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AD5755-1 Iout accuracy issue

		
		

		

			

									
						Verti
					
							

			
				
									
											
			        
                                                			
				
			
		


				
		

		    
		    
		    		    
			
Dear Specialists,

 
         We are using AD5755-1 and now meet Iout accuracy issue. We design to get 4-20mA Iout.

 
Based on the datasheet, the Maximum Iout error should be 0.05%.We follow the reference design and use external Rset for better accuracy.

 
       We test the Iout with one series digital multimeter (keithley2000) ,

 
        Now if input data "0" , we get 3.99740mA output. If input"65535", we get 20.2269mA

 
        Does any problems exist with our test method? I also tie the schematic  FYR~

 
 

 

 

								


				
				
		

			
						
									
									
						
						
			
			
			
			
		


					

			





	

		
	











	

	

					



			


				
	









	
    
	

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    Hi Verti,
    
 
       
    
 
       The test setup used to measure the accuracy specifications using external RSET didn't use resistors. The specifications were taken assuming we have an ideal resistor connected to the RSETx pins. The accuracy of the output is affected by the Rset tolerance and drift as you can see on the spec table. When using the internal Rset, the accuracy is worse compared to the external RSET(ideal resistor) specifications. Hence, when using an external resistor which has some error, this would directly affect the output accuracy and the specifications will no longer apply.
    
 
       You can try to confirm that the issue is caused by RSET by checking the output using the internal RSET. If the output measurements are within the TUE limits(internal) then the cause of the accuracy issue is the external RSET used. Can you share with us the part number of your RSET?
    
 
    Best regards,
    
 
    Rainier
    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
						
				
				
		
		
    
			
		
    
	
    

    

    

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    Dear Rrosario,
    
 
             Really thank you for your help. We do several tests with more boards these days, below is log data.
    
 
             We use two channels. Both output 4-20mA current. We use internal resistor to  test, the error suddenly becomes large among 12mA, we wonder whether it's normal performance, can you kindly share your advice? Really sorry for  troubling you.
    
 
             We will purchase external ±0.1% precision resistor to test, do you think ±0.1% precision can be used as Rset?
    
 
     
    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
						
				
				
		
		
    
			
		
    
	
    

    

    

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    Hi Verti,
    
 
    The Rset that you should use should also be a low drift resistor. You can use the 15kohm low drift resistor just like in the AD5755-1 evaluation board user guide (part number:PCF0805-13-15K-B-T1). 
    
 
    http://www.analog.com/media/en/technical-documentation/user-guides/UG-244.pdf
    
 
    Regarding the error (TUE) of the AD5755-1, the typical error should be maximum at the higher codes as shown on the figure below:
    
 
    
 
    Best regards,
    
 
    Rainier
    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
				
						
						
				
				
		
		
    
			
		
    
	
    

    

    

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