ADL5391 fundamental feedthrough

Dear ADI Engineers,

I have a question regarding the ADL5391.

In the data sheet the fundamental feedthrough is specified as -35 dB. Is this measured in full differential mode? I did several test measurements

  1. In full single-ended operation using matching circuits, I got a feedthrough of -10 dB
  2. Operating one input differentially (right now, I only have one balun available) and the other one in single-mode, I got a feedthrough of -15 dB
  3. Operating one input differentially and the other input is directly connected to GND, I got a feedthrough of -25 dB.

This means that the feedthrough gets lower, if the ADL is operated in differential mode and if one input is connected to GND (which is not possible as I need both inputs). The output is always operated in single-ended mode. In the data sheet, I saw the feedthrough was measured with the other input port at level 0V. Does this, that the other input is directly connected to GND or left open?

From this and from my measurements, I assume that the feedthrough is higher, if a voltage is present at the other input port. As far as I understand, in single-ended mode, there is a voltage present, which is the bias voltage. In differential mode, there should be no voltage present, as the bias voltage ist he same for both ports. As the difference is taken, the effective bias voltage is 0V. Is this assumption correct?

I prefer to operate the ADL in single-ended mode. Is there any way the feedthrough can be minimized in single-ended mode?

I already tried to connect a shunt inductance to the input ports. For DC the inductance behaves like GND, meaning the DC voltage at ports is 0V. However, this didn’t result in an improvement. In some configurations, I even got a feedthrough of +5 dB. As for DC, the inputs are connected to GND, the current flowing through the ADL is up to 200 mA. Is this too high or is this value still ok? For reference, with shunt 50 Ohm terminations the current is 185 mA and without any components present at the inputs the current is 145 mA.

Thanks for helping me out and best regards,

Alexander