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simplest possible voltage doubler?

Discussion in 'Electronic Design' started by Michael Noone, Jul 24, 2005.

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  1. Hi - I need to take a signal coming from a 3K potentiometer that ranges
    from 0.44-1.77V and double it (so I'd have an output of 0.88-3.54V). That
    output will be then fed into an ADC. I have available on the board a very
    stable 5V supply, and a less clean 6V supply. The ADC is 10b with a 4.096
    reference voltage, and I'd like to keep this circuit accurate enough to a
    single bit. My first thought is a non inverting amplifier - but that's one
    chip and 4 resistors per circuit, and since I need three of these on the
    same board, 1 chip and 12 resistors total. That's quite a bit... Is there
    any better way? I've seen some dedicated chips designed for current
    monitoring that would amplify a signal by a much higher gain - 50 or 100
    without any external components at all, is there anything like this that
    would work for me?

    Thanks for your help!

    -MJ Noone
     
  2. mmm

    mmm Guest

    why four resistor ?

    a non inverting-amplifier is only 2 resistor and an OpAmp

    you must use precise equal resistor or calibrate ( the gain stage ) in
    software
     
  3. Guest

    Theat's six resistors, not twelve - since your 3k potentiometer doesn't
    present a constant to the amplifier, balancing the input impedances
    doesn't win you much.

    And they don't have to be discrete resistors - in fact if you want
    precise and stable gain you are much better off using an array of thin
    film precision resistors in a single package, or three precision
    dividers.

    Farnell carry the Vishay RMKMS816 with 8 10k 0.1% resistors in a 16-pin
    DIP - order code 309-8667, for about $15, and the MPM range of
    precision dividers in an SOT23 package - the 10k+10k 1:1 divider (order
    code 309-8643) costs about $4 and you'd need three of them.

    I'd go for the SOT23 dividers myself - they are marginally cheaper, and
    the PCB layout would come out better.

    Not all that cheap, but quite precise - the dividers offer a ratio
    tolerance of +/-0.05%, witha temperature stability of +/-2ppm/C,
    appreciably better than the eight resistor array.
     
  4. Argh. That is what happens when I post when I'm about 90% asleep. I was
    thinking of a difference amplifier, which I had originally planned on using
    instead of a non-inverting amplifier.

    -MJ
     
  5. wrote in @f14g2000cwb.googlegroups.com:
    yes - 2 resistors, quite right. I think I need some sleep... If I were
    to go the op-amp route those vishay dale voltage dividers look pretty
    nice. Something in a DIP package would be much, much too big for this
    board.

    Can you suggest an op-amp to use? Ideally a quad op-amp as I will need 3
    channels. I'm just never sure on how to choose an op-amp... If it
    matters, I'm currently planning on sampling at 100hz, but it's quite
    possible that I'll increase that.

    -MJ
     
  6. Isn't it 2 resistors per circuit, 6 total?

    R
    +---/\/\---+
    | _ |
    +--|- \ |
    | |324>---+-->2x Vin
    Vin, 0.44-1.77V +------|+_/
    |
    \
    /R
    \
    |
    0v-----+------------0v

    For a 324 make 2R low enough (1k-ish?) so that they
    provide an output pulldown current, keeping the o/p
    stage in Class A.

    If it is not smd, reduce the floor plan by standing
    resistors on end, on a 0.1 pitch.

    Try to power the 3k pots from a voltage derived from
    the ADC's 4.096V.
     
  7. CLC417 would do, except that I don't know whether it would work
    from a single sided 5V supply.

    Regards,
    Mikko
     
  8. Just checked from the NSC website (who acquired Comlinear,
    the original maker of the CLC417, some years ago): they seem
    to have a number of new types, including the LMH6718 dual which
    is specified for a single sided 5V.

    Regards,
    Mikko
     
  9. Guest

    The CLC417 is intended for video application. It is inappropriate for
    the OPs application - in order to maximise the bandwidth of the part,
    Comlinear ran a lot of curren through the input transistors, which
    leads to large input bias currents and input offset currents - not
    something you'd want in Michael Noone's application.

    Video is not usually too picky about DC levels - not least because if
    the DC level is critical, they fix it with a "black level clamp" -
    which menas that the CLC417 has a pretty poor DC offset specification
    of +/-5mV at 25C. Not what you'd normally chose for an instrument
    application.
     
  10. Pooh Bear

    Pooh Bear Guest

    If you take the wiper of your pot directly to the op-amp +input ( without an R
    to ground ) you need only 2 resistors per amplifier stage.

    How did you get 4 ?

    Use a quad op-amp to keep the IC package count low.

    I don't understand why you think this is 'large' number of components. Are you
    extremely short of space ?

    Graham
     
  11. MSC

    MSC Guest

    Can't you just feed the ADC with half the reference voltage?

    Mike
     
  12. Your 3.54 max output from a 5 volt supply just squeaks you by with
    some non rail to rail types. Since the frequency appears to not be
    very high, offset voltage and output voltage range appear to be the
    main concerns.

    Take a look a at the LT1014
    http://www.ldeo.columbia.edu/~tolstoy/research/

    A rail to rail type with a similar offset spec is the LT6012
    http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1009,C1021,P2315,D2626

    To get a better offset than that would probably take an auto zero
    type. For instance the LTC2052 has a typical offset spec of +-0.5 uV.
    http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1009,C1021,P1855,D1799

    Keep in mind, that when you take layout into account, the dual
    versions may actually save space compared to the quads because there
    may be less of a tangle of traces.
     
  13. Ken Smith

    Ken Smith Guest

    If you want to avoid 0.1% resistors, consider this:

    -------------------------- ADC
    !
    ---O
     
  14. mike

    mike Guest

    Sanity Check...
    Why do you need to double it?
    Can you halve the reference voltage?
    You say signal???? What's a signal??? AC? DC? Varying DC? How fast?
    The responses you've had so far assume a rock-solid non-varying DC
    voltage. Unless that's the case, you have little to gain by doubling
    the signal. Just multiply the ADC output by 2. The accuracy/stabiltiy
    of your gain stage plus the noise introduced by it may outweigh the
    additional bit of precision. If the signal is noisy, you may gain
    something by averaging the ADC output.
    You mentioned the pot, so it must be significant. If it's user
    settable, you may find that the mechanical stiction and hysteresis in
    the pot make it impossible to set it as accurately as you
    expect...especially as it ages.

    Can't tell what you're trying to accomplish, but it sounds like you're
    trying to do it the hard way.
    mike

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  15. Yes - I meant 2. My bad on that.
    Very much so!

    -MJ Noone
     
  16. Possibly - I'm currently looking into that. The ADC is multiplexed and 3 of
    the other signals going into it need the 4.096 reference voltage. I'm
    working on finding a very small shunt resistor for those circuits that
    would allow me to use a 2.048V reference. (I just made a new post about
    exactly this)

    -MJ Noone
     
  17. John Larkin

    John Larkin Guest

    The problem with the SOT-23's is that they're almost impossible to
    get. We've given up on them for a few current designs where they would
    be ideal, and gone to discrete 0603 thinfilms.

    Do you know of anybody who actually stocks parts? It would seem to us
    that a few basic ratios should be popular enough to be stocked, but
    we've run into absured leadtimes and quantity requirements on the
    SOT-23s.

    Mouser and Digikey stock 0.1% 0603 thinfilms at 25 and 10 ppm/k.

    John
     
  18. Fred Bloggs

    Fred Bloggs Guest

    There's that 3rd resistor, a pull-up to V+.
     
  19. Fred Bloggs

    Fred Bloggs Guest

    4.096/1024=4mV resolution. Neglecting differential non-linearity, you
    will measure voltage within the 0.44-1.77V to within 4mV accuracy.
    Assuming perfectly errorless scaling into the 0.88-3.54V range, measured
    with 4mV accuracy, you then get 2mV accuracy on the original signal. If
    you want 2mV accuracy, then you can alternately sum 2mV and 0mV with the
    original signal to obtain the same estimate. So you don't need a
    doubler, you need a summer- and that is two resistors.
     
  20. John Larkin

    John Larkin Guest

    Even more fun is to sum with noise or an asynchronous triangle, smear
    the samples several LSBs, and software lowpass filter. If the native
    noise is enough (dicey at 10 bits, almost guaranteed at 14-16) just
    filter.

    John
     
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