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lock-in amplifier

Discussion in 'Electronic Design' started by scot, Mar 6, 2013.

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  1. scot

    scot Guest

    Anyone have experience with lock-in amplifiers? I am thinking of purchasing a used one but don't know enough about them. I know they are extremely effective at finding small signals buried in lots of noise but what are the downsides to a lock-in besides expense?

    Also, this is probably over my head but I have read about the AD630 chip from Analog Devices and supposedly that can be used to make a very low cost lock-in amplifier. Anyone ever try that? Was it worth it for the money? Ishould mention a friend of mine has a couple of these AD630 IC's so I would be willing to solder something up pretty quick if it's not a waste of time.


  2. As Phil said, there are better IC's these days,

    But for learning how a lockin works the AD630 will be fine.
    There's at least two kinds of lockin's (all analog techniques can be
    done in digital.) Multiplying and switched gain. (gain = +1/-1) The
    AD630 is a switched gain lockin.

    George H.
  3. scot

    scot Guest

    Thanks Phil for the suggestion on the ADG1236. I read up on Lock-in's lastnight and learned a lot. I found this interesting article and it looks like the principles are pretty understandable.

    The article specifies a DG303A chip but I think the ADG1236 can be substituted instead. My main goal here is to learn about Lock-in's by building onemyself since reading only takes you so far. What do you think? Is this agood starting point?

  4. Bill Sloman

    Bill Sloman Guest

    The last sentence should be wrong, if you are using lock-in right. The
    basic idea is that you start off by modulating whatever it is you are
    measuring at some convenient frequency, and the detect only the
    modulation frequency in the output that you are looking at (usually
    after amplification and a certain amount of filtering).

    For added extra credit you can detect just some of the higher
    harmonics of the modulation frequency, but it takes sophisitcated
    experimental design to make this a useful approach.

    One of the great virtues of this approach is that you can stay well
    away from the frequencies where 1/f noise (and drift can be seen as a
    form of 1/f noise) is a problem.
  5. I think Phil was talking about 1/f noise elsewhere in the signal
    (I assume he knows how to pick a modulation frequency.)
    But in answering for someone else I've likely stuck my foot in my
    mouth. (again :^)

    George H.
  6. Re: front ends
    With a nice front end I find a DSO with an average function, and sync
    trigger is 'good enough' lots of times. (a poor man's lockin.)

    George H.
  7. Well it's not phase sensitive, but it does select frequency, so 128
    averages is like a band pass with a Q of 128(?)

    Ya know I've never seen a good article on the many ways to use the
    average button on a DSO.

    George H.
  8. Could you expand on that, or suggest a favorite part?

    I found the ADG1236 too for low charge injection. Also DG636.
  9. Same here. Unfortunately the DG1236 does not work at
    cryogening temperatures, unlike e.g. the TS5 series
    from Texas Instruments. I have been looking for a
    low-injecting switch to operate in LHe, and Phil's
    suggestion of using dual-gste MOSFETs is good. At least
    such devices as the 2N7000 (from some manufacturers) and
    the FDV301P do work, so I might try throwing in eg. the

  10. Well let us know what you think of the Owon. We bought one (probably
    a different model) years ago, but it now lives in Europe for trade
    shows over there. I didn't like the screen, it had a very restricted
    viewing angle. (It seemed a bit noisier than a low end tek too... but
    no real data.)

    George H.
  11. OK, thanks.

    Looks like something I will have to breadboard some time.

    I seem to recall someone (John L?) talking about using opto-fets as zero
    charge injection switches. But I wonder are they really? They have fets
    inside, with gates that charge and discharge presumably. Hmm, not sure I
    understand how they work, now I come to think of it. Photovoltaic mode
    photodiode(s) connected to the gate I guess.
  12. I'd be interested to know what you think of it. The display is a lot
    bigger and higher resolution than my Tek, and some of the models have
    an interesting 16-channel LA function at an interesting price point
    ( < $1K ).
  13. Not even sure it was him.

    But do *you* know how they work? Is there charge injection?
  14. Yes, I imagine an isolated little battery connected between source and
    gate, magically charging itself up. But does that inject charge out the
    drain, say? Must do I think.

    The datasheets I have seen don't show anything much. I use a lot of
    LCA710 (for power switching, so probably not the best choice for this).

    Actually I think it might have been a Jim Williams app note, not
    JL. Sorry to be so vague; really can't remember.
  15. OK, nice, thank you.

    I just went on digikey and I see that is the same one I already clicked
    on, when I idly looked one up after you mentioned them!

    Ordered some for a rainy day.

  16. Hi John & Jim,

    Just think a little before flaming someone else, please.

    opto-fets can never have a charge injection, because the charge they use
    to load the gate comes from the chanel that the charge flows in. so the
    sum will allways be zero. I would say, this is as obvious as Kirchhoffs
    law can be.


  17. Flaming? Really? First time I've been accused of that!

    It was an honest question. Just thinking aloud.
    Well, I don't think it is that obvious.

    Even Jim "charge is always conserved" Thompson doesn't seem to think so

    (And please, folks, we dont really need to go there again!)
  18. Hi John,
    Well? You know how reliable Larkin's advice is >:-}
    What a question?
    Where do you expect the injected charge comming from?
    May be Jim woul tell his statement, too.

    I don't see any source for charge-injection.

  19. Well you snipped it didn't you?
    The voltage at the gate changes, relative to the other terminals. There
    is a capacitance between the gate and the other terminals. dQ=C.dV.
    The photodiode? (or whatever equivalent generates the change in gate
  20. Ahh, (a light dawns) you've mentioned the double shot noise in
    photoconductors, but I never got it.

    I've got this 'weird' dark noise from the reverse biased LED's I've
    been playing with. I'm pretty sure it's after pulsing of the
    avalanche. Which they attribute to some sort of traps in the avalnche
    There's an observable 'bunchiness' in the counts.

    George H.

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