AD8367 working in cascaded mode

Document created by analog-archivist Employee on Feb 23, 2016
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I am using AD8367 AGC  kit which is working properly for 0 to -30 dBm input
level . But my requirement of dynamic range is from 0 to -90dB. Please give me
the solution how I can increase the dynamic range, using two or three AD8367
AGC kit .

 

This is not the first time cascaded AD8367 have been used to extend the dynamic
range.  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 (or all three
VGAs if you need it, right now, I’ll just speak to two cascaded VGAs). 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. In order to answer your question about
sensitivity, we need to know the bandwidth of your signal. If we cascade two
AD8367's with a 10dB pad in between the max gain will be ~(2x42.5dB-10dB)=
75dB. The noise figure of each VGA will be ~7dB at max gain (dependent on
actual center frequency). Due to the high gain of the first stage the overall
cascaded NF will be dominated by the first stage and will also be ~7dB. From
the NF we can calculate a minimum detectable signal level (MDS). The MDS is
equal kT+NF-10Log(BW). Where kT is -174dBm/Hz and BW is the signal bandwidth.
MDS corresponds to the signal level that is detected with 0dB of SNR. If you
need several dB of SNR than your sensitivity level needs to be adjusted to
account for you SNR requirement. As an example, say your signals are 1MHz wide
bandwidth QPSK modulated waveforms which require ~6dB of SNR for adequate
bit-error-rate performance. The cascaded 7dB of NF corresponds to a MDS of
-107dBm for a 1MHz BW. In order to have 6dB of SNR the minimum input signal
level needs to be greater than -101dBm at this gain setting. From this example
perhaps you can calculate what the minimum signal level should be for your
system. If you provide some more information regarding your application then we
may be able to provide some more representative calculations to help you design
your system. At the -20dBm input level, the gain will certainly need to be
reduced to ensure the individual stages are not overdriven. What is the target
output level you are trying to achieve? You will probably have to either LC
matching or baluns/transformer on the input of the first VGA and the output of
the second VGA to get the 50 Ohms to 200 Ohm match.  This will help with the
MDS and the Noise Figure of the system. 

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