PLEASE HELP! Frequency to Voltage Converter LM2917N 14-pin

Discussion in 'Electronic Basics' started by electro, Apr 6, 2006.

  1. electro

    electro Guest

    im trying to explore the operation of the chip in zener regulated F>V
    circuit with 250mVpp @ 1khz,12V Vcc as prescirbed in the datasheet.my
    threshold levels are rated +-50mVpp so im quite sure that proper output
    Voh(8.3) and Vol(2.3) shoudl be produced at pin 2 or 3...
    my R1 is 100k C1 is 20nF and C2 is 1uF and a 10k transistor
    output(pin5)...

    my problem is:

    1.my reading at pin 2 is 580 mVdc max
    2.my reading at pin 3 is 520mVdc max
    -> shouldnt it be equal to ic x R1 = 185uA x 100k = 18.5V where my ic
    or I2=I3 is rated to be about 185uA @ 25degree celsius as based from
    the datasheet graph
    3.my reading at pin 5 is 420mVdc max
    -> the output voltage which says Vout = 66Hz/volt is nowhere to be
    attained. changing the input frequency from my function gen. doesnt
    alter the Vout in anyway...why?
    4.my application is in metal detedector. i have a lc tuned oscillator
    and i want to use the frequency change as my indicator of metal
    presence as opposed to "mixing"...or dou you have any other way to do
    this? meaning a simple switch in which when a certain threshold
    frequency is attained, it outputs a certain voltge level?

    thanks!
    help please!


    datasheet is here:http://www.national.com/pf/LM/LM2917.html
    electro, Apr 6, 2006
    #1
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  2. electro

    electro Guest

    sorry forgot to connect pin 12 to ground..however my values for the
    said pins are now equal

    pin 5,4,3 an2 = 80mV

    please help
    electro, Apr 6, 2006
    #2
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  3. electro

    Noway2 Guest

    electro wrote:
    > sorry forgot to connect pin 12 to ground..however my values for the
    > said pins are now equal
    >
    > pin 5,4,3 an2 = 80mV
    >
    > please help


    Are you sure that the chip is functional? Did you possibly damage
    something by accident when you didn't have the ground connected?

    When faced with unexpected results, I try to back up and see if I can
    get things working in a known environment. What I am trying to say
    here, is perhaps you are biting off too much unknown in one pass. Try
    implementing the F-V converter on the bench, using one of the sample
    circuits in the data sheet and see if you can get "good" results that
    way first.

    The LM2917N has been around a long time and should be a reliable
    device. I was planning on using one in the project I am working on,
    but the requirements were changed on me and I no longer need the
    functionality of it, otherwise I would offer you better specifics on
    the circuit implementation.
    Noway2, Apr 6, 2006
    #3
  4. electro

    Chris Guest

    electro wrote:
    > im trying to explore the operation of the chip in zener regulated F>V
    > circuit with 250mVpp @ 1khz,12V Vcc as prescirbed in the datasheet.my
    > threshold levels are rated +-50mVpp so im quite sure that proper output
    > Voh(8.3) and Vol(2.3) shoudl be produced at pin 2 or 3...
    > my R1 is 100k C1 is 20nF and C2 is 1uF and a 10k transistor
    > output(pin5)...
    >
    > my problem is:
    >
    > 1.my reading at pin 2 is 580 mVdc max
    > 2.my reading at pin 3 is 520mVdc max
    > -> shouldnt it be equal to ic x R1 = 185uA x 100k = 18.5V where my ic
    > or I2=I3 is rated to be about 185uA @ 25degree celsius as based from
    > the datasheet graph
    > 3.my reading at pin 5 is 420mVdc max
    > -> the output voltage which says Vout = 66Hz/volt is nowhere to be
    > attained. changing the input frequency from my function gen. doesnt
    > alter the Vout in anyway...why?
    > 4.my application is in metal detedector. i have a lc tuned oscillator
    > and i want to use the frequency change as my indicator of metal
    > presence as opposed to "mixing"...or dou you have any other way to do
    > this? meaning a simple switch in which when a certain threshold
    > frequency is attained, it outputs a certain voltge level?
    >
    > thanks!
    > help please!


    > sorry forgot to connect pin 12 to ground..however my values for the
    > said pins are now equal
    >
    > pin 5,4,3 an2 = 80mV
    >
    > please help


    Hi, Electro. It looks like you're doing the 14-pin version of the app
    circuit, "Zener Regulated Frequency To Voltage Converter" on the top of
    p.9 of the data sheet.

    It _does_ work. That happens to be the first one I tried the first
    time I got the IC.

    You can't expect an output voltage greater than the supply voltage in
    this circuit -- actually, your maximum frequency should be about 360Hz
    (Vz - 2V * (1V/66Hz) ). Anything over that will show about 5.5V.

    Here's what to do. First, turn the power off. Then, copy the circuit
    shown on p.9 on a sheet of paper, and transpose the pin numbers for the
    14 pin IC. Make a Xerox of your artwork. Now get a cup of coffee and
    think about something else for 10 minutes. Once your mind is fresh,
    re-trace your wiring point-to-point, using a colored marker to track
    out each node of the circuit on a piece of paper. Once every line is
    colored in, you know the circuit is wired correctly. It's important to
    draw out the entire circuit before you wire it up.

    You have already indicated that you had at least one wiring problem.
    Sometimes running an IC without the GND pin (12 on this IC) connected
    is fatal, sometimes not. You'll definitely have some interesting
    current paths.

    I don't know if you remembered that the 8-pin version has the charge
    pump connected to the input of the output op amp, where you have to
    make that connection externally with the 14-pin version. If you
    haven't made this connection (pin 3 to pin 4 on the 14-pin IC), that
    might be the source of your problem.

    If you triple-check the wiring, and it still doesn't work, there's a
    possibility you smoked the IC. Try a new one and see if it works.

    You haven't given enough information to determine whether this is the
    way to go here. Metal detectors typically operate at RF (radio
    frequency). This is too high a frequency for the LM2917 to detect.
    Are you mixing your sensing signal with a known oscillator frequency?
    I know it's sometimes easier to read a meter than listening, but your
    ears are very sensitive to small changes in tone, actually much better
    than a voltmeter. Not only that, but your ear can normalize for slow
    drift in the reference frequency, where you'll have to be constantly
    nulling the voltmeter. There's a reason why most of these circuits use
    an output tone.

    Do you have a link, or can you provide a schematic for your proposed
    circuit?

    Good luck
    Chris
    Chris, Apr 6, 2006
    #4
  5. electro

    electro Guest

    thank you for your kind thoughts...
    yes im referring to the zener F>V converter circuit

    initially, i made a circuit that operates in mixing mode.i have a fixed
    oscillator of about 480khz and then another one
    with almost the same frequency and is made variable by the loop
    inductance..my problem however was that i cant get
    my signal mix or extract the difference signal. i tried single ended
    bjt mixer as well as diode but cant get a good output..
    also, i would like to indicate metal presence not by an audio tone but
    by a pic mcu.my initial setup was to rectify and extract
    the input frequency and input it to my pic mcu which is then compared
    to a reference voltage which will be pre-calibrated.
    but having found of chips that can do frequecny to voltage conversion,
    i opted for this chip now...
    my application is very simple.i will just calibrate my pic reference
    voltage and then from there any change in frequency will correspond to
    a certain voltage...
    these are my problems:
    1. the maximum input frequency based on datasheet is fmax =
    I2/(C1xVcc)...from datasheet, for a supply voltage of 12V,my I2=I3 is
    about 185uA..also, C1 should be at least 500pf due to timing
    considerations...so that will give me about 30.8khz maximum..my plan is
    to further increase my loop inductance to b e in the range of
    20-30khz.now my question is how sensitive will this be with the
    presence of metal?
    2.what waveform will i expect from pin 2 and 3?
    I2 is mirrored to I3 right? isnt it i should be seing a waveform in
    which its voltage changes from about 8.3 then discharges to about 2.3
    which indicated the Voh and Vol for the pin3?
    3.my Vout should be varying when i vary my input frequency right?
    it seems im not gettng the sensible outputs.....
    please help...
    _i connected pin3 and 4

    lastly,

    any idea on a simpler way to realize my application?
    electro, Apr 6, 2006
    #5
  6. electro

    Chris Guest

    electro wrote:
    > thank you for your kind thoughts...
    > yes im referring to the zener F>V converter circuit
    >
    > initially, i made a circuit that operates in mixing mode.i have a fixed
    > oscillator of about 480khz and then another one
    > with almost the same frequency and is made variable by the loop
    > inductance..my problem however was that i cant get
    > my signal mix or extract the difference signal. i tried single ended
    > bjt mixer as well as diode but cant get a good output..
    > also, i would like to indicate metal presence not by an audio tone but
    > by a pic mcu.my initial setup was to rectify and extract
    > the input frequency and input it to my pic mcu which is then compared
    > to a reference voltage which will be pre-calibrated.
    > but having found of chips that can do frequecny to voltage conversion,
    > i opted for this chip now...
    > my application is very simple.i will just calibrate my pic reference
    > voltage and then from there any change in frequency will correspond to
    > a certain voltage...
    > these are my problems:
    > 1. the maximum input frequency based on datasheet is fmax =
    > I2/(C1xVcc)...from datasheet, for a supply voltage of 12V,my I2=I3 is
    > about 185uA..also, C1 should be at least 500pf due to timing
    > considerations...so that will give me about 30.8khz maximum..my plan is
    > to further increase my loop inductance to b e in the range of
    > 20-30khz.now my question is how sensitive will this be with the
    > presence of metal?
    > 2.what waveform will i expect from pin 2 and 3?
    > I2 is mirrored to I3 right? isnt it i should be seing a waveform in
    > which its voltage changes from about 8.3 then discharges to about 2.3
    > which indicated the Voh and Vol for the pin3?
    > 3.my Vout should be varying when i vary my input frequency right?
    > it seems im not gettng the sensible outputs.....
    > please help...
    > _i connected pin3 and 4
    >
    > lastly,
    >
    > any idea on a simpler way to realize my application?


    If you want to operate a metal detector at low frequencies, you'd
    better bring a plastic wheelbarrow to carry the thing. You'll need a
    lot of windings to get the inductance you'll need for a large air coil
    to oscillate at lower frequencies.

    Small pieces of metal will make small changes in the oscillating
    frequency -- possibly tens or hundreds of Hz out of hundreds of KHz. A
    tenth of a percent or less. Dividing it down to a frequency accessible
    by a PIC will mean you lose the information you need.

    Metal detection is inherently an RF enterprise -- hate to say it, but
    get used to it. It's an analog world. And the LM2917 is made for
    tachometer-speed applications (RPMs rather than KHz).

    If you really want a voltage output, get the beat frequency oscillator
    working, and then work on changing the audio beat frequency into a
    voltage. You can do that easily by using a comparator to change the
    audio into a digital output, and then use a 555 to output a brief pulse
    for every beat oscillation. You can then use an R-C filter to get a DC
    voltage you can read with a meter or your PIC. You also might want to
    try just counting the beats with a PIC, but that will have limitations,
    too. You have to watch out for maximum frequency.

    If you are allergic to the whole mixer concept, you might just want to
    get a handheld frequency counter, and pick off the RF oscillator
    output. Inexpensive handheld frequency counters can give you the
    resolution you'll need, but it gets tiring looking at the display. The
    idea is to wander blithely down the beach waving the finder back and
    forth, and having an excuse to scope out the scenery while you're
    nominally being industrious. ;-)

    Good luck
    Chris
    Chris, Apr 6, 2006
    #6
  7. electro

    electro Guest

    i have a working lc tuned oscillator now.my loop is about 5 inches wide
    with 20 windings,im getting a 37.9uH
    from that setup...can i simply port that to the ADC unit of my
    PIC876A?how will the digital value relate to my frequency?
    also, since resonance is dependent on capacitor and inductor, why cant
    i select capacitor values and still have a tolerable amount of
    inductance air coil winding?

    thanks!
    electro, Apr 6, 2006
    #7
  8. electro

    Ban Guest

    electro wrote:
    > i have a working lc tuned oscillator now.my loop is about 5 inches
    > wide with 20 windings,im getting a 37.9uH
    > from that setup...can i simply port that to the ADC unit of my
    > PIC876A?how will the digital value relate to my frequency?
    > also, since resonance is dependent on capacitor and inductor, why cant
    > i select capacitor values and still have a tolerable amount of
    > inductance air coil winding?
    >
    > thanks!


    maybe you first read up a bit how such a thing works. when you have 400kHz
    oscillator, it will maybe change its frequency 100Hz with a big chunk of
    metal, lets say a treasure buried a few inches below ground, that is 0.025%.
    http://home.clara.net/saxons/bfo.htm that is why another oscillator is used.
    It is very easy to mix the two outputs, just with resistors.
    Since both oscillators are similar the frequency drift gets cancelled and
    you can listen to the more stable beat frequency. A PIC will not help, when
    you do not have the front end right.
    --
    ciao Ban
    Apricale, Italy
    Ban, Apr 6, 2006
    #8
  9. electro

    electro Guest

    or cant i use the built-in comparator of my pic...sine i have a sine
    wave signal at zero dc offset,
    i will count the interval between successive zero crossings that is i
    will set my zero volt level as my reference...
    will that be possible?
    electro, Apr 7, 2006
    #9
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