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The ADuM1250 / ADuM1251 are hot swappable digital isolators with non-latching, bidirectional communication channels compatible with I2C interfaces. This...
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ADuM1250 Side 2 Logic Low is 800mV issue

		
	
				

			

									
					    NeoVoyager
					
							

            
            			
				
									
											
			
			
				
			
			
			    	
			                    
                
				                on Jun 25, 2026 
				
			
			
		

		
		                     
				
        		

		    
		    		    

		      		            		        
Category: Hardware

    		        		            		    
Product Number:  ADUM1250  

    		        		    		    

		    		    
		    			

			    
I am working on a prototyping circuit which can be found in file “EquipLvlTest_Circuit_for_ADI_20260625.pdf”. The external MCU “Raspberry Pi Pico 2” is connected to “ADuM1250” side 1. The “ADuM1250” side 2 is connected to a multiboards environment with at least 3 EEPROMs and 1 master i2c controller. During the test, the waveform is captured as seen in the attachment. (Yellow Color Channel 1: ADuM1250 Side 1, Blue Color Channel 2: ADuM1250 Side 2). At Channel 2, the SDA logic LOW voltage is around 800mV. If include the noise, I have doubt that this ADuM1250 Side 2 I2C signal will give a clean signal to EEPROM Microchip 24LC64 and hence the EEPROM “ACK” is not response properly with a looked like floating signal in Channel 2 waveform.


 


From above info, can you advise is it I did not use ADuM1250 properly and caused the above issue?




EquipLvlTest_Circuit_for_ADI_20260625.pdf




 







			

			
									
		                             
                        

    

                                            
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    •  Analog Employees  
    
            
						
                            
            
            				
            
                on Jun 26, 2026 9:10 AM
            
            
						
			            		
    
		
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    Hello,
    

       Looking at the two waveforms, the low level voltage for the blue trace appears to be 800mV and the yellow closer to 0.  Assuming that these traces are the two sides of the ADuM1250 isolator, then the blue trace would be side 1 and yellow side 2 which is the reverse of what you describe above.
    

       In the schematic, the ADuM1250's side 1 is connected to the Raspberry Pi Pico 2.  It appears that this board uses the RP2350 microprocessor and digging through the RP2350's datasheet in Section 14.9.4, the input low threshold depends on the IOVDD supply and ranges from 630mV when IOVDD=1.8 to 800mV when IOVDD=3.3V.  I'd assume that your pico 2/RP2350 is running on 3.3V (but don't know) and so for the Pico 2 to recognize the low levels, it needs to be driven below 800mV and your measurements suggest that the low voltage is 800mV, so it would only marginally operate.  Since the output low voltage for the ADuM1250's side 1 can be as large as 900mV over all temperature, part and loading variations, even if a particular Pico 2 is able to work a particular ADuM1250 ok, there will likely be combinations (or conditions) where it doesn't work and so this isn't a good design/combination.
    

      There are 2 options for fixing this.
    

    1. swap the ADuM1250 sides so that the Pico 2 sees side 2 which has a maximum output low of 400mV and so will be compatible with the Pico 2 and the multiboards bus side sees the 900mV output low of Side 1.  This will work only if all of the devices which need to see data from the Pico 2 will accept a 900mV low voltage.   Its unclear to me if your Pico 2 is operating as an I2C controller or just as a target device.  If there is only one controller in your system, and its on the multiboards side then only that device needs to tolerate a 900mV low.   If, however, the Pico 2 will be the bus controller and will talk with other devices like the EEPROM, then they need to be checked for the 900mV low voltage compatibility.
    

     2. You can replace the ADuM1250 with the ADuM1252 which offers a lower maximum side 1 VOL of 690mV.  For the Pico, this will work for IO supply voltages above 2.5V.  While it appears in your situation, this should work, if the 690mV were to be too high, option 1's swapping sides could also be performed with the ADuM1252.
    

    Eric
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
						
						
				
				
		
		
    
			
		
    
	
    

    

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