AD8367: impedance matching and working in cascaded mode

Document created by analog-archivist Employee on Feb 23, 2016Last modified by analog-archivist Employee on Aug 10, 2016
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The device AD8367 is 200 ohms and is requires matching for 50 ohms, in the
datasheet, 190MHz input and output reactance values are provided. 1. I would
like to know the reactance values for 182MHz. 2. Another point is I want to
connect the two AD8367 in cascade and with a common node of resistor divider to
get a combined gain variation please suggest me the resistor values.


For impedance matching, we have simulated the results for you. You could take
them for reference and adjust the values according to your test. Please see
figure 1 and figure 2 in the attached document.
For cascade of AD8367, there are a few things one must keep in mind while
cascading the VGAs to extend the dynamic range:
1) The input linearity of the cascade will be very low at high gain settings.
2) The sensitivity will be poor at low gain settings.
3) High max gain and high bandwidth devices like the AD8367 may be potentially
unstable, and it may be necessary to apply some resistive padding between the
stages to ensure a stable solution.
Starting with the third point, it will likely be necessary to add ~10dB of
padding between the two VGA's. The AD8367 needs to be sourced and loaded in a
200-Ohm impedance environment to ensure stability. However, the output of the
AD8367 presents a 50-Ohm impedance. If you directly cascade the devices the 2nd
stage VGA will be presented by the 50-Ohm impedance of the first stages output,
and may cause the 2nd stage to oscillate. As a result a 200-Ohm to 200-Ohm
resistive pad will likely be needed between the two stages. A 10dB pad should
be enough to isolate the 50-Ohm source impedance of the first stage VGA from
the input of the second stage. If the cascade is properly stabilized, you
should be able to achieve ~90dB of gain adjustment range.