i wish to design the impedance analyzer which can analyze impedance starting from 1Hz to 20Mhz, plz recommend a ADC which can digitize the signal up to 20Mhz and a variable gain amplifier.
Can you tell me what range of signal amplitudes that the VGA input will see? What do you need for dynamic range. Do you need more than one channel? If your bandwidth is limited to 20MHz, I don't think you need an RF VGA like 5330 or IF VGA like AD8272. I think that our digitally controlled VGAs that are aimed towards cable line driver applications might work best for you. They all have 3dB bandwidth in the range of 100-150MHz. Take a look at the AD8321, AD8324, AD8325, AD8327, let me know if you think these are OK.
As far as the ADC goes, I think you can get by with something that samples at ~100MSPS, depending on your dynamic range maybe you can get by with a 12 or even 10 bit converter. Let me know what you think,