Post by thread:OT: pressure sensor -> PIC

I guess I should've mentioned that I tried that. The voltage was at about 1v and would rise to about 2.5v. Proper output should be about 4.5v dropping to about 2.5v, but even reversed it still started at 1v. By "starting", I mean atmospheric pressure. Wagner Lipnharski wrote: {Quote hidden}> I think the subject is of general interest to the list. > Just reverse the polarity of the sensor output or the INA122 inputs. > Wagner. > > Greg Hartung wrote: > > > > I know this is way off topic, but it really has quite a bit to do > > with my last coupla posts... I am trying to connect a sensym baro > > sensor to an F877 thru a BurrBrown INA122 instrument amp, and it appears > > to inverting my voltage so that when pressure goes up, the voltage goes > > down. Anyone with any experience in this area could e-mail offlist... > > Thanks.

Greg, I've used SenSym's SCX series absolute sensors in a PIC-based barometer (interfaced to an AD620 IA). If you are using an absolute sensor, the polarity is reversed. As far as your results below, the output is biased between ground and your bridge excitation voltage or whatever the voltage is at the top of the bridge. The output increases with pressure. I would have to see your circuit to be of further help. - Tom At 08:16 AM 1/31/00 -0700, Greg Hartung wrote: {Quote hidden}> I guess I should've mentioned that I tried that. The voltage was at about >1v and would rise to about 2.5v. Proper output should be about 4.5v dropping >to about 2.5v, but even reversed it still started at 1v. By "starting", I >mean atmospheric pressure. > >Wagner Lipnharski wrote: > >> I think the subject is of general interest to the list. >> Just reverse the polarity of the sensor output or the INA122 inputs. >> Wagner. >> >> Greg Hartung wrote: >> > >> > I know this is way off topic, but it really has quite a bit to do >> > with my last coupla posts... I am trying to connect a sensym baro >> > sensor to an F877 thru a BurrBrown INA122 instrument amp, and it appears >> > to inverting my voltage so that when pressure goes up, the voltage goes >> > down. Anyone with any experience in this area could e-mail offlist... >> > Thanks. ------------------------------------------------------------------------ Tom Handley New Age Communications Since '75 before "New Age" and no one around here is waiting for UFOs ;-)

part 0 3563 bytes
x-html>    I finally got the AD working and the input from this circuit fluctuates 20% also.
   I can't believe I'm doing ASCII art, but I'm desperate...
   Thanks
   When I try reversing Out- and Out+, the output voltage still hovers around 1V from the amp at atmospheric pressure, but it won't go up or down under a vacuum.

                        47K
SX15             ___/\/\/\____
_______          |   _______   |
|   Out-|______   |__|Rg   Rg|__|
|   Vdd |__Vcc |_____|In- Vdd|____Vcc
|   Out+|____________|In+ Vo|____to PIC AN1
|   Gnd |____________|Gnd Ref|____
|_______|     |      |_______|    |
              |        INA122     |
             Gnd                 Gnd

Tom Handley wrote:

   Greg, I've used SenSym's SCX series absolute sensors in a PIC-based
barometer (interfaced to an AD620 IA). If you are using an absolute sensor,
the polarity is reversed. As far as your results below, the output is biased
between ground and your bridge excitation voltage or whatever the voltage is
at the top of the bridge. The output increases with pressure. I would have
to see your circuit to be of further help.
   - Tom
At 08:16 AM 1/31/00 -0700, Greg Hartung wrote:
>   I guess I should've mentioned that I tried that. The voltage was at about
>1v and would rise to about 2.5v. Proper output should be about 4.5v dropping
>to about 2.5v, but even reversed it still started at 1v. By "starting", I
>mean atmospheric pressure.
>
>Wagner Lipnharski wrote:
>
>> I think the subject is of general interest to the list.
>> Just reverse the polarity of the sensor output or the INA122 inputs.
>> Wagner.
>>
>> Greg Hartung wrote:
>> >
>> >    I know this is way off topic, but it really has quite a bit to do
>> > with my last coupla posts... I am trying to connect a sensym baro
>> > sensor to an F877 thru a BurrBrown INA122 instrument amp, and it appears
>> > to inverting my voltage so that when pressure goes up, the voltage goes
>> > down. Anyone with any experience in this area could e-mail offlist...
>> > Thanks.
------------------------------------------------------------------------
Tom Handley
New Age Communications
Since '75 before "New Age" and no one around here is waiting for UFOs ;-)

Tom Handley wrote: {Quote hidden}> Greg, I've used SenSym's SCX series absolute sensors in a PIC-based > barometer (interfaced to an AD620 IA). If you are using an absolute sensor, > the polarity is reversed. As far as your results below, the output is biased > between ground and your bridge excitation voltage or whatever the voltage is > at the top of the bridge. The output increases with pressure. I would have > to see your circuit to be of further help. > > - Tom > > At 08:16 AM 1/31/00 -0700, Greg Hartung wrote: > > I guess I should've mentioned that I tried that. The voltage was at about > >1v and would rise to about 2.5v. Proper output should be about 4.5v dropping > >to about 2.5v, but even reversed it still started at 1v. By "starting", I > >mean atmospheric pressure. > > > >Wagner Lipnharski wrote: > > > >> I think the subject is of general interest to the list. > >> Just reverse the polarity of the sensor output or the INA122 inputs. > >> Wagner. > >> > >> Greg Hartung wrote: > >> > > >> > I know this is way off topic, but it really has quite a bit to do > >> > with my last coupla posts... I am trying to connect a sensym baro > >> > sensor to an F877 thru a BurrBrown INA122 instrument amp, and it appears > >> > to inverting my voltage so that when pressure goes up, the voltage goes > >> > down. Anyone with any experience in this area could e-mail offlist... > >> > Thanks. > > ------------------------------------------------------------------------ > Tom Handley > New Age Communications > Since '75 before "New Age" and no one around here is waiting for UFOs ;-) Are you using an unamplified pressure transducer? If you are, you may be seeing a sensor offset effect. The Sensym SX series of transducer, as an example, have very large zero pressure offset voltages. For the SX15A (a 15PSI absolute sensor) the offset can be as much as -35mv. With a 3 volt power supply to the sensor the differential output voltage at 15PSI can be as little as 45mv! This is 1mv /volt of exitation /PSI. The symptoms that you described may be caused by the amplifier going into saturation caused partly by the offset voltage. To see if this is the case try injecting a current into one leg of the bridge. Adjust the current to bring the output of the amplifier to about 2.5 volts then see if the voltage changes in the expected manner when the pressure is changed -- Rich

Well, I found my first mistake. I trusted someone else's drawing. The datasheet calls for about a 5K not a 50K for the correct amplification of this baro sensor. Now to figure out why it's going the wrong way. I am sure if I keep reading. Thank goodness for Hooked on Phonics! Perhaps reversing the inputs would be more meaningful now. Alice Campbell wrote: {Quote hidden}> Date sent: Tue, 1 Feb 2000 11:18:22 -0700 > Send reply to: pic microcontroller discussion list <spam_OUTPICLISTTakeThisOuTMITVMA.MIT.EDU> > From: Greg Hartung <.....ghartungKILLspam@spam@ADT.COM> > Subject: Re: OT: pressure sensor -> PIC > To: PICLISTKILLspamMITVMA.MIT.EDU > > um, what happens when you connect vref on inamp to vcc instead > of ground?? > > alice > > > I finally got the AD working and the input from this circuit fluctuates 20% > > also. > > I can't believe I'm doing ASCII art, but I'm desperate... > > Thanks > > When I try reversing Out- and Out+, the output voltage still hovers around 1V > > from the amp at atmospheric pressure, but it won't go up or down under a vacuum. > > > > > > 47K > > SX15 ___/\/\/\____ > > _______ | _______ | > > | Out-|______ |__|Rg Rg|__| > > | Vdd |__Vcc |_____|In- Vdd|____Vcc > > | Out+|____________|In+ Vo|____to PIC AN1 > > | Gnd |____________|Gnd Ref|____ > > |_______| | |_______| | > > | INA122 | > > Gnd Gnd > > > > > > > > Tom Handley wrote: > > > > > Greg, I've used SenSym's SCX series absolute sensors in a PIC-based > > > barometer (interfaced to an AD620 IA). If you are using an absolute sensor, > > > the polarity is reversed. As far as your results below, the output is biased > > > between ground and your bridge excitation voltage or whatever the voltage is > > > at the top of the bridge. The output increases with pressure. I would have > > > to see your circuit to be of further help. > > > > > > - Tom > > > > > > At 08:16 AM 1/31/00 -0700, Greg Hartung wrote: > > > > I guess I should've mentioned that I tried that. The voltage was at about > > > >1v and would rise to about 2.5v. Proper output should be about 4.5v dropping > > > >to about 2.5v, but even reversed it still started at 1v. By "starting", I > > > >mean atmospheric pressure. > > > > > > > >Wagner Lipnharski wrote: > > > > > > > >> I think the subject is of general interest to the list. > > > >> Just reverse the polarity of the sensor output or the INA122 inputs. > > > >> Wagner. > > > >> > > > >> Greg Hartung wrote: > > > >> > > > > >> > I know this is way off topic, but it really has quite a bit to do > > > >> > with my last coupla posts... I am trying to connect a sensym baro > > > >> > sensor to an F877 thru a BurrBrown INA122 instrument amp, and it appears > > > >> > to inverting my voltage so that when pressure goes up, the voltage goes > > > >> > down. Anyone with any experience in this area could e-mail offlist... > > > >> > Thanks. > > > > > > ------------------------------------------------------------------------ > > > Tom Handley > > > New Age Communications > > > Since '75 before "New Age" and no one around here is waiting for UFOs ;-) > >

Greg, the first thing I notice is that you are using the SX series which is a basic sensor element and you will have a lot of work ahead of you to compensate and calibrate it. It also sounds like you don't have SenSym's data book which has a `wealth' of application information. First, order their free data book at: http://www.sensym.com/ I'm still not sure which version you are using. Is it an Absolute, Gauge, or Differential sensor? What is the part number? What is the application? One obvious problem from your schematic is that you are using a fixed voltage on the bridge with no temperature compensation. In this case, you should use something like an LM334 constant current source. The data book goes into this in detail. From the data book, the full-scale span, when using +5V, is 110mv with a zero-pressure offset of 0 to 35mv (20mv typical). As far as your IA gain setting resistor, a 50K resistor would give a gain of 9 and 5K would result in a gain of 45. Tell us more about your application. I would strongly recommend considering the SCX compensated, calibrated, series unless cost is critical. Also, take a look at Lucas NovaSensor's NPC-1220 series: Lucas : http://www.novasensor.com/ - Tom At 11:18 AM 2/1/00 -0700, Greg Hartung wrote: > Well, I found my first mistake. I trusted someone else's drawing. The datasheet >calls for about a 5K not a 50K for the correct amplification of this baro sensor. Now >to figure out why it's going the wrong way. I am sure if I keep reading. Thank >goodness for Hooked on Phonics! Perhaps reversing the inputs would be more meaningful {Quote hidden}>now. > > I finally got the AD working and the input from this circuit > fluctuates 20% also. > I can't believe I'm doing ASCII art, but I'm desperate... > Thanks > > When I try reversing Out- and Out+, the output voltage still hovers >around 1V from the amp at atmospheric pressure, but it won't go up or down >under a vacuum. > > > 47K > SX15 ___/\/\/\____ > _______ | _______ | > | Out-|______ |__|Rg Rg|__| > | Vdd |__Vcc |_____|In- Vdd|____Vcc > | Out+|____________|In+ Vo|____to PIC AN1 > | Gnd |____________|Gnd Ref|____ > |_______| | |_______| | > | INA122 | > Gnd Gnd > ------------------------------------------------------------------------ Tom Handley New Age Communications Since '75 before "New Age" and no one around here is waiting for UFOs ;-)

I am using an SX15A, which is an absolute, uncompensated. This is just for testing, I am sure I will go with a compensated DIP package like the SDX15A2 (the SCX are way too many $). So for now, I'm not worried about temperature compensation, but this is all for a rocket altimeter so it will become important later. I spent a LOT of time messing around with it last night. I finally ripped it out of the board with the PIC and put it in a separate board: just a 78L05, the INA122, and a 4.7K resistor for about 49x gain and a pot and resistor for messing around with the input voltage. Surprisingly, it worked fine, no reversal of inputs required. Then replaced the pot with the sensor: still works good. Output voltage is within a few milliamps of the calculated value. Put it all back in the PIC board (not even connected to the PIC, just sharing the same voltage regulator, really), and it exhibits the following: about .75v output from the INA122 no matter what I do (either the sensor connected, nothing connected to the INA inputs, or +5V/gnd). The INA was run straight to the voltage regulator, no daisy chaining. It got too late to try any more, and I was sick of wire wrapping. I'll try pulling the PIC out of its socket tonight and checking the current drain on the whole circuit to see if something's shorted or whatnot. I can't imagine the amp has to be decoupled very well. Maybe it's not getting enough current? Argggh. I have the sinking feeling this is gonna be an "I did something really stupid" story. And I might as well ask now: anybody know how to measure quickly changing temperature? I have a thermistor on an A/D port that is finally working beautifully, but a bit too slow in getting to the target temp. And for those who couldn't care less about the newbie's pressure woes: sorry 'bout the long winded message. Tom Handley wrote: {Quote hidden}> Greg, the first thing I notice is that you are using the SX series which > is a basic sensor element and you will have a lot of work ahead of you to > compensate and calibrate it. It also sounds like you don't have SenSym's > data book which has a `wealth' of application information. First, order > their free data book at: > > http://www.sensym.com/ > > I'm still not sure which version you are using. Is it an Absolute, Gauge, > or Differential sensor? What is the part number? What is the application? > > One obvious problem from your schematic is that you are using a fixed > voltage on the bridge with no temperature compensation. In this case, you > should use something like an LM334 constant current source. The data book > goes into this in detail. > > From the data book, the full-scale span, when using +5V, is 110mv with > a zero-pressure offset of 0 to 35mv (20mv typical). > > As far as your IA gain setting resistor, a 50K resistor would give a gain > of 9 and 5K would result in a gain of 45. > > Tell us more about your application. I would strongly recommend > considering the SCX compensated, calibrated, series unless cost is critical. > Also, take a look at Lucas NovaSensor's NPC-1220 series: > > Lucas : http://www.novasensor.com/ > >

First of all, analog electronics is a little bit more complicated and full of tricks than digital. Using an op-amp with a gain of 49 increase those factors. Dealing with a gain above 15 put your experiment inside a lion's cage. There are several factors (if not everything) that can make your life miserable. Small capacitances and resistances in protoboards can make your night a pure hell, with the devil being your personal technical support. What you did was a very smart thing, taking all of it out of the PIC board, so you isolated the problem. There are some professional boards that sometimes need to use an op-amp lead not soldered at the board at all, it is bent to the air with the component lead soldered directly to it, as coupling and feedback capacitors. During years (translate to "lack of money") I did prototypes just in air, all components soldered one to another in that great mess, without using any protoboards (solderless) or perforated boards at all. To change a capacitor in the middle of that "spider web" was a difficult task. One day I thought it was all wrong and I should invest in a nice and expensive protoboard... who remember the PB104? I did it. Today dealing with sensible analog electronics sometimes I need to go back to the air prototypes, since "air" is one of the best dielectric and isolation we can have. Your PIC board connections to this INA122 input pins should have some problems that is changing the chip input offset (externally)... try to lift the INA122 input leads carefully from the PIC board and solder whatever needs to be there directly to the leads, avoiding any contact with the board. All other INA122 pins would be doing contact with the board, of course. If this works nice, you get the proof that your board is not so innocent. Wagner Greg Hartung wrote: {Quote hidden}> > I am using an SX15A, which is an absolute, uncompensated. This is just for > testing, I am sure I will go with a compensated DIP package like the SDX15A2 > (the SCX are way too many $). So for now, I'm not worried about temperature > compensation, but this is all for a rocket altimeter so it will become important > later. > I spent a LOT of time messing around with it last night. I finally ripped it > out of the board with the PIC and put it in a separate board: just a 78L05, the > INA122, and a 4.7K resistor for about 49x gain and a pot and resistor for > messing around with the input voltage. Surprisingly, it worked fine, no > reversal of inputs required. Then replaced the pot with the sensor: still works > good. Output voltage is within a few milliamps of the calculated value. Put it > all back in the PIC board (not even connected to the PIC, just sharing the same > voltage regulator, really), and it exhibits the following: about .75v output > from the INA122 no matter what I do (either the sensor connected, nothing > connected to the INA inputs, or +5V/gnd). The INA was run straight to the > voltage regulator, no daisy chaining. > It got too late to try any more, and I was sick of wire wrapping. I'll try > pulling the PIC out of its socket tonight and checking the current drain on the > whole circuit to see if something's shorted or whatnot. I can't imagine the amp > has to be decoupled very well. Maybe it's not getting enough current? > Argggh. I have the sinking feeling this is gonna be an "I did something > really stupid" story. > And I might as well ask now: anybody know how to measure quickly changing > temperature? I have a thermistor on an A/D port that is finally working > beautifully, but a bit too slow in getting to the target temp. > And for those who couldn't care less about the newbie's pressure woes: sorry > 'bout the long winded message.

I have been using a Motorola MPX4115 which is a compensated absolute pressure sensor. You feed it +5v and get an analog output that has a small equation to related it to barometric pressure. No OpAmps, no calibration. Unfortunely, the Motorola web page for documentation of this part is down at the moment. The web page for the part is ebus <ebus> .mot-sps.com/ProdCat/psp/0,1250,MPX4115A~98716,00.html#documentation A quick look on Newark Electronics web site has the part from $16 to $22 each. David V. Fansler Network Administrator TriPath Imaging, Inc. (Formerly AutoCyte, Inc) 336-222-9707 Ext. 261 .....dfanslerKILLspam.....autocyte.com <EraseMEdfanslerspam_OUTTakeThisOuTautocyte.com> Now Showing! http://www.mindspring.com/~dfansler <http://www.mindspring.com/~dfansler> Updated December 16, 1999 Ann's Cancer & David's Observatory {Original Message removed}

Well, I connected the pressure circuit back to the PIC board, and it immediately stopped working correctly so I started pulling things until it did work. This board has an LED, a piezo, a 7805, a 16F877, a 20Mhz ceramic resonator, a relay and MAX232. Removing the MAX232 did the trick, but the MAX isn't connected at the other end during this problem, only to the PIC and the MAX works just fine when it's in the board. I reread the MAX datasheet, and I don't see any glaring mistakes. I have absolutely no idea where to go next. > Your PIC board connections to this INA122 input pins should have some > problems that is changing the chip input offset (externally)... try to > lift the INA122 input leads carefully from the PIC board and solder > whatever needs to be there directly to the leads, avoiding any contact > with the board. All other INA122 pins would be doing contact with the > board, of course. If this works nice, you get the proof that your board > is not so innocent.

My guess: a) Max232 has some unused pin, so it is floating causing lots of noise at the Max unused output, so to the supply. b) There is a wrong connection at the Max, even that it is working, as for example, a wrongly grounded pin. Read here also wrong capacitors...and connections c) Missing 200µF + 0.1µF capacitors at the +Vcc d) Missing a 10 Ohms resistor in series to the INA122 Vcc with an extra decoupling 10µF capacitor at the INA122 side... filtering noises... e) What is the voltage *before* the 7805? Isn't that too marginal close to the 5V? (when you remove the max, reduce consume, 7805 input voltage goes a little bit up and it regulates ok. f) Try to feed power to the Max from any other +5V except from this 7805. g) Max generates some oscillation noise from its charge pump, but not enough to create problems to the INA122, except if you have a terrible grounding problems. Try to route another ground wire from Max to PS (PowerSupply), another from the PIC to PS and another from the INA122 to the PS. Avoid to use "Bus Ground Sytem", go to a "Star Ground System" (single ground point for analog and another single ground point for digital, then a single wire connecting both grounds). we are starting to get curious now.. <g> Wagner. Greg Hartung wrote: > > Well, I connected the pressure circuit back to the PIC board, and it immediately > stopped working correctly so I started pulling things until it did work. This board > has an LED, a piezo, a 7805, a 16F877, a 20Mhz ceramic resonator, a relay and MAX232. > Removing the MAX232 did the trick, but the MAX isn't connected at the other end during > this problem, only to the PIC and the MAX works just fine when it's in the board. I > reread the MAX datasheet, and I don't see any glaring mistakes. > I have absolutely no idea where to go next.

Greg I had forgot about the SDX series and that would be a better choice over the SCX. For a model rocket altimiter, you might want to check out Lucas NovaSensor's NPP-301-100A which is a 15 PSI absolute sensor in a SO-8 package. It's low cost and the temperature compensation typically consists of two resistors. There is no pressure port (just a hole in the package) so the chip will fit in a very small space and it weighs only 0.10 grams. Lucas sent an application kit and I made a simple barometer. Though not as accurate as the SCX/SDX or NovaSensor's NPC-1220 series, the results were still very good. They have two related application notes: Basic Temperature Compensation for the NPP-301 Designing an Altimeter using the NPP-301 I also checked out Motorola's MPX4115 series that David Fansler mentioned. This seems ideal if the cost is right. You give it +5V and you get a conditioned voltage output from ~0.5V to 4.8V. Back to your current `woes', I'd recommend selecting the sensor you are going to use in the altimiter, breadboard the circuit and test it with a DVM. Once your are satisfied with the results, then go on to the A/D interface. It sounds like you are going around in circles with the basic SX element considering the calibration and compensation issues. As far as why the circuit stops working with the PIC hooked up, I've been following this but I don't have a clue at this point... - Tom http://www.novasensor.com/ http://mot-sps.com/products/sensors/ At 09:35 AM 2/2/00 -0700, Greg Hartung wrote: > I am using an SX15A, which is an absolute, uncompensated. This is just for >testing, I am sure I will go with a compensated DIP package like the SDX15A2 >(the SCX are way too many $). So for now, I'm not worried about temperature >compensation, but this is all for a rocket altimeter so it will become important >later. ------------------------------------------------------------------------ Tom Handley New Age Communications Since '75 before "New Age" and no one around here is waiting for UFOs ;-)

Well, it looks like I've learned a little about decoupling caps. A .1u at the MAX improved things, but a .1u at the INA was like pure magic. Thanks for everyone's help. I guess the irony is that I've decided on the MPX chip which negates the need for an amp, but I think the lesson here was almost worth the pain. I must use my scope more! I am curious if anyone could explain what kind of cap would be best for decoupling (electrolytic, etc.) Most drawings I see seem to freely interchange polarity sensitive and insensitive caps for decoupling... Does anyone know a source for the MPX4115ASX? I tried Arrow, Newark, Digikey and FutureActive. Some stock some of the other MPX's but not this one. (I sure can pick 'em, eh?) Also, anyone know where I can get a 16C774? Everyone seems backordered... And lastly, for those interested, I called Microchip, and the 16F628 should be available in samples in April and to our doors in June. {Quote hidden}> Greg I had forgot about the SDX series and that would be a better choice > over the SCX. For a model rocket altimiter, you might want to check out > Lucas NovaSensor's NPP-301-100A which is a 15 PSI absolute sensor in a SO-8 > package. It's low cost and the temperature compensation typically consists > of two resistors. There is no pressure port (just a hole in the package) so > the chip will fit in a very small space and it weighs only 0.10 grams. Lucas > sent an application kit and I made a simple barometer. Though not as > accurate as the SCX/SDX or NovaSensor's NPC-1220 series, the results were > still very good. They have two related application notes: > > Basic Temperature Compensation for the NPP-301 > Designing an Altimeter using the NPP-301 > > I also checked out Motorola's MPX4115 series that David Fansler > mentioned. This seems ideal if the cost is right. You give it +5V and you > get a conditioned voltage output from ~0.5V to 4.8V. > > Back to your current `woes', I'd recommend selecting the sensor you are > going to use in the altimiter, breadboard the circuit and test it with a > DVM. Once your are satisfied with the results, then go on to the A/D > interface. It sounds like you are going around in circles with the basic SX > element considering the calibration and compensation issues. As far as why > the circuit stops working with the PIC hooked up, I've been following this > but I don't have a clue at this point... > > - Tom > > http://www.novasensor.com/ > http://mot-sps.com/products/sensors/ > > At 09:35 AM 2/2/00 -0700, Greg Hartung wrote: > > I am using an SX15A, which is an absolute, uncompensated. This is just > for > >testing, I am sure I will go with a compensated DIP package like the SDX15A2 > >(the SCX are way too many $). So for now, I'm not worried about temperature > >compensation, but this is all for a rocket altimeter so it will become > important > >later. > > ------------------------------------------------------------------------ > Tom Handley > New Age Communications > Since '75 before "New Age" and no one around here is waiting for UFOs ;-)