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AD595 Not Recommended for New Designs
The AD594/AD595 is a complete instrumentation amplifier and thermocouple cold junction compensator on a monolithic chip. It combines an ice point reference...
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Strange AD595 behavior

		
	
				

			

									
					    jonahz
					
							

            
            			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Jan 5, 2012 
				
			
			
		
    
		
		                     
				
        		
    
		    
		    		    
		    			
    
			    
    Hello,
    
 
    I'm using a K-type thermocouple and an AD595 to measure temperatures in the range +20 C to -100 C.  The system that I'm measuring has rapid temperature changes, covering the entire temperature range in as little as 2 seconds.
    
 
    I'm powering the AD595 with +/- 12V (from two lead acid batteries), my thermocouple has an ungrounded junction and I've made the ground connection as described in the AD595 spec sheet.  The output of the AD595 goes directly into a NI USB data acquisition card.  The batteries are also powering 2 pressure transducers.
    
 
    The problem that I have is that once the temperature drops below 0 C, I get very large oscillations on the output.  I've atached a screenshot of some of the results.  The frequency of oscillations is larger than my sampling frequency (10 kHz).  I have no idea what's causing it or how to fix it.  If it weren't for the fact that this only occurs at negative temperatures I would just assume I need some low pass filtering and be on my way, but I'm confused by this situation.
    
 
    Any help that anyone can provide would be very much appreciated!
    
			
    
			
									
		                             
                        

    
    
                                            
                 Analog Temperature Sensors
                
                                                    
                 Temperature Sensor and Control Devices
                
                                                    
                 ad595
                
                                
            
    
    
    
		
    

							
    
				
				
			
    
					
				        
        		
    
			
						
									
												
			
			
                        
                                    
                        
                        
                        
            
			            
						
						
			
			
			
			
		
    

        
        
        
    
					
    
			
    


    
















    
	
    
	
    
					



			


				
	









	
            
						
                            
            
            				
            
                on Jan 5, 2012 3:45 AM
            
            
						
			            		
    
		
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    Jonahz,
    
 
      It would help if you also posted your schematic with component values.  I suspect oscillation, so currentleak may be on the
    
 
    right track.  Put the 50 ohm in series with the output of the 595.  Also read page 5 of the d.s. on adding caps depending on
    
 
    what your gain is.  Finally, the AD595 is over ten years old.... You may want to look at the AD8495.
    
 
    Harry
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
						
				
				
		
		
    
			
		
    
	
    

    


	

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                on Jan 6, 2012 1:06 AM
            
            
						
			            		
      
		
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      Harry & currentleak,
      
 
      Thank you both for replying.
      
 
      I tried adding the 50 Ohm resistor and it's definitely improved the response.  I've attached two plots of some results, along with a schematic.
      
 
      Harry - if I'm using the default gain of 247.3, I shouldn't be adding capacitors for additional frequency compensation, right?
      
 
      Thanks again,
      
 
      -Jonah
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
						
				
				
		
		
      
			
		
      
	
      

      

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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Jan 6, 2012 8:01 PM
            
            
						
			            		
      
		
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      Hello jonahz,
      
 
      Is the reference junction (cold junction) temperature pretty stable or is it also moving around some?
      
 
      One idea is to place a 1MOhm resistor between the thermocouple and where you ground it (so ground that leg of the thermocouple through a 1MOhm resistor), see if this makes a difference.
      
 
      Also, as Harry suggested, take a look at the AD8495...
      
 
      We do also recommend at filter at the inputs of the amplifier, this might help you.
      
 
      https://ez.analog.com/cfs-file/__key/communityserver-discussions-components-files/382/e998adadd58b9c7876c151f7b8e6e2cf.html
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
						
				
				
		
		
      
			
		
      
	
      

      

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                on Jan 7, 2012 12:24 AM
            
            
						
			            		
        
		
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        Hi Reemo,
        
 
        I should have mentioned that the reference junction temperature is farily constant (17 C +/- 1 C).
        
 
        I originally had the 1 MOhm resistor -> ground as you described, but I found that I had much lower noise when I grounded it directly.
        
 
        So I took a look at the AD8495 and for my application it doesn't seem like it would be a substantial improvement because of my small measurement range, the +/- 12V supplies I already have in use for this system, and everything is done on strip boards so I'd need an adapter for the MSOP anyway. 
        
 
        However, I did like the details in the spec sheet about filtering at the inputs.  I implemented that (setting corner frequencies to ~200 Hz) and now my response looks beautiful.  I've attached a plot.
        
 
        Thank you all for your help!
        
 
        -Jonah
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
				
						
				
				
		
		
        
			
		
        
	
        

        

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        •  Analog Employees  
        
            
						
                            
            
            				
            
                on Jan 9, 2012 2:52 PM
            
            
						
			            		
        
		
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        Excellent, happy to hear that you found a solution.
        
 
        Thanks for asking the question.
        
 
        Reemo
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
				
						
				
				
		
		
        
			
		
        
	
        

        

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                on Apr 29, 2019 6:35 AM
            
            
						
			            		
          
		
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          This question has been closed by the EZ team and is assumed answered.
          
			
          

		
			
	      

		
		
	
          
	
          
		
          
			
		
          
	
          
	
          
		
						
				
						
						
				
				
		
		
          
			
		
          
	
          

          

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