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Using an ADL5513 as a replacement for the AD8310 in an existing 10MHz-500MHz power detector product that has undergone a revision.

The ADL was chosen due to its improved temperature stability.

Unfortunately the input impedance behaves very differently with capacitive loading. Would be grateful for an explanation.

Attached is partial schematic.

When the capacitor C10 is present, the input of the ADL5513 is no longer modelled by ~1300Ohm || 0.5pF!

The circuit works exactly as expected with the AD8310 (modelled as 900Ohm || 1.1pF).

Does the ADL5513 use some form of VHF feedback that doesn't work with a large capacitive load across the differential input?

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