ADL5391 bias voltage

Dear ADI Engineers,

I have a question regarding the ADL5391.

In the data sheet I see that all pins are internally biased to Vpos/2, being 2.5 V in my case (Vpos = 5 V). This means that in DC coupled mode input voltages below 2.5 V are treated as negative voltages and above 2.5 V as positive voltages, correct?

I applied some DC voltages to one input to test this. When doing so, I ran into a strange effect. I couldn’t apply a input voltage lower than 1.8 V. Whenever I set my input voltage to a value lower than 1.8 V, the voltage output of my power supply jumped back to 1.8 V. I didn’t use any kind of matching circuit and operated the ADL5391 in single-ended mode. Could you please tell me the cause of this effect and how to fix it?

Given the case, I later want to use a input signal with voltage levels belwo 1.8 V, does this mean all those voltage levels are „clipped“ to 1.8 V and my input waveform is disturbed/destroyed?

Thanks for helping me out and best regards,

Alexander