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AD7687 Production
The AD76871 is a 16-bit, charge redistribution, successive approximation, analog-to-digital converter (ADC) that operates from a single power supply, VDD...
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AD7687 on Analog.com

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AD7687 Operates Single-Ended, not differential.

		
	
				

			

									
					    ArnieK
					
							

            
            			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Oct 31, 2011 
				
			
			
		
    
		
		                     
				
        		
    
		    
		    		    
		    			
    
			    
    I have a board with three AD7687's that are acting like single-ended ADC, not differential.
    
 
    The parts are marked C03 #535.
    The devices operating in Chain Mode With Busy Indicator, connected as shown in the datasheet.
    Sample rate is 5K samples/second.
    SCK period is 110ns (9MHz) 50% duty cycle.
    
 
    VDD and VREF are both 5.0 Volts. VIO is 3.3V.
    
 
    I have verified all signals with an oscilloscope and all clocking looks valid, and the data being read by the software matches what I see on SDO with the scope.
    
 
    All three devices are acting similarly. The following data was collected from one device, and is repeatable.
    
 
    With a differental input:
    IN+ at 1.8V, IN- at 3.25V (a differential of -1.45 V) I get an output around 15XX Hex.
    IN+ at 3.7V, IN- of 1.7V (Differential of +2.0V), Output is around 55XX Hex.
    Varying between these two conditions, I get ONE intermediate step around 2BXX hex.
    
 
    If instead, I operate single ended:
    
 
    IN+ 1.8V, IN- GND, Output is 5BXX.
    IN+ 3.6V, IN- Ground, Output is B7XX.
    These outputs are about what I would expect from a single-ended ADC.
    Also, when I vary between these two conditions, I get nice uniform steps as would be expected.
    
 
    Going one step further, Grounding IN+ and applying any positive value to IN-, the output is 0000.
    
 
    Anybody have any clues? Or are my parts not really AD7687's?
    
			
    
			
									
		                             
                        

    
    
                                            
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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Nov 2, 2011 6:49 PM
            
            
						
			            		
      
		
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      Hello Arnie,
      
 
      It seems weird that it works in psedo-differential mode, but not in differential mode.
      
 
      It's hard to figure out the solution to your problem without actually looking at your scope screenshots and schematic.
      
 
      The parts are indeed AD7687 as per thier markings. I don't think they are missmarked parts.
      
 
      
 
      You need to ensure that the AD7687 inputs IN+ and IN- are completely anti-phase (180 degree out of phase) of each other between 0 V and VREF.
      
 
      The AD7687 voltage reference input, REF, has a dynamic input impedance and should therefore be driven by a low impedance source with sufficient decoupling 10uF (X5R, 0805 size) ceramic chip capacitor between the REF and GND pins for the optimum performance. 
      
 
      
 
      
 
      An optional reference buffer such as AD8031 is recommended as shown in figure 34. 
      
 
      Have you followed the layout guidelines from the datasheet?
      
 
      
 
      The input impedance of the AD7687 is not very high (like few MΩ for traditional opamps). We recommend that its inputs be driven using a low impedance source driver amplifier as shown in table 10.  The RC filter is also recommended as shown in figure 27. The key is the analog front (OpAmps, etc.) must settle within the acquisition time (1.8uS min) of the ADC.
      
 
      
 
      BTW, What's your end application and estimated annual volume?
      
 
      Regards,
      Maithil
      
 
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
						
				
				
		
		
      
			
		
      
	
      

      

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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Nov 2, 2011 6:49 PM
            
            
						
			            		
      
		
										0
					
	
      

		
		
      
		
      
                                
                                                            
      
					
      Hello Arnie,
      
 
      It seems weird that it works in psedo-differential mode, but not in differential mode.
      
 
      It's hard to figure out the solution to your problem without actually looking at your scope screenshots and schematic.
      
 
      The parts are indeed AD7687 as per thier markings. I don't think they are missmarked parts.
      
 
      
 
      You need to ensure that the AD7687 inputs IN+ and IN- are completely anti-phase (180 degree out of phase) of each other between 0 V and VREF.
      
 
      The AD7687 voltage reference input, REF, has a dynamic input impedance and should therefore be driven by a low impedance source with sufficient decoupling 10uF (X5R, 0805 size) ceramic chip capacitor between the REF and GND pins for the optimum performance. 
      
 
      
 
      
 
      An optional reference buffer such as AD8031 is recommended as shown in figure 34. 
      
 
      Have you followed the layout guidelines from the datasheet?
      
 
      
 
      The input impedance of the AD7687 is not very high (like few MΩ for traditional opamps). We recommend that its inputs be driven using a low impedance source driver amplifier as shown in table 10.  The RC filter is also recommended as shown in figure 27. The key is the analog front (OpAmps, etc.) must settle within the acquisition time (1.8uS min) of the ADC.
      
 
      
 
      BTW, What's your end application and estimated annual volume?
      
 
      Regards,
      Maithil
      
 
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
						
				
				
		
		
      
			
		
      
	
      

      

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