequency counter with an input stage that consists of an AD8034 in voltage following setup. The purpose of that is to burden the signal source as little as possible (ideally the source should not notice the counter is there).
    

    The circuit simulated in LTspice draws very little current (in the µA range), while in real life that is not the case. The input is a quasi-square wave AC with 8V peak (16Vpp), the desired frequency range is 0.1Hz -10MHz.
    

    So much for the intro.
    

    Putting it together on a breadboard I noticed the AD8034 draws lots of current (>30mA if I let it) and gets extremely hot when simply shorting the feedback from VOUT to IN-. Adding the resistors helped taming the current consumption to 8mA at load (with +-12V power supply) and temperature on the chip's surface to about 45ºC/110ºF while keeping the signal good for the LT1720 that follows.
    

    So the question is: Is that kind of heat generation to be expected or is there something fundamentally wrong with how I'm doing this?
    

    Thanks for any pointers!
    

    Regards
    

    NB
    

    PS: Yes, I'm a rookie, so please don't laugh when you look at this:
    

     
    

     
    

    
			
    
			
									
		                             
                        

    
    
                                            
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                 ad8034
                
                                                    
                 High Speed Comparators (<100ns Propagation Delay)
                
                                                    
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                on Nov 13, 2024 6:46 AM
            
            
						
			            		
        
		
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        Reducing the voltage on IN+ to 500mV about halfs the total supply current to 4mA, only very little more than running it idle without any signal. Then, the temperature of the chip surface is less than 35ºC (down from 45ºC).
        

        Works for me, but is such significant heat production at almost idle current to be expected?
        

        Regards
        

        NB
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
				
						
				
				
		
		
        
			
		
        
	
        

        

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        •  Analog Employees  
        
            
						
                            
            
            				
            
                on Nov 14, 2024 7:04 PM
            
            
												
						in reply to NotBenoit
					
										
			            		
        
		
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        Hi  ,
        

        1st cause may be the use of breadboard. A poor PCB layout can degrade the performance of the part and cause it to draw more current.
        

        Another thing is that you may be violating some parameters in the op amp. What is your signal amplitude and supply? Please make sure that you do not exceed the Input Common-Mode Voltage Range.
        

        By the way, I'm seeing that you have used a voltage divider at the op-amp output. I suggest adjusting the voltage divider at the input instead of voltage divider at the output, the lower output impedance of the op-amp driving the next component will reduce noise pickup and other circuit effects.
        

        Thanks,
        

        Gio
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
						
						
				
				
		
		
        
			
		
        
	
        

        

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                on Nov 18, 2024 9:45 AM
            
            
												
						in reply to GioBalgemino
					
										
			            		
          
		
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          Hi Gio,
          

          

            many thanks for your reply and suggestions.
          

          I followed your suggestion to remove the output voltage divider and added one at the input, so input voltage got down from 16Vpp to less then 1Vpp. The output change increased the chip temperature slightly from 32ºC to 35ºC.
          

          But now that the input voltage is low I reduced the supply voltage from +-12V to +-5V and that seems to make a huge difference with heating, the temperature is now down to less than 30ºC.
          

          I somehow falsely assumed that it's a good idea to subject the opamp to my signal directly so long as it is within the generally acceptable operating boundaries. Apparently high input and supply voltage not only causes extra heat but also makes the circuit more susceptible to noise pickup. At least the signal looks much cleaner now, even on the breadboard.
          

          I shall see how much difference a PCB implementation makes in terms of noise.
          

          Thanks again
          

          NB
          
			
          

		
			
	      

		
		
	
          
	
          
		
          
			
		
          
	
          
	
          
		
						
						
				
						
				
				
		
		
          
			
		
          
	
          

          

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