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

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.

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  • Hi Jebas,

    The ADG407 is an older part and it looks like the off isolation plot was made with some measurements that were then extrapolated for the plot. The ADG1207 off isolation was measured on a network analyzer and so full measurement data is available and the low frequency values in the plot show the equipments noise floor as opposed to the performance of the part.

    At the frequencies you are operating I would think that there would not be much difference in real isolation between the ADG407 and the ADG1207. The only advantage for the ADG407 is that it is a physically larger part and so there is more separation between channels but at 200Hz this should not create a significant difference between the ADG1207 and the ADG407.

    If you are ok to accept the higher charge injection of the ADG407 and the timing specifications are acceptable, this part should be suitable for the applications you have suggested.

    Regards,

    Joe

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