Analog switch

Discussion in 'General Electronics Chat' started by nado, Dec 20, 2012.

  1. nado

    nado Thread Starter Member

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    Someone please explain how these analog switch works. U301, U302, U303
    Or point to an article that describe the same switch. It doesn't have to be quad. Single is better for understanding, I guess.

    Thanks again for your help.
     
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  2. crutschow

    crutschow AAC Fanatic!

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    I don't see a part number so I can't give a specific answer but, in general, an analog switch has a low resistance between the two output pins when ON, allowing a signal to pass through, and a very high resistance when OFF, preventing the signal from passing. Within their limits they act similar to a relay.
     
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  3. Ron H

    Ron H AAC Fanatic!

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    The JFETs used as analog switches are Q303, Q304, Q310, Q311, and Q315. Note that the 1Meg pullup resistors on the LM339 comparators all are returned to TP302. LM339 is a quad comparator with open collector outputs. When the comparator output transistors are off, those JFETs will be on. When a comparator output goes low (-5V, see U305), the driven JFET will be off.
    The JFETs with source shorted to drain are used as low-leakage voltage limiting diodes.

    Depending on part number, a JFET ON resistance is typically on the order of a few ohms to a few hundred ohms. The OFF resistance will be many Megohms.
     
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  4. nado

    nado Thread Starter Member

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    @Ron H,
    thanks again for detailed answer.
    Could you tell me, as an example, one of U301-303 IC switches, how it works in the circuit? from input signal to what and how they control?
    I am trying to get a clue what and how this circuit does?
     
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  5. Ron H

    Ron H AAC Fanatic!

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    You are being fast and loose with your reference designators.:D As I said, U301 and U302 are NOT being used as analog switches (and they are not ICs, they are individual JFET transistors). They are connected as diodes, to limit the voltage between them to ≈±24.5V.
    It is difficult to know what the circuit does without a full schematic, and knowledge of what product the schematic is for.
     
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  6. nado

    nado Thread Starter Member

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    I am talking about 3 ICs on the left bottom of schematics, page 9-3. U301, U302, U303.
    I think each IC has 4 switches which is being used individually as a switch on the different part of the circuit and it is being diagrammed as a circle with x in it. I think U is for IC and Q is for transistor? No? Sorry, confused.

    I understand that JFET are used as diodes from the other thread. I am talking about different things this times, I think.
     
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  7. tubeguy

    tubeguy Well-Known Member

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    I believe it's the Fluke 8840 from his other thread
     
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  8. nado

    nado Thread Starter Member

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  9. Ron H

    Ron H AAC Fanatic!

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    Sorry.:( You are correct. I was zeroed in on the JFETs.:rolleyes:
     
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  10. nado

    nado Thread Starter Member

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    @Ron H,
    That's ok. Actually I am glad you are in this thread because I like your explanations a lot.
     
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  11. nado

    nado Thread Starter Member

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    Anyone? Just point to me where I can get more information about these ICs like the ones U301, U302, U303 on the bottom of left side in the schematic I attached.
    At least if I get the datasheet, I can try to study it. There are more than 10,000 analog switch ICs at Digikey. I checked the hundreds of the datasheet. None of them look alike.

    I fixed the machine almost but I want learn how it works.

    Thank you in advance.
     
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  12. Ron H

    Ron H AAC Fanatic!

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    If you're looking for a datasheet, you may not find a current one. I suspect the parts are obsolete.
    I think they are similar to DG411, but with internal AND gates controlling the inputs.
     
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  13. nado

    nado Thread Starter Member

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    Thanks. So the circle with X is the switch symbol? I guess it's switch but I coudn't fine any references. I just wanted to make sure. I searched the Internet but all I got is this. http://www.kpsec.freeuk.com/symbol.htm It says it is lamp. Weird. They call the switch as a lamp.
     
    Last edited: Dec 21, 2012
    #13
  14. Ron H

    Ron H AAC Fanatic!

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    Yes, I believe it represents a SPST switch, probably normally open.
     
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  15. nado

    nado Thread Starter Member

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    @Ron H;
    Thank you very much.
     
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