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Analog switch with different ref signal

thierry venet
Intellectual 465 points
Other Parts Discussed in Thread: TS5A3166

Hello,

I don't know where to ask my question, I assume it is here, sorry if it is not the right place...

I have a question about Analog Switches, some people say it can be use in place of relays, but It seems that the analog output must be between Vss and Vdd. 

So, what happens if I want to switch a signal which is not referenced to the switch GND ? 

For example, if I have a 4-20mA signal from the external world, I want to switch it but I don't have access to his ref GND, can I use a analog switch in this case ?

I assume not, because it may have large common mode voltage on the signal (for exemple if I have 20mA signal on 1ohm switch, I have only 20mV ddp on the switch, but maybe I have +5V difference between the two ref)

I don't know if my reasoning is right.

Thanks for any suggestion

  • 0
    TI__Genius 9860 points

    If your signal does not violate the absolute maximum ratings of the part, then the switch will operate. Therefore, you should find out the worst-case difference in reference between your signal voltage and the switch GND and make sure it does not violate the spec.

    Also, make sure the switch can handle 20 mA. Many of our switches are rated for higher currents than 20 mA. Which parts have you been looking at?

     

    -Ryan

  • 0 in reply to C. Ryan Land
    Intellectual 465 points

    Hello, thanks for answer

    I'm looking at SPST switch in small package.

    And as I want to switch current regulated signal, I think I might use small ROn to avoid large voltage on the switch.  

    A part like TS5A3166 seem well suited...

    But the problem is : If I put a mechanical switch, or a relay to switch the signal, I'm not concerned by the GND reference of the switched signal.

    It seems it not the case of theses "Analog Switches", so I can not really put a analog switch in place of a mechanical switch or relay.

    So I think If I have not access to the switched signal reference (= I cannot connect the two ref), it is not a good idea to put a analog switch because of the "not known common mode voltage".

    I have made a small diagram to explain the subject :

    Isolation between the two devices is not required but I cannot connect the two ref because I have no access to external device electrical reference.

    Best regards,

    thierry

  • 0 in reply to thierry venet
    TI__Genius 9860 points

    Hi Thierry,

    You are correct - if you cannot know the worst-case difference between the references, there is no way of knowing if the absolute maximum conditions were violated - therefore the switch will be unreliable in this configuration.

    -Ryan

  • 0 in reply to C. Ryan Land
    Intellectual 465 points

    OK thanks for answer !

    thierry