Two questions about Op Amp4s
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"jferrero" <jferrero@u...> wrote:
>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
>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...
>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?
Hi Jorge,
Your amilifier has a minimum gain of x101, when the 2k potentiometer is
turned to its full resistance. That means that it only takes a 49.5
millivolt change at the input to make the output swing a full 5 volts. To
start with, be sure that gain set for its minimum value, that is, the
maximum value of the potentiometer. To help you "find" the signal, you
might want to start with a lower gain, say by putting a 1k resistor in
series with the 2k potentiometer and turn the gain down to x70. Also, be
sure that you have the + output of your sensor connected to the + input of
the differential amplifier, (+V2 in your circuit).
-- The LM324 can only swing up to about 3.5 volts (as Al pointed out).
The input signal is probably greater than 35 millivolts, and the changes in
pressure at the input have no effect because the amplifier is in
saturation. You could use a higher supply voltage for the op amp (9
volts?), or choose a different op amp that has "rail-to-rail" output.
-- The SX15A has an "offset" of tyically +/- 20 millivolts at zero
pressure, and that must be added to the signal. Also, you want to be able
to set your "zero" value at some value you choose, like 10 PSI. You will
need to add an "offset adjustment" to your original circuit. Here is a
simple one:
10k pot, 15 turn "offset"
Vss
/\/\/\
Vdd=5.0 volts regulated
|
| 100k,1%
'---/\/\
to +V1 (~4k impedance)
Start with the trimmer in the middle of its range. If you monitor the
differential voltage (V2-V1) on a multimeter, you will see that adjusting
the potentiometer trims the differential input. At the output of the
amplifier, you should see that you get a good signal within the 0--3.5 volt
range, when the input (V2-V1) is in the range of 0 to about 35 millivolts.
You will want to calibrate it so that the output is 0 when the pressure is
10 PSI (offset adjustment) and the output is 3 volts when the pressure is
15 PSI (gain adjustment). The offset circuit can be improved by putting
stable resistors in series with the 10k pot, once you know what range of
adjustment you need.
Also check out the application notes at Sensym. There are some barometer
circuits there. Also, check out Matt Parnell's site,
<http://www.weatherstamp.com/>
regards,
-- Tracy Allen
Electronically Monitored Ecosystems
http://www.emesystems.com
>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
>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...
>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?
Hi Jorge,
Your amilifier has a minimum gain of x101, when the 2k potentiometer is
turned to its full resistance. That means that it only takes a 49.5
millivolt change at the input to make the output swing a full 5 volts. To
start with, be sure that gain set for its minimum value, that is, the
maximum value of the potentiometer. To help you "find" the signal, you
might want to start with a lower gain, say by putting a 1k resistor in
series with the 2k potentiometer and turn the gain down to x70. Also, be
sure that you have the + output of your sensor connected to the + input of
the differential amplifier, (+V2 in your circuit).
-- The LM324 can only swing up to about 3.5 volts (as Al pointed out).
The input signal is probably greater than 35 millivolts, and the changes in
pressure at the input have no effect because the amplifier is in
saturation. You could use a higher supply voltage for the op amp (9
volts?), or choose a different op amp that has "rail-to-rail" output.
-- The SX15A has an "offset" of tyically +/- 20 millivolts at zero
pressure, and that must be added to the signal. Also, you want to be able
to set your "zero" value at some value you choose, like 10 PSI. You will
need to add an "offset adjustment" to your original circuit. Here is a
simple one:
10k pot, 15 turn "offset"
Vss
/\/\/\
Vdd=5.0 volts regulated
|
| 100k,1%
'---/\/\
to +V1 (~4k impedance)
Start with the trimmer in the middle of its range. If you monitor the
differential voltage (V2-V1) on a multimeter, you will see that adjusting
the potentiometer trims the differential input. At the output of the
amplifier, you should see that you get a good signal within the 0--3.5 volt
range, when the input (V2-V1) is in the range of 0 to about 35 millivolts.
You will want to calibrate it so that the output is 0 when the pressure is
10 PSI (offset adjustment) and the output is 3 volts when the pressure is
15 PSI (gain adjustment). The offset circuit can be improved by putting
stable resistors in series with the 10k pot, once you know what range of
adjustment you need.
Also check out the application notes at Sensym. There are some barometer
circuits there. Also, check out Matt Parnell's site,
<http://www.weatherstamp.com/>
regards,
-- Tracy Allen
Electronically Monitored Ecosystems
http://www.emesystems.com
Comments
Thank you very much for your good advices: I have now several things to try.
May be in a couple of days (I have not too much time this days), I´ll make
new questions, or I will comment about good results...
Best regards
Jorge
Original Message
From: Tracy Allen <emesys@c...>
To: <basicstamps@egroups.com>
Sent: Monday, July 31, 2000 2:25 PM
Subject: [noparse][[/noparse]basicstamps] Two questions about Op Amp4s
> "jferrero" <jferrero@u...> wrote:
> >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
> >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...
> >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?
>
>
> Hi Jorge,
>
> Your amilifier has a minimum gain of x101, when the 2k potentiometer is
> turned to its full resistance. That means that it only takes a 49.5
> millivolt change at the input to make the output swing a full 5 volts.
To
> start with, be sure that gain set for its minimum value, that is, the
> maximum value of the potentiometer. To help you "find" the signal, you
> might want to start with a lower gain, say by putting a 1k resistor in
> series with the 2k potentiometer and turn the gain down to x70. Also, be
> sure that you have the + output of your sensor connected to the + input of
> the differential amplifier, (+V2 in your circuit).
> -- The LM324 can only swing up to about 3.5 volts (as Al pointed out).
> The input signal is probably greater than 35 millivolts, and the changes
in
> pressure at the input have no effect because the amplifier is in
> saturation. You could use a higher supply voltage for the op amp (9
> volts?), or choose a different op amp that has "rail-to-rail" output.
> -- The SX15A has an "offset" of tyically +/- 20 millivolts at zero
> pressure, and that must be added to the signal. Also, you want to be able
> to set your "zero" value at some value you choose, like 10 PSI. You will
> need to add an "offset adjustment" to your original circuit. Here is a
> simple one:
>
> 10k pot, 15 turn "offset"
> Vss
/\/\/\
Vdd=5.0 volts regulated
> |
> | 100k,1%
> '---/\/\
to +V1 (~4k impedance)
>
> Start with the trimmer in the middle of its range. If you monitor the
> differential voltage (V2-V1) on a multimeter, you will see that adjusting
> the potentiometer trims the differential input. At the output of the
> amplifier, you should see that you get a good signal within the 0--3.5
volt
> range, when the input (V2-V1) is in the range of 0 to about 35 millivolts.
> You will want to calibrate it so that the output is 0 when the pressure
is
> 10 PSI (offset adjustment) and the output is 3 volts when the pressure is
> 15 PSI (gain adjustment). The offset circuit can be improved by putting
> stable resistors in series with the 10k pot, once you know what range of
> adjustment you need.
>
> Also check out the application notes at Sensym. There are some barometer
> circuits there. Also, check out Matt Parnell's site,
> <http://www.weatherstamp.com/>
>
> regards,
> -- Tracy Allen
> Electronically Monitored Ecosystems
> http://www.emesystems.com
>
>
>
>
>
>> 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...
Buenos dias otra vez Jorge,
If you look in the spec sheet for the SX15A, you will read that its "output
impedance" is typically 4.1 kohms. That is the datum Ray and I needed to
calculate the potentiometer/resistor value needed to use to force +/- 50
millivolts of offset in the bridge. All the sensor is, really, is an
arrangement of 4 resistors, arranged (as a "bridge") so that physical
strain causes their resistances to change slightly. The data sheet shows
the "equivalent circuit" on the first page. See? It is just 4 resistors,
and the nominal value of each of those resistors is 4.1 kohms. The added
potentiometer and resistor forces one side or the other of the bridge to a
lower or higher voltage, to bring the differential output of the bridge to
near zero at your desired "zero point" pressure.
The disadvantage of this particular offset circuit is that it has a small
effect on the span also. That is, the offset adjustment and the span
adjustment interact slightly. More important is that this particular
offset circuit upsets the "temperature coefficient of offset". If you look
at the data sheet, that parameter is typically 4 microvolts per volt per
degree Celsius. Sensym has gone to a lot of trouble to make that a small
value in the design of the chip, and it is best to keep it small, so you
only have to deal with the "temperature coefficient of the span". The upset
will be larger as you go for larger values of offset, like, setting your
zero point for the barometer at 10PSI. Fancier circuits apply the offset
later on, after the first differential amplifier stage, so as not to affect
the "temperature coefficient of offset". Are you going to want your
barometer to be accurate (how accurate?) over a wider temperature range
(how wide?)?
> 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.
You uploaded your modified circuit, "DiffAmpl-2.JPG", to this list, right?
There you are using the LM334 current source to drive the sensor bridge,
for temperature compensation. The "temperature coefficient of span" is
large -2150 ppm per volt per PSI. That means that without compensation,
the full scale reading would change by about 0.215% for every one degree
change in temperature. The LM334 drives the sensor from a current source.
Why? Those 4.1kohm resistors that make up the sensor bridge have a
"temperature coefficient of resistance" of typically +750 ppm per degree C.
See that in the data sheet. By driving the sensor from a current source,
the voltage from the top terminal to the bottom terminal of the sensor will
increase with temperature. That counteracts the large negative temperature
coefficient of span. By adjusting the temperature coefficient of the LM334
current source, the circuit can acheive a zero overall temperature
coefficient of span at the output. I didn't check the values in your LM334
circuit. Did those come from an app note? I am wondering, because I am
suprised that the LM334/SX15A combination in series are being powered from
5 volts.
The fancier circuits (e.g. using the LM10 and LT1014) you see in the
Sensym app notes take all this stuff into consideration. The current
source drive is sometimes achieved by putting the SX15 in the feedback loop
of an op-amp, and the offset voltage is applied to the circuit at a later
stage, where it will not affect the temperature coefficient of offset.
Designing these bridge circuits for maximum accuracy is a highly
quantitative undertaking, and provision must be made to calibrate the
characteristics of each individual sensor.
You are almost there with your existing circuit. You have one more section
in your LM324 op amp. You could set that up as another differential
amplifier stage to follow the output of your existing circuit. (4-resistor
gain=1x) The + input comes from your existing Vout, and the - input comes
from a variable voltage (potentiometer). That would be better than the
original input side offset circuit.
I hope that helps,
-- Tracy Allen
Electronically Monitored Ecosystems
http://www.emesystems.com