Adum2250 VOL too high

Question

Dear ADI 
 We are using Adum2250 for I2C isolation, the VDD1 and VDD2 are all 3.3V. 
 
 We input SDA from SDA2 , use internal pull up resistor, output to SDA, the VIL to SDA2 is SDA2 < 0.4V, , the 
 VOH from SDA1 is 0.8V < SDA1 < 0.7V . 
 
 Why VOH higher than VIL so much ? How can we reduce VOH ? 
 
 Regards 
 TigerCong

ericb · Accepted Answer

Hello TigerCong, 
 Because the ADuM2250 provides bi-directional data flow which means that the forward and reverse data paths could form a latch, there needs to be a mechanism to prevent the latch up which occurs when one side is driven low. The ADuM2250 (and nearly all other I2C isolators and many other I2C parts) solves this by making one side's driven low be higher than its input threshold. This is why side 1's SDA/SCL's output low is approximately 0.8V while side 2 is < 0.4V. This is discussed in Application Note AN-913 . In particular, see Figure 7. 
 In many applications, it often occurs that the devices which are incompatible with the higher VOL of side 1 are only on one side of the isolator and so by flipping the orientation of the isolator to put side 2 to the bus with the lower VOL requirement and side 1 with the higher VOL tolerant bus the issue is resolved. Since the ADuM2250 is bidirectional, this works just fine. 
 If both buses have devices which require a lower VOL, then the LTC4308 can be used to reduce the side 1 VOL of the ADuM2250. Connect the "xOUT" side of the LTC4308 to the ADuM2250's side 1 and the "xIN" of the LTC4308 to the bus. 
 Eric)