I first want to summarize my need. I have a prototype portable instrument and need to produce more units. The unit has an 18 MHz source and applies a stimulus voltage to an unknown and a signal returns. I only need to measure the amplitude of this pure sine wave which I can range to be from zero to a ~ 1 volt rms or 3 v p-p.
Currently using AD8361 for AC to DC conversion. The problem is that an operator using my unit sees 0.013 v on the display with zero signal due to AD8361 error. Also nothing happens on the 8361 output until about 44 mv p-p is applied to its input and the output is useless until 75 p-p is applied to its input. So I added a dc offset to make the operator happy but this adds an offset to all other values.
Was hoping that ADL5501 or other "linear in voltage" device might do the job. I believe you were checking on this to see if it would work at 18 MHz and if this device does not have same problems as 8361 below 75 mv p-p input.
An alternate solution might be a high speed opamp with diode from out via resistor back to - input and a second diode reversed with resistor back to - input. Then one takes the difference between the two diodes where they tie to the resistors. This is the classical Burr Brown AC to DC or absolute circuit. I am having difficulty getting this to work at 18 MHZ. Any help appreciated