# Multiplexer - Deriving Cds & Switch OFF resistance value from OFFisolation Vs frequency curve

Question asked by jebaspaul on Sep 6, 2013
Latest reply on Sep 18, 2013 by JoeCreech

Hi Bros,

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.

Thanks,

Jebas.