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LTC6253 Last Time Buy
The LTC6252/LTC6253/LTC6254 are single/dual/quad low power, high speed unity gain stable rail-to-rail input/output operational amplifiers. On only 3.5mA...
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LTC6253 on Analog.com

LTC6254 Last Time Buy
The LTC6252/LTC6253/LTC6254 are single/dual/quad low power, high speed unity gain stable rail-to-rail input/output operational amplifiers. On only 3.5mA...
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LTC6254 on Analog.com

LTC6252 Last Time Buy
The LTC6252/LTC6253/LTC6254 are single/dual/quad low power, high speed unity gain stable rail-to-rail input/output operational amplifiers. On only 3.5mA...
Datasheet
LTC6252 on Analog.com

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LTC6253 - GBP 720MHz, but gain of 10 = BW of ~35MHz??

		
	
				

			

									
					    QBort
					
							

            
            			
				
									
											
			
			
				
			
			
			    	
			                    
                
				                on Mar 14, 2019 
				
			
			
		

		
		                     
				
        		

		    
		    		    
		    			

			    
Hello,


I am trying to design a circuit to achieve a gain of 10 for input signals up to 50MHz. I have looked at LTC6253 as a possibility, since it has quite a large GBP of 720MHz. If my understanding is correct, for a gain of 10, it should handle up to 720/10 = 72MHz


I have done a simulation on LTSpice, of non-inverting op-amp with a gain of 10 but it shows that the bandwidth would end up being around 35Mhz.... See screenshot below. 


The datasheet recommends to use a capacitor in the feedback loop to compensate stray capacitances (C1 in my simulation). Through trial and error I have found this to be around 0.5pF.


It seems there are other parameters I need to consider when selecting my op-amp for this application. Does anyone have any suggestions as to what causes a reduction in effective bandwidth vs what I would have expected from the GBP?


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Thanks!




Qbort

			

			
									
		                             
                        

    

                                            
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    •  Analog Employees  
    
            
						
                            
            
            				
            
                on Mar 14, 2019 12:15 PM
            
            
						
			            		
    
		
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    Hi Qbort:  The 2GHz spec is the gain-bandwidth-product.  It is the product of gain and frequency at a relatively low frequency.   In this case, the GBW product was specified at 10MHz, and indicates a typical gain of 200 (46dB) there. 
    

    In a gain of 10, consulting the Gain-Phase curve, you should expect a bandwidth of about 85MHz (on a 5V supply, less on a 3V supply).  I suspect the problem is that your resistor values are too high, so you are up against the feedback pole and necessary compensation.  I suggest you try again with resistor values of 200:450 instead of the 1k:9k, and you should be able to get significantly higher bandwidth.
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
						
				
				
		
		
    
			
		
    
	
    

    

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                on Mar 14, 2019 3:27 PM
            
            
												
						in reply to ADIApproved
					
										
			            		
    
		
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    Dear GlenBrisebois,
    

    Thank you for your reply.
    

    I have tried as you suggested of lowering the resistor values (I assume you meant 50:450, for a gain of 10), but saw no noticeable difference. I have even removed the compensation capacitor but it just means the output overshoots (See screenshot below), and maybe marginal increase in bandwidth to 45MHz, but I would expect a bandwidth of at least 72MHz if the GBP is 720MHz and the gain is 10?
    

    Also, just to point out, I think you might be getting confused with a different part? There is no mention of 2GHz in the datasheet of the LTC6253. The GBP is 720MHz as shown on the front page (see extract below).
    

    

     
    

     
    

    

    Thanks again,
    

    Qbort.
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
						
				
				
		
		
    
			
		
    
	
    

    

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                on Mar 14, 2019 3:27 PM
            
            
												
						in reply to ADIApproved
					
										
			            		
    
		
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    Dear GlenBrisebois,
    

    Thank you for your reply.
    

    I have tried as you suggested of lowering the resistor values (I assume you meant 50:450, for a gain of 10), but saw no noticeable difference. I have even removed the compensation capacitor but it just means the output overshoots (See screenshot below), and maybe marginal increase in bandwidth to 45MHz, but I would expect a bandwidth of at least 72MHz if the GBP is 720MHz and the gain is 10?
    

    Also, just to point out, I think you might be getting confused with a different part? There is no mention of 2GHz in the datasheet of the LTC6253. The GBP is 720MHz as shown on the front page (see extract below).
    

    

     
    

     
    

    

    Thanks again,
    

    Qbort.
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
						
				
				
		
		
    
			
		
    
	
    

    

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