ADG1407 Drain and Source Capacitance

Hi

For  a new project I have selected the ADG1407 dual 8:1 mux. Three of these are used as part of a programmable 6-pole Butterworth filter (8 cut off frequencies) and there are multiple channels per instrument, with each channel having it's own filter.The filter is a cascade design, 3 stages of 2-pole Sallen-Key.

During prototyping I have noticed that the cut-of frequency fc of all filters is low. For example, for the 1kHz filter it is ~910Hz. This is true for all channels and the fc variation across channels is low.

I believe that the on capacitance of the switches are affecting the filter. The ADG1407 datasheet gives typical values of 70pF for CdON and CsON. This however is at f=1MHz while my frequency range is 100Hz to 20kHz.

I have some questions:

1) Do CdON and CsON vary with frequency (an approximate answer would be ok)?

2) How do CdON and CsON vary with temperature  (again an approximate variation or even a guesstimate would be ok)?

3) Can you provide me with a reasonable model for the ADG1407 mux when one switch is closed? I looked at MT-088 Figure 8. It shows CDS(ON) across the switch. But the datasheet doesn't give a value for this. What I have is 8 inputs connected to S1a to S1h and one output DA. So say S1a is switched to DA. Should I include CD(OFF) for the 7 switches which are not closed as they are internally connected to the drain? If I could get a reasonable model I could probably determine the resistor values needed for each stage.

4) I looked but could not find a SPICE model, I guess it is not available?

Thanks for any help you can provide.

regards

Steve

  • 0
    •  Analog Employees 
    on Sep 16, 2020 11:00 AM 2 months ago

    Hi Steve,

    Thanks for getting in touch.

    Hopefully I can help out.

    Firstly the capacitance should not vary much with frequency or temperature.

    Unfortunately we don't currently have a model for the ADG1407.

    The capacitances we list in our datasheets are all with respect to GND.

    I think the Capacitance you are referring to in MT-088 is the capacitance between the source and drain pins with the switch off. We do not spec this capacitance in the datasheet, instead we provide an off isolation spec and plot. This off isolation spec is effectively set by that capacitance.

    The CD, CS (on) that is listed in the datasheet is the capacitance seen at the source or drain node to GND with a switch channel on. This spec will include all the internally connected off channels.

    I'm not sure what way you have the switches configured so I'm wondering if the ADG1207 might be a solution for you. this is the same configuration as the ADG1407 but has lower capacitance. The trade off is that Ron is higher. so this will be depend on where the switches sit in your design.

    Regards,

    David

  • Hi David

    Thanks for getting back to me so quickly & with a detailed response. I did consider the ADG1207 but was put off by the higher RdsON. At this stage I have some more testing to do as the problem may also be due to the 5% NP0 capacitors (there are 6 of them in the circuit!). I also have some ideas about to solve this. It would just have been nice if I could have accurately modelled the actual circuit :-) . Anyway, again thanks for your response.

    regards

    Steve 

  • +1
    •  Analog Employees 
    on Sep 16, 2020 3:26 PM 2 months ago in reply to LPool

    Hi Steve, 

    No problem.

    Just so you know, we are currently working getting LTSpice models for all our devices.If you're a user of LTSpice you will see new switch additions almost every week. The ADG1407 is on the list but I cannot accurately give you a release date for it at the moment.

    If you come across any other questions during your testing just let me know.

    David