I am working on 16 bit 24 channel DAS. So, I am using MUX (ADG407) to select the proper channel.
I derived Cds and Switch OFF resistance in the following way.
reference fig: 23 in the datasheet of ADG407
@10Khz (F) OFFisolation = 100 dB
it means that, RL / ( RL + Xcds ) = 10^(-100/20)
Here, RL = 1Kohm
Xcds = RL/ 10^-05 = 100 *10^6
1/(2*pi*F*Cds) = 100*10^6
Cds = 159.15 fF.
now, @ fz = 1Khz, a zero is occuring.
fz = 1/( 2 * pi * Rcoff * Cds) ; Where Rcoff - OFF resistance of MUX switch
Rcoff = 1/( 2 * pi * fz * Cds)
= 1 Gohm
I have these following questions;
1. Are the values calculated correct
2. If the calculations are correct - I have gone through several other multiplexers datasheet, e:g ADG5209, only this part ADG407 gives higher OFF resistance. For, ADG5209 it is 5 Mohm only.