Can we use the AD8367 in cascaded ?

I am getting the input signal as a -20dB expected signal after the VGA is -70dB , So in order to get this output signal i am using the AD8367 ( VGA) in cascaded mode but i am able to see -40dB at the first VGA output without any noise in the signal, but at the second VGA i am getting able to see the -50dB signal with a full noise. Could some one can suggest me the better way to get the perfect output.

You can cascade the AD8367, but it's a little tricky due to the amount of possible gain of the AD8367 and the impedance matching considerations that make it stable.

Firstly, when you say "dB", so you mean "dBm". Secondly, are you referring to signal levels or the gain of the VGA?

dBm is the logarithmic ratio of the power in mW, to a 1mW reference (10*log10(Power_mW/1mW). So 1mW would be 0 dBm, 10 mW would be +10 dBm, 100mV would be +20 dBm and 1000mW (1W) would be +30 dBm. Going the other way, 0.1 mW is -10 dBm, 0.01mW is -20 dBm, and 0.001 mW (1000uW) is -30 dBm.

dB is purely are log ratio, 10*log10(power_gain) or 20*log10(voltage_gain). Note that the power gain and voltage gain are equivalent if the input and output impedances are the same.

Are you using the ADI evaluation boards with the L-pad resistive matching on the board (s)? Could you provide the schematic of your circuit that you are using?

Customers have had success with cascading the AD8367 in the past doing the following:

1) Using a broadband 4:1 match on the input of the first VGA; a TC4-1W balun from mini-circuits works nicely

2) Using a combination of low pass filtering and resistive padding between the output of the first VGA and the inptu of the second VGA.

3) Either using a 4:1 impedance match on the output of the second VGA or 200 Ohm impedance or higher.

Hope this helps,

Joel