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
