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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 Eric,

    Thanks for your response. The discussion with You and Joe is much useful.

    I have another one doubt to be cleared.

    The first figure taken from ADG407 data-sheet.

    Whereas, the second figure taken from ADG1207 data-sheet.

    While comparing these two images, the one coressponds to ADG407 looks more ideal and gives a impression that it might not be from practical measurement.

    Can I go with this data given in the datasheet of ADG407.

    Also, as you said in the previous post, we are little worried about charge injection with ADG407 even our architecture is differential. Thanks for suggesting ADG1207.

    But, ADG407 overlooks ADG1207 interms of OFFisolation, Crosstalk, Rdson etc.,. So, If I am using ADG407 with bottom plate  sampling inorder to discard the effect of charge injection, Whan can be your command on this.

    Thanks,

    Jebas.

Reply
  • Hi Eric,

    Thanks for your response. The discussion with You and Joe is much useful.

    I have another one doubt to be cleared.

    The first figure taken from ADG407 data-sheet.

    Whereas, the second figure taken from ADG1207 data-sheet.

    While comparing these two images, the one coressponds to ADG407 looks more ideal and gives a impression that it might not be from practical measurement.

    Can I go with this data given in the datasheet of ADG407.

    Also, as you said in the previous post, we are little worried about charge injection with ADG407 even our architecture is differential. Thanks for suggesting ADG1207.

    But, ADG407 overlooks ADG1207 interms of OFFisolation, Crosstalk, Rdson etc.,. So, If I am using ADG407 with bottom plate  sampling inorder to discard the effect of charge injection, Whan can be your command on this.

    Thanks,

    Jebas.

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