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AD8629 Production
This amplifier has ultralow offset, drift, and bias current. The AD8628/AD8629/AD8630 are wide bandwidth auto-zero amplifiers featuring rail-to-rail input...
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AD8629 on Analog.com

LTC2052 Production
The LTC2051/LTC2052 are dual/quad zero-drift operational amplifiers available in the MS8 and SO-8/GN16 and S14 packages. For space limited applications...
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LTC2052 on Analog.com

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Problem using AD8629 as an ADC driver



		

			

									
					    PeterClark
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Jul 25, 2024 
				
			
			
		
    

				
		
    
		    		    
		    
		    	
			
    I wanted a low drift ADC driver in a precision thermometer circuit so I used an AD8629 (please refer to attached pdf). The op-amp +ve input was fine (i.e. a dc level proportional to the thermistor resistance) but the output was oscillating at several hundred mV, at about 800kHz. The same thing happened using an LTC2052, but problem was fixed when I used an LMP7702. What was I doing wrong?
    
									
			                
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    Tags: 
                                            
                 Operational Amplifiers (Op Amps)
                
                                                    
                 amplifiers
                
                                                    
                 Zero-Drift Op Amps
                
                                                    
                 ltc2052
                
                                                    
                 Low Input Bias Current Op Amps (≤100 pA)
                
                                        
                                            
                
                
                                
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							FormerMember
					
      
		
      
			
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									FormerMember
							        
			
			
			
    
            
						
                            
            
            				
            
                on Apr 28, 2015 5:28 PM
            
            
					
    
	
    

		
		
    
					
    Peter,
    
 
      I would suspect the LMP will drive a little more C.  Try changing the 10 nF to1 nF and see what happens.
    
 
    Or with the 10 nF, change the 20 ohm to 100 ohm.  What frequency is the oscillation?
    
 

    Harry
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
						
				
				
		
		
    
			
		
    
	
    

    


	

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													0
							
			
				
									
							        
			
			
			
      
            
						
                            
            
            				
            
                on Apr 29, 2015 9:27 AM
            
            
					
      
	
      

		
		
      
					
      Hi Harry,
      
 
      I was using a 1nF cap originally, but the AD8629 still oscillated. The frequency is about 800kHz. Subsequently, after I got the circuit working with the LMP7702, I increased the cap to 10nF. I'm averaging 4096 successive ADC samples at a rate of 100k samples/s, and I needed the increased capacitance to keep the ADC input stable.
      
 
      I've found an article in EDN, October 16, 2008 that describes this kind of op amp stability problem - I guess I need to learn a bit more op amp theory.
      
 
      By the way, there's an error on the schematic - pin 5 of the op amp is connnected to a +2.5V reference, not +5V(A) as shown. 
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
						
				
				
		
		
      
			
		
      
	
      

      

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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Aug 2, 2018 11:00 AM
            
            
					
      
	
      

		
		
      
					
      This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

      Thank you,
      EZ Admin
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
				
						
						
				
				
		
		
      
			
		
      
	
      

      

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