I need a switchable 0 /40dB attenuator for a 10Hz-20MHz 1Vp-p signal into a buffer amplifier (10k|3pF input loading). First I thought of using a SPDT analog switch and resistor divider, but even 'low capacitance' devices come with 15~20pF capacitance on the common pin and have poor off isolation for my purpose.
in --.--SW1--.--> to buffer | | 1kR | |--SW2--' 10R | gnd
When I simulated a 2N5484 J-FET, which have been around for at lease 30 yea rs, they beat the pants of any modern CMOS analog switch for off isolation and shunt capacitance. They only downside is more complex driving circuit because of negative gate drive and should have good AC grounding for best o ff isolation.
Is the small signal JFET best suited for this application ? There seems ver y little information on comparing the JEFTs analog switch to the CMOS switc h, especially AC performance.
Adam Seychell