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AD8436 DC Coupled Single Supply - strange voltage on IGND

Question asked by m.mayhew on Jul 28, 2017
Latest reply on Jul 31, 2017 by jstaley



I am looking at using the AD8436 for an application that measures RMS of both DC and DC+AC sources centered at 2.5V. I did some simulations with the AD8436 in DC coupled, single supply setup, and I am getting an unexpected result. I am not sure if it's the simulation or if this is the expected result of the part itself. I will need to better understand before continuing with the part in my design.


First, I will show an expected result: 2.5V DC input produces 0V output. That makes sense. 2.5V DC corresponds to 0V RMS: 


Now, if I apply a 3.5V DC input, I would expect a 1V output, since 3.5V DC corresponds to 1V RMS. However, I do not get this, and I have a strange value on the IGND pin. I would expect the IGND pin to be midscale 2.5V, however it is sitting at 3.362V:


Now, if I tie the IGND pin directly to a midscale 2.5V supply, I do in fact get the expected 1V output. 


I have two questions:

1. Is this the expected result? Maybe it's just a problem with the SPICE model.

2. Is it ok to drive the IGND pin to half VCC (2.5V) in the application?