Two questions about Op Amp´s
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Posts: 46,084
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
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
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
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
>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
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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
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?
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
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
> 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,
>