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Common Mode Question (AD8138 part, single supply)

Question asked by ddurachk on Mar 8, 2017
Latest reply on Mar 9, 2017 by harryh

Good afternoon,

 

I am working with the AD8138 part and am having some issue with the common mode reference in my simulations.  Due to constraints on the board, we are only able to use a single supply to operate from, the AD8138 has been a relatively good part in the past for me, although I've never used it without a dual supply. 

I might be missing something obvious here, or perhaps the model that I have doesn't support a single supply applications very well.  Thinking I was setting this up wrong, I copied the schematic from Figure 42 from the datasheet.  The problem I am seeing is that the output voltage levels are ~ 2.1V above the common mode set point.  (i.e., set the common mode to 2.5V, the Vpos and Vneg are sitting at 4.65V.  For an input I am applying a 100kHz sine wave with a DC offset of 0.5V, and a peak-peak amplitude of 0.5V. 

 

I also ran DC sweeps on the same circuit to see where the outputs were in relation to the common mode reference point.  They are always ~ 2V greater than the common mode set point.  This is not a complicated circuit, I don't think I'm doing anything wrong with the setup.  What made me start suspecting the model is if I set the common mode down to around 500mV, the AD8138 current draw sky rockets up to 350MA (not milli, but MEGA).  If I run the same sweep with a -5V rail, it performs as expected.

 

Has anyone else reported any strange issues with the common mode at the outputs when using a single supply?

 

As always, thanks for your time,

Dave

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