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AD7771 Recommended for New Designs
The AD7771 is an 8-channel, simultaneous sampling analog-to-digital converter (ADC). Eight full Σ-Δ ADCs are on-chip. The AD7771 provides an ultralow...
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AD7771 on Analog.com

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Troubleshooting AD7771



		

			

									
					    Chritron
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Mar 26, 2025 
				
			
			
		
    

				
		
    
		    		    
		    
		    		      		            		        
    Category: Hardware
    
    		        		            		    
    Product Number:  AD7771  
    
    		        		    		    	
			
    I am trying to optimize the quality of the output from the AD7771 and have encountered a result (figure below) that I was wondering if may provide a clue to errors and optimization.
    

    The AD7771 is interfaced with an ESP32 via SPI reading 15kHz single ended conversions from four channels simultaneously. Both the AD7771 analog and digital is powered by 3.3V, and the internal (2.5V) reference is used (connected to Ref+ and Ref- to GND) and the chip is configured to run in low power mode (high resolution mode gives more noise all over the voltage range). The plot below shows a dependency between input signal voltage level and noise, using a ~100Hz triangle wave test signal of ~200mV to 1400mV from a function generator. The plot shows 1000 conversions for one channel as an example, and the error registers (GEN_ERR_REG1,2, STATUS_REG_1,2,3) except for the Channel status register (e.g. 0x4C) which gives seems to alternate between 1001101 and 1001000, although the signal is well within the reference voltage range. 
    

    Do you have any hint on what the error could be?
    

     
    
									
			        
		
    

				
						
        

    
    Tags: 
                                            
                 hardware
                
                                                    
                 Simultaneous Sampling A/D Converters
                
                                                    
                 ad7771
                
                                                    
                 Precision A/D Converters
                
                                
            
    
    
    

		
    
			
						
									
									
						
						
			
			
			
			
		
    

        
        

					
    
			
    


    
















    
	
    
	
    
					



			


				
	









	
      
	

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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Mar 26, 2025 10:25 PM
            
            
					
      
	
      

		
		
      
					
      Hi  ,
      

      Can you share the schematic and register settings?
      

      What is the value of the error and status registers? AIN- is grounded and AIN+ is connected to the test signal with amplitude of 200mV to 1400 mV, is that correct?
      

      Thanks,
      Janine
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
						
				
				
		
		
      
			
		
      
	
      

      

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                on Mar 27, 2025 1:37 PM
            
            
												
						in reply to JEstayo
					
									
        
	
        

		
		
        
					
        Hi Janine,
        

        Thank you for the quick response.
        

        Here is a quick drawing of the schematic, hope it is clear enough:
        

         
        

        

        Here is the registers configuration for this example (everything else unchanged from default):
        

        DEC_RATE_MSB 0b00000000
        

        DEC_RATE_LSB 0b11111111
        

        DEC_RATE_IF_MSB 0b11111111
        

        DEC_RATE_IF_LSB 0b11111111
        

        GENUSERCFG1 0b00110100
        

        GENUSERCFG2 0b00001001
        

        DOUT_FORMAT 0b11001110
        

        ADC_MUX_CFG 0b00000000
        

        GENUSERCFG3 0b10010000
        

        

        The error registers checked after the 1000 conversions (GEN_ERR_REG1,2, STATUS_REG_1,2,3) are zero except for the Channel status register (e.g. 0x4C) which gives seems to alternate between 1001101 and 1001000. 
        

        

        It is correct as you say, AIN- is grounded and AIN+ is connected to the test signal with amplitude of 200mV to 1400 mV. 
        

        

        Thank you!
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
				
						
				
				
		
		
        
			
		
        
	
        

        

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        •  Analog Employees  
        
            
						
                            
            
            				
            
                on Mar 27, 2025 10:28 PM
            
            
												
						in reply to Chritron
					
									
        
	
        

		
		
        
					
        Hi  ,
        

        Thank you for sharing this information.
        

        Just a few notes below. Please try these and let me know how it goes.
        

          

        • Connect XTAL1 to GND and CMOS output to XTAL2 pin
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        • Configure FOMAT0 and FORMAT1 pins - connect to IOVDD for SPI control
          • 

        • Connect AVSS2B to GND
          • 

        

        If the above doesn't work
        

          

        • Try to set the ADC reference to internal reference
          • 

        

        Thanks,
        Janine
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
				
						
				
				
		
		
        
			
		
        
	
        

        

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                on Mar 28, 2025 11:54 AM
            
            
												
						in reply to JEstayo
					
									
          
	
          

		
		
          
					
          Hi Janine,
          

          Thank you for the suggestion.
          

          I am sorry, the schematic was drawn in a hurry. XTAL1 is actually connected to GND and XTAL2 to the CMOS output (was an error in the drawing but not in the actual PCB), and AVSS2B is connected to GND. 
          

          I tried connecting FORMAT0 and 1 to IOVDD, but it had no effect on the result.
          

          Setting ADC reference to internal (0b01000000 to ADC_MUX_CFG) provides the same result, however with a slightly skewed(?) waveform. 
          

           
          

          The error registers are clear most of the time in this case, except for the occasional GEN_ERR_REG1 = 11111111, CH0CH1_DSP_err = 11111111 and ERR_STAT_REG2:11111111). 
          

          

          Thank you again.
          
			
          

		
			
	      

		
		
	
          
	
          
		
          
			
		
          
	
          
	
          
		
						
						
				
						
				
				
		
		
          
			
		
          
	
          

          

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                on Mar 31, 2025 11:28 AM
            
            
												
						in reply to JEstayo
					
									
            
	
            

		
		
            
					
            Hi Janine,
            

            Thank you for the suggestion.
            

            I am sorry, the schematic was drawn in a hurry. XTAL1 is actually connected to GND and XTAL2 to the CMOS output (was an error in the drawing but not in the actual PCB), and AVSS2B is connected to GND. 
            

            I tried connecting FORMAT0 and 1 to IOVDD, but it had no effect on the result.
            

            Setting ADC reference to internal (0b01000000 to ADC_MUX_CFG) provides the same result, however with a slightly skewed(?) waveform. 
            

             
            

            The error registers are clear most of the time in this case, except for the occasional GEN_ERR_REG1 = 11111111, CH0CH1_DSP_err = 11111111 and ERR_STAT_REG2:11111111). 
            

            I also tried to sample a sine was produced by the microcontroller DAC to rule out any problems related to the function generator as a signal source, reproducing the noise dependency on voltage level:
            

             
            

            Another clue perhaps is that I measure the DC current consumption of the supply to the AD7771 to be about 49 mA in low power mode and about 83 mA in high power mode which seems high compared to the datasheet. In high power mode, the noise seems to be independent on voltage level, but there is a high noise level overall.
            

             
            

            Do you have any further suggestions for the troubleshooting?
            

            Thanks
            
			
            

		
			
	      

		
		
	
            
	
            
		
            
			
		
            
	
            
	
            
		
						
						
				
						
				
				
		
		
            
			
		
            
	
            

            

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