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Question about LTC2986



		

			

									
						AkiraO
					
							

			
				
									
											
							 		
		        	            							
			
			    	
			                    
                
				                on Feb 21, 2019 
				
			
			
		


				
		

		    		    
		    
		    	
			
Hello,


I received a question about LTC2986 from our customer. He is checking LTC2986 by following setting.


 


When the customer sets YOKOGAWA 2553A to 0.0 deg C. The measurement result of LTC2986 is -0.5 deg C.


Then he sets 2553A to 100.0 deg C. The measurement result of LTC2986 is 99.5 deg C. There is -0.5 deg C offset.


Do you have any ideas for a cause of the offset? Can we correct this offset by resistor setting of LTC2986?


Best regards,


Akira

								


				
				
		

			
						
									
									
						
						
			
			
			
			
		


					

			





	














	

	

					



			








	
	
	
	



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                                 Analog Employees 
                                
                            
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                on Feb 27, 2019 4:37 PM
            
            
					
    
	
    

		
		
    
					
    Hi Akira - what is the configuration of the LTC2986 - is this without cold junction correction? Likewise, is the 2553A set to use an external correction or no CJC?
    

    If they both have cold junction correction turned on this may just be the difference between the temperature at the 2553A jack vs. the board temp of the LTC2986. 
    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
						
				
				
		
		
    
			
		
    
	
    

    

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                on Feb 28, 2019 10:36 AM
            
            
												
						in reply to LoganC
					
									
    
	
    

		
		
    
					
    Hi,
    

    Thank you for your reply. He didn't use CJC both LTC2986 and 2553A. Is this a cause of 0.5 deg C offset? Can we adjust like this offset by resistor setting of LTC2986?
    

    Thanks & regards,
    

    Akira
    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
						
				
				
		
		
    
			
		
    
	
    

    

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                                 Analog Employees 
                                
                            
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                on Feb 28, 2019 11:40 PM
            
            
												
						in reply to AkiraO
					
									
    
	
    

		
		
    
					
    Unless they are using a resistive sensor for cold junction, no resistor offset here. 
    

    Could you send a photo of the test setup? Are they using the demo board DC2508 and DC2210 or a different setup? 
    

    They could be seeing a combination of offsets from thermocouple effects and the combined accuracy limits of the 2553A (0.11C specified accuracy 1yr from cal) and the reference of the LTC2986. What is the ambient temperature in the test lab?
    

    I would also disable open-circuit detection if enabled in the LTC2986 - the 2553A specifies an output resistance of 1ohm but it is not clear if it can sink the pulsed open-circuit detect current. 
    

    0.5C is only 20uV - thermocouple effects are possible, air drafts from HVAC can cause this if the terminals are separated.
    

    -Logan
    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
				
						
						
				
				
		
		
    
			
		
    
	
    

    

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													+1
							
			
				
									
							        
            
                                                                                                        
      
                            
    • 
                                 Analog Employees 
                                
                            
      • 
                        
     
                                                           			
			                            
                        
                on Feb 28, 2019 11:40 PM
            
            
												
						in reply to AkiraO
					
									
    
	
    

		
		
    
					
    Unless they are using a resistive sensor for cold junction, no resistor offset here. 
    

    Could you send a photo of the test setup? Are they using the demo board DC2508 and DC2210 or a different setup? 
    

    They could be seeing a combination of offsets from thermocouple effects and the combined accuracy limits of the 2553A (0.11C specified accuracy 1yr from cal) and the reference of the LTC2986. What is the ambient temperature in the test lab?
    

    I would also disable open-circuit detection if enabled in the LTC2986 - the 2553A specifies an output resistance of 1ohm but it is not clear if it can sink the pulsed open-circuit detect current. 
    

    0.5C is only 20uV - thermocouple effects are possible, air drafts from HVAC can cause this if the terminals are separated.
    

    -Logan
    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
				
						
						
				
				
		
		
    
			
		
    
	
    

    

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