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Meter with a log scale

Discussion in 'Electronic Design' started by Dave, Jun 13, 2005.

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  1. Dave

    Dave Guest

    Is there a simple way of getting an analogue meter (50uA or so) to give
    a deflection proportional to the log of the input? It need not be
    precise at all.

    I want to have a 'light level' meter, which will handle a large range of
    light levels.

    I was thinking that perhaps a forward biased diode in parallel with the
    meter might do this, as for increasing input levels, more current will
    go through the diode and less through the meter.

    I expect there is a chip that will do this sort of thing, but I just
    want a simple solution. The instrument has a A/D converter and can
    collect the exact data into a file. I just want a simple analogue meter
    to show things are not too bad, without having a computer program running.

    If it can be done with a diode as I suspect, has anyone tried optimising
    the layout of resistors that might be needed to give the most accurate
    log response?
     
  2. BobG

    BobG Guest

    Regular old ohmmeters have a nonlinear scale... I think its a
    reciprocal.... there is a bridge in the meter.
     
  3. Joerg

    Joerg Guest

  4. Dave

    Dave Guest

    Does the reciprocal scale not result from simply the fact that the
    current through a resistor is proportial to 1/R. I don't think you need
    anything clever for that.
     
  5. John  Larkin

    John Larkin Guest

    A silicon solar cell already has a log voltage output.

    John
     
  6. A forward biased (voltaic mode) photo diode produces a voltage
    proportional to the light level, so you can use a linear meter to read
    the self generated voltage. You can add a linear DC voltage amplifier
    if you need more voltage to match the meter.
     
  7. Dave

    Dave Guest

    I can't take that approach.

    I have (and must use, as light levels are very low) a silicon avalache
    photodiode module from Hamamatsu. The output voltage is 7500 V / Watt of
    light on the device. With further amplifiation, the voltage level should
    be 1V or a little less, before feeding into an A/D.

    The source is a 50mW laser at 780 nm, but this is attenuated a lot,
    hence light levels are very low, and I need the sensitivity of the APD.
    (I also need a bandwidth of DC-100MHz, which I'm not sure if solar cell
    would provide anyway).

    This part is just one part of a more complex project. I just wanted an
    indication that the light level was about right, and was not too low, or
    too high.
     
  8. Guest

    This is a well-known solution. My Ph.D. thesis includes a reference to
    J.F.Gibbons and H.S. Horn's paper on that circuit in the IEEE
    Transactions on Circuit Theory CT-11, from page 378 (1964). I didn't
    use the circuit in my Ph.D. work, but I did put together a logging and
    anti-logging circuits on the same basis at EMI around 1977. They worked
    fine, but didn't do anything useful for our ultrasound images.

    At Cambridge Instruments, the electron microscopes vacuum monitors used
    much the same circuit to compress some five orders of magnitude onto
    one analog meter.
     
  9. John Larkin

    John Larkin Guest

    Wow, I couldn't *afford* and APD module from Hamamatsu!

    OK, dump the diode current into a good silicon diode and measure the
    diode voltage drop with a high-impedance opamp, or put the diode in
    the opamp feedback. The b-e junction of a small-signal transistor is
    an excellent diode down to very low currents.


    John
     
  10. Guest

    And if you drive the current into the collector, while maintaining the
    collector at much the same voltage as the emitter, it works even
    better. See the Gibbons and Horn paper I cited earlier in the thread.
     
  11. Joerg

    Joerg Guest

    Hello Bill,
    In medical ultrasound you really need log detection though. There are
    some nice log amps but their prices have gone up quite a bit so often it
    becomes necessary to "roll your own". That also solves potential single
    source issues.

    Regards, Joerg
     
  12. Joerg

    Joerg Guest

    Hello Dave,
    If cost isn't critical and here I mean one or two $20 bills look at the
    Analog Devices log amps. AD606 and others. During layout be careful that
    the input never "sees" the output but that goes for every fast log amp.

    Regards, Joerg
     
  13. Mike Monett

    Mike Monett Guest

    Joerg wrote:

    [...]
    Heh - as long as we are getting into that price range, there's maybe good
    reason to look further. Analog methods drift, aren't too accurate, and get
    slooooow at low signal levels.

    A perfect log converter (cost be dammed) would be a precision A/D, PROM lookup
    table to convert to log, followed by a suitable D/A.

    This would have no trim adjustments, very tiny tempco, a constant conversion
    rate independant of signal level, and no long-term drift. Perfect for the
    photographer who has everything.

    Mike Monett
     
  14. Joerg

    Joerg Guest

    Hello Mike,
    Not necessarily. I have used an AD log amp in a design a while ago. IIRC
    it was the AD630 and two of them in series to achieve the necessary
    dynamic range. Nice wide BW down to low levels and very precise.
    However, isolating input from output in such a high gain setup is no
    small feat.
    That works nicely but when you need a dynamic range north of 80dB and
    several MHz of bandwidth it can really become cost prohibitive. The
    other downside I have experienced here was that top notch AD converters
    do not nearly have the production life of a typical log amp. Innovation
    happens so fast that it may be only a few years and you'll have to
    redesign the AD section which pretty much means the whole board.

    Regards, Joerg
     
  15. Robert Baer

    Robert Baer Guest

    No there isn't.
    The basic circuit is extremely complex: from one lead to a battery to
    a selected resistor, to the meter and to the other lead.
     
  16. Mike Monett

    Mike Monett Guest

    I thought the AD630 was a lock-in amplifier. Maybe it was the AD640?

    Yes, high gain and wide bandwidth are a recipe for late nights:)
    I thought the OP's app was a light sensor for photography.
    That's called a "lifetime buy". The new devices usually have better
    specs and cost less, and you may end up redesigning anyway to keep
    up with the competition or the customer's requirements may change.
    That's called a "sales opportunity".

    So you need to keep the soldering iron plugged in, and have Eagle
    and LTSpice running on the desktop:)
    Mike Monett
     
  17. Joerg

    Joerg Guest

    Hello Mike,
    You are right, I believe it was the AD640. The 630 is a commutating
    mod/demod.
    Yes but the commercial chips are usually high BW so some care is
    necessary. Many people who use them in apps where only a few Hz are
    needed tend to overlook that they are dealing with the temperament of a
    Ferrari when using commercial log amps.
    CFOs and corporate boards hate lifetime buys and the resulting surge in
    inventory. In the med world many designs just don't need any improvement
    over years, often over much more than a decade. Just look at the xray
    machines of many dentists. These can be 20+ years old and work just fine
    for them. Sometimes it's the same with automotive, for example the Ford
    F150.
    Interesting. That's the same combination I use. I decided to switch from
    OrCad to Eagle a half year ago. I still treasure the cloth covered
    manuals of the old PSpice but now prefer LTSpice.

    Regards, Joerg
     
  18. Mike Monett

    Mike Monett Guest

    Hello Joerg,

    [...]
    OT, but I just had two wisdom teeth pulled. The dentist said he wanted to
    take X-Rays and asked me if it was OK. I figured he was concerned about
    possible radiation damage, and said sure.

    You know how they used to give you a film plate to hold in your mouth, then
    bring the X-Ray head next to the tooth? This was completely different.

    He told me to get up out of the chair and took me to another room the size
    of a closet. The nurse put my chin on an adjustable rest and went into
    another room. When she pushed the button, a mechanism on either side of my
    head started rotating. When it stopped, she took me back to the other room.

    A minute later, the dentist came back with a small viewer. It held a long
    negative that showed a 360 degree view of my entire mouth and teeth.

    I have never seen such a clear X-Ray. It made it easy to see the damage was
    unrepairable and the teeth had to go. The next time I have to visit a
    dentist, I will make sure he uses a machine like this.

    This is an example of how fresh designs can vastly outperform older
    technology.

    [...]
     
  19. Joerg

    Joerg Guest

    Hello Mike,
    Wow. That is high tech. Mine still uses the "hold this plate, please"
    technology but he said it's fine for him. With root canals he has some
    modern gizmo to see that the nerve is really dead.
    It sure does. The only question is, what did he charge you for this
    x-ray? Mine charges $20.

    Regards, Joerg
     
  20. Mike Monett

    Mike Monett Guest

    Joerg wrote:

    Me wrote:

    [...]
    I don't know. The whole thing was paid by a medical plan and I never saw
    the bill. It couldn't be too much - this is a small town in Canada with
    mostly retired people on a pension.

    Even if it costs a bit more, it sure is worth it. The amazing thing is
    you get the entire picture all at once. I could see the beginning of
    infection on the roots of some teeth on the other side, so I know I am in
    for a return visit.

    I found a site that shows what it looks like. It's called Panorex and
    gives a panoramic view to show the general condition of all the teeth. It
    is usually taken every 5-7 years. Other types of X-Rays are needed for
    more specific diagnosis. (The panoramic image is much larger and better
    than shown here:)

    http://www.dentalgentlecare.com/necessary_x-rays.htm

    Mike Monett
     
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