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Two questions about Op Amp´s — Parallax Forums

Two questions about Op Amp´s

ArchiverArchiver Posts: 46,084
edited 2000-08-02 14:40 in General Discussion
Hello respectable list:

I wish to measure atmospheric pressure with a Stamp I.
I already have an SX15A sensor from Sensym, wich nicely measures the
pressure between 0 and 15 PSI = 1034 mili Bar, giving good readings from 0
to 100 mV, with a very good sensitivity.
As I need to transform this voltage variation in 0 to 5 V, in order to input
the ADC 0831, I have used an LM324 quad op amp as a differential amplifier
connected as in the attache schematic. (High input impedance, good CMRR,
etc)
Carefully wired on the breadboard, previously matched 1 % resistances, etc.,
and fed the Op Amp with 5 V.

1. Under these circumstances, I have some 3,5 V in the Op Amp output, but
with a terrible sensitivity, since almost there is no voltage variation with
pressure. ¿What did I do wrong? ¿Is there a better way to transform 0-100 mV
dc in 0-5 V dc?

2. Since the ADC0831 has only 8 bits, hence 255 possible readings, and as I
don´t need the readings between 0 and 10 PSI, I would like to enhance the
sensitivity sending to the ADC, 0 to 5 V, but corresponding to 10 - 15 PSI
readings. Is this possible with a reasonably simple electronics?

Any help will be strongly appreciated...

Best regards

Jorge Ferrero

Comments

  • ArchiverArchiver Posts: 46,084
    edited 2000-07-31 16:24
    Two questions Jorge:

    1) Where do you have the connections to the sensor? The sensor's output is
    100mV, but the input is differential. This implies you would have one of the
    amp's inputs grounded?

    2) You should really consider feeding the opamp with more voltage than +5 on
    the supply rail. Most op amps can't reach the supply rails. The more current
    you draw out, the less voltage you can get. So with a supply of 5V, if you
    draw 100uA you might get 4.7V. But if you draw 1mA, you might get 4.3V (just
    example numbers -- depends on the op amp).

    Some op amps can go to one rail or the other -- usually the ground rail as
    this allows you to use a single ended power supply.

    If memory serves, the 324 can go to ground, but not to the +V rail.

    Regards,

    Al Williams
    AWC
    * Floating Point Math for the Stamp, PIC, SX, or any microcontroller:
    http://www.al-williams.com/awce/pak1.htm
  • ArchiverArchiver Posts: 46,084
    edited 2000-07-31 18:50
    If you check the specs(http://www.national.com/pf/LM/LM324.html) on
    this op amp you will see that it operates from 0 to V-1.5V so 3.5 is
    the most you are going to get out. You need to use a little less
    gain. I made the same mistake on a recording altimeter I made for my
    bicycle. For more sensitivity use the first stage to subtract the mv
    that represents something close to 10psi then amplify the difference
    being carefull not to exceed 3.5V.


    --- In basicstamps@egroups.com, "jferrero" <jferrero@u...> wrote:
    > Hello respectable list:
    >
    > I wish to measure atmospheric pressure with a Stamp I.
    > I already have an SX15A sensor from Sensym, wich nicely measures the
    > pressure between 0 and 15 PSI = 1034 mili Bar, giving good readings
    from 0
    > to 100 mV, with a very good sensitivity.
    > As I need to transform this voltage variation in 0 to 5 V, in order
    to input
    > the ADC 0831, I have used an LM324 quad op amp as a differential
    amplifier
    > connected as in the attache schematic. (High input impedance, good
    CMRR,
    > etc)
    > Carefully wired on the breadboard, previously matched 1 %
    resistances, etc.,
    > and fed the Op Amp with 5 V.
    >
    > 1. Under these circumstances, I have some 3,5 V in the Op Amp
    output, but
    > with a terrible sensitivity, since almost there is no voltage
    variation with
    > pressure. ¿What did I do wrong? ¿Is there a better way to
    transform
    0-100 mV
    > dc in 0-5 V dc?
    >
    > 2. Since the ADC0831 has only 8 bits, hence 255 possible readings,
    and as I
    > don´t need the readings between 0 and 10 PSI, I would like to
    enhance the
    > sensitivity sending to the ADC, 0 to 5 V, but corresponding to 10 -
    15 PSI
    > readings. Is this possible with a reasonably simple electronics?
    >
    > Any help will be strongly appreciated...
    >
    > Best regards
    >
    > Jorge Ferrero
  • ArchiverArchiver Posts: 46,084
    edited 2000-07-31 20:54
    At 05:50 PM 7/31/00 +0000, jimsp@y... wrote:
    >If you check the specs(http://www.national.com/pf/LM/LM324.html) on
    >this op amp you will see that it operates from 0 to V-1.5V so 3.5 is
    >the most you are going to get out. You need to use a little less
    >gain. I made the same mistake on a recording altimeter I made for my
    >bicycle. For more sensitivity use the first stage to subtract the mv
    >that represents something close to 10psi then amplify the difference
    >being carefull not to exceed 3.5V.

    Hiya gang.

    I use the LM324 in several designs and usually DON'T have the problem that
    you are running into. My technique is simple - I power the '324 from the
    unregulated supply. Include a resistor in series with the output so that
    an output above 5V does no harm.

    Using even a 6V gell cell will result in usable output to at least 4.5 -
    4.75V - a full 25% improvement in dynamic range. But I suspect most of you
    are running from a 9 to 12 V supply. Running the '324 from that
    unregulated input gives you full dynamic range.

    Note that the quiescent current is relatively constant over the entire
    supply voltage range. It doesn't cost you anything at all to run the '324
    from the unreg supply.

    Just a suggestion.

    dwayne



    Dwayne Reid <dwayner@p...>
    Trinity Electronics Systems Ltd Edmonton, AB, CANADA
    (780) 489-3199 voice (780) 487-6397 fax

    Celebrating 16 years of Engineering Innovation (1984 - 2000)

    * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
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  • ArchiverArchiver Posts: 46,084
    edited 2000-07-31 21:27
    Hello Al:

    Thank you for your answer.


    >
    > 1) Where do you have the connections to the sensor? The sensor's output is
    > 100mV, but the input is differential. This implies you would have one of
    the
    > amp's inputs grounded?

    No. The sensor is a bridge type, so the input to this bridge are +Vb (some 3
    V) and GND; the outputs of the bridge are V+ and V-, and this are the inputs
    for the diff. amplifier. I have measured the voltage at V+ and V-, and I
    have there some 0 to 100 mV (making void to the sensor)

    > 2) You should really consider feeding the opamp with more voltage than +5
    on
    > the supply rail. Most op amps can't reach the supply rails. The more
    current
    > you draw out, the less voltage you can get. So with a supply of 5V, if you
    > draw 100uA you might get 4.7V. But if you draw 1mA, you might get 4.3V
    (just
    > example numbers -- depends on the op amp).
    >
    Data sheet of 324 indicates + 5V single voltage and GND to feed the 324;
    anyway, it works also to greaters voltages. I´ll try it.

    Best regards

    Jorge
  • ArchiverArchiver Posts: 46,084
    edited 2000-07-31 22:03
    Jorge, some additional comments:

    The 324 opamps have an input stage designed to include gnd, using PNP
    transistors. This means that you must sink their input current to gnd. The
    circuit you have appears to float the inputs with no gnd return, so the amps
    will not be properly biased.

    You have a gain of 101, but you only need a gain of 50 to achieve 5 volts
    from 100 mv. As others have pointed out, the 324 output can only rise to
    3.5V with a 5V supply.

    Can't you use a single non-inverting amp with a gain of 50, grounding one
    end of the sensor? This configuration still has high input impedance. You
    could still inject the required offset into the inverting terminal through
    an adjustable resistance. I am not familiar with that sensor, so maybe
    these ideas are not applicable.

    Regards,
    Ray McArthur

    Original Message
    From: jferrero <jferrero@u...>
    To: <basicstamps@egroups.com>
    Sent: Monday, July 31, 2000 10:52 AM
    Subject: [noparse][[/noparse]basicstamps] Two questions about Op Amp´s
    > I wish to measure atmospheric pressure with a Stamp I.
    > I already have an SX15A sensor from Sensym, wich nicely measures the
    > pressure between 0 and 15 PSI = 1034 mili Bar, giving good readings from 0
    > to 100 mV, with a very good sensitivity.
    > As I need to transform this voltage variation in 0 to 5 V, in order to
    input
    > the ADC 0831, I have used an LM324 quad op amp as a differential amplifier
    > connected as in the attache schematic. (High input impedance, good CMRR,
    > etc)
    > Carefully wired on the breadboard, previously matched 1 % resistances,
    etc.,
    > and fed the Op Amp with 5 V.
    >
    > 1. Under these circumstances, I have some 3,5 V in the Op Amp output, but
    > with a terrible sensitivity, since almost there is no voltage variation
    with
    > pressure. ¿What did I do wrong? ¿Is there a better way to transform 0-100
    mV dc in 0-5 V dc?
    >
    > 2. Since the ADC0831 has only 8 bits, hence 255 possible readings, and as
    I don´t need the readings between 0 and 10 PSI, I would like to enhance the
    > sensitivity sending to the ADC, 0 to 5 V, but corresponding to 10 - 15 PSI
    > readings. Is this possible with a reasonably simple electronics?
  • ArchiverArchiver Posts: 46,084
    edited 2000-08-02 01:13
    Hi great list:

    First, thank you very much to all those who has answer me, very useful all
    your comments!

    Now, what I did:

    1) Really the specs for LM334 from National establishs that the output
    voltage will be Vcc-1,5 V, so I powered the 334 chip with 9 V and still I
    don´t have good sensitivity at atmospheric pressure (the sensor outputs 55
    mV and the 324, some 3,5V), but if I make some void to the sensor (when it
    outputs some 20 mV) then the 324 outputs < 3 V and now the sensitivity is
    good! This clearly indicates that I must input not more than 20/30 mV to the
    op amp...

    2) I have read the Sensym sensor specs, and I think I must use a
    differential amplifier because its high input impedance and its good CMRR;
    there is in specs a suggested circuit with LT1014CN and LM10CN, but I can´t
    found this chips here.

    3) It seems that the circuit I have used (from the LM334 specs), has the two
    inputs (V2 and V1)isolated from gnd, still must I sink their input current
    to gnd as Ray McArthur said?

    4) I think the solution is to make the "offset adjustment" proposed by Tracy
    Allen, because in that way i´am going to measure from 10 to 15 psi (as I
    want), with a smaller input to the 324, but I have not clear how to wire it.
    (See in the attache, how the sensor and the op amp must be connected).

    Again, thanx a lot to all...

    best regards

    Jorge
    Original Message
    From: rjmca <rjmca@w...>
    To: <basicstamps@egroups.com>
    Sent: Monday, July 31, 2000 6:03 PM
    Subject: Re: [noparse][[/noparse]basicstamps] Two questions about Op Amp´s


    > Jorge, some additional comments:
    >
    > The 324 opamps have an input stage designed to include gnd, using PNP
    > transistors. This means that you must sink their input current to gnd.
    The
    > circuit you have appears to float the inputs with no gnd return, so the
    amps
    > will not be properly biased.
    >
    > You have a gain of 101, but you only need a gain of 50 to achieve 5 volts
    > from 100 mv. As others have pointed out, the 324 output can only rise to
    > 3.5V with a 5V supply.
    >
    > Can't you use a single non-inverting amp with a gain of 50, grounding one
    > end of the sensor? This configuration still has high input impedance.
    You
    > could still inject the required offset into the inverting terminal through
    > an adjustable resistance. I am not familiar with that sensor, so maybe
    > these ideas are not applicable.
    >
    > Regards,
    > Ray McArthur
  • ArchiverArchiver Posts: 46,084
    edited 2000-08-02 04:33
    Hi Jorge,

    Original Message
    From: jferrero <jferrero@u...>
    To: <basicstamps@egroups.com>
    Sent: Tuesday, August 01, 2000 8:13 PM
    Subject: RE: [noparse][[/noparse]basicstamps] Two questions about Op Amp´s

    > 2) I have read the Sensym sensor specs, and I think I must use a
    > differential amplifier because its high input impedance and its good CMRR;

    ** I didn't realize the sensor was a bridge, so you DO need a diff-amp.
    Your circuit looks like it could be made to work.
    >
    > 3) It seems that the circuit I have used (from the LM334 specs), has the
    two
    > inputs (V2 and V1)isolated from gnd, still must I sink their input current
    > to gnd as Ray McArthur said?

    ** Your bridge should take care of that.
    >
    > 4) I think the solution is to make the "offset adjustment" proposed by
    Tracy
    > Allen, because in that way i´am going to measure from 10 to 15 psi (as I
    > want), with a smaller input to the 324, but I have not clear how to wire
    it.
    > (See in the attache, how the sensor and the op amp must be connected).

    ** I took a quick look at the Sensym SSC (don't remember if that is your
    device); they show an offset adjustment circuit. I lost Tracy's circuit,
    but you could use something like:

    A 10K pot, one end grounded, the other end to 2.5K which then connects to
    +5V.
    Connect a 100K resistor to the wiper.
    Connect the other end of the 100K to either diff-amp input (V1 or V2).

    This should give about +/- 50 mv offset adjustment.
    If you need more offset, reduce the 100K.

    Hope this helps,
    Ray McArthur
  • ArchiverArchiver Posts: 46,084
    edited 2000-08-02 14:40
    Hello Ray:

    > Hope this helps,
    Really this help!

    > A 10K pot, one end grounded, the other end to 2.5K which then connects to
    > +5V.
    > Connect a 100K resistor to the wiper.
    > Connect the other end of the 100K to either diff-amp input (V1 or V2).
    >
    > This should give about +/- 50 mv offset adjustment.

    I did exactly this, and it worked!! I don´t understand well how offset
    works, but it seems that it is the same to connect the 100 K resistor to V1
    or to V2; in both cases I have reduced to 35 mV the sensor signal, and with
    R2 = 3,2 K in the op amp, I have a gain of 63,5, and an output of 2,2 V from
    the Op amp. Sensitivity, in this case, is VERY GOOD. This indicates, I
    think, that the diff amp is working properly...

    > ** I took a quick look at the Sensym SSC (don't remember if that is your
    > device); they show an offset adjustment circuit.

    My sensor is Sensym SX15 for absolute pressure. Effectively there is an
    offset circuit in the SX15 specs, but quite more complicated, and with Op
    Amp´s that I can´t get easily here.

    Well Ray, thanx a lot to you, to Tracy, Al, Dwayne and all that nice people
    of this great list!

    best regards

    Jorge

    Original Message
    From: rjmca <rjmca@w...>
    To: <basicstamps@egroups.com>
    Sent: Wednesday, August 02, 2000 12:33 AM
    Subject: Re: [noparse][[/noparse]basicstamps] Two questions about Op Amp´s


    > Hi Jorge,
    >
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