(AD8418A)Vout ≠ Vref

Question

Hi ! 
 I have a question about AD8418A. 
 This is my test circuit. 
 
 I measured the Output voltage but it is not Vref level. 
 
 => I don't have AD8418A so I measured AD8418. 
 Why this happens? 
 Our customer said that this won't happen when they are using AD8206 
 but I measured AD8206 at my Lab and same thing happened. 
 But the output voltage offset is smaller than AD8418. 
 I think that maybe the +IN pin leakage current is big 
 Can you explain that why this happens? 
 Best regards 
 Kawa

krisf · Accepted Answer

Hi Kawa, 
 Yes, I agree. The shunt resistor is too big. If, as you say, the resistor is 47kohms, with a 1.5uA bias current at the inputs (which you would very likely get), your numbers now look valid. 
 I just wanted to provide some additional information on your questions. 
 The current you suspect is not leakage, but is inherent to the part. Figure 8 gives you an idea of the bias current over common-mode voltage. 
 
 The inputs of the AD8418 are tied to an internal floating power supply, which powers the device at high common mode voltages. This is the reason the bias current increases largely as the common-mode voltage increases. At low common-mode voltages, on the other hand, there is still a leakage current from the floating power supply, and this is what you see as bias current. 
 In your configuration, you would be operating roughly at 0V common-mode. Fortunately, we do have data available to give you an idea of the spread of the bias current at that common-mode voltage. See below:

Based on this data, you can see that the Vin+ pin can have a little over 4uA worst case, while at the Vin- pin, you would roughly have 1uA. That's 3uA going through a 47kohm resistor worst case, and you get 2.8V at the outputs, so this definitely will not work. 
 Let me know if you have additional questions. 
 Thanks, 
 Kris