currently working on builiding Blood pressure monitor and currently working on pressure sensning circuit.
I am using MPX2050GP pressure sensor to sense the blood pressure from cuff wrapped around the arm. pressure sensor based on the pressure applied to it gives an output in millil volts(around 8-10mVolts- based on pressure). My task is to amplify that signal using AD622 amplifier. I connected my milli volt output from pressure sensor and got it directly into input of the amplifier. It sounds very simple but i am facing few problems with it.
I set amplifier to amplify signal 100 times. When i check my output on the scope it only shows some DC crap on the signal.I need a clean AC signal whos AMPLITUDE would vary with change in pressure on the sensor.
Would appreciate ur suggestion and help. project is due in weeks time.
I am also interested in the concept of how this mechanism will find blood pressure from the signals measured.
The versions I am familiar with, use a combination of a slowly changing cuff pressure (and a low pass filtered measurement of that cuff pressure), and an acoustic pickup that listens for the opening and closing of an artery that is being collapsed by the cuff pressure and forced open by blood pressure (mimicking what a human would do with a cuff and stethoscope).
Hi, John. I'd guess he's trying something like the BP setup shown in Motorola/Freescale AN-1571, "Digital Blood Pressure Meter"
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If he is, the OP has chosen unwisely in using the MPX2050 instead of the MPX5050 shown in the app note. The -2050 has a 0-40mV differential strain gage-type output (unbalanced, some output impedance), whereas the -5050 has a built-in buffer/amplifier which gives an 0.2-4.7V single voltage output for the same range. Sadly, it's likely none of this would have been necesssary if he'd gone with the right part. And amplification of this signal buried in the "DC crap" is going to be much more difficult from farther away.
According to the appnote, the OP would be trying to find a buried signal of approximately 1Hz which is around 120uV to 360uV in amplitude.
Possibly the OP should be a little more forthcoming with what he's doing, and also take the time to describe what he's doing as far as filtering -- which is the technically significant part on the analog end. If he were to post a schematic on a.s.b.e or do an ASCII sketch here, that might be helpful, too.
Apart from suggesting that he looks like an excellent candidate for a PHB, I'll refrain from editorial comment on the original post. Blood pressure, you know. ;-)
John i am working on oscillometric technique were in the cuff pressure is measured.for the first semester my lecturer wants me to capture output from pressure sensor circuit to view the pulse component(Systolic and diastolic points in BP). He wants me to use DSPACE card to capture the pulse component from output from pressure sensor!
Chris i used MPX 2050GP as UNi gave it to me for free, i agree with u that output is max 40mv but i dont have any other option. I appreciate ur suggestion and will try get hold of 5050.The filter i have designed is a 1st order RC network low pass filter using 741 op amp. Havin operationg frequency from DC to 50 hz. is that filter good enough chris?or i should use 2nd order filters. I agree with u, i am finding it difficult to get my AC signal. any suggestions for that?
"excellent candidate for a PHB". Sorrry guys i didnt quit understand that. Did i post something which i shoudnt be posting on the forum?
Will await ur valued suggestion! Thanks once again guys!
Hi, Priyank. I guess you've got a bit of a conundrum here. Sorry about the brusque answer before -- some students just want others to do their work for them (like future PHBs). Hearing your explanation, it just seems you're stuck. My bad.
Believe it or not, some fossils read TXT as not even taking the time to bother to express yourself clearly, with all that implies. Odd, but true. I guess we'll both have to deal with it.
You are actually heading in the right direction if you' *have* to use a
-2050. You'll want to use an instrumentation amp to amplify your
0-40mV differential sensor output signal into a 0-4V signal. But I'm not sure the AD622 is the best choice. You've got power-up considerations, and having the possibility of a negative input voltage present at the input to your card and the filter might mean problems down the line, if they can't handle negative voltages. The thing is, you probably don't need to get the output of your instrumentation amp right down to zero, anyway (if someone's BP is zero, they've got more important things to worry about, like dying). It might be better to use a single supply instrumentation amp like the LT1789-1 rather than the AD622. The output can go to within a few mV of GND with a single supply if your card can't handle negative input voltages. Since the LT1789-1 can handle an indefinite short circuit, all you have to do to protect the uC and filter input is put a 4.7V zener across the output, and you're good to go. Choose a resistor for appropriate gain, and you'll have pretty much an exact duplicate of the other sensor. (Go to the Linear.com site and download the data sheet. While you're there, download AN-43, "Bridge Circuits". You'll be glad you did.)
The sensor uses a recommended 10V supply, so you could go with
3-conductor shielded wire to the cuff (+10V, GND, and the IA output signal). Be sure to use good grounding practices, and ground the shield. Noise is going to be the real issue in this little toy.
If you go with the AD622, you'll need an extra line for a -10V supply, and you should place a reverse-biased shottky in parallel with the zener if your card can't handle negative. Again, the IC is short circuit protected. (Download AN-43 at Linear anyway).
Once you get past this point, you can pretty much follow the Freescale appnote on the filter. Read AN-1571 carefully. The herbs, the spices
-- it's in there. They explain what they did, and also why the did it. Throw an LM324A into the breach, and just use the filter in Fig. 1. If your card can handle negative voltages, you don't have to worry about the two biasing resistors for the Voffset -- just use GND and your venerable LM741, if you must. Since the oscillations you're looking for aren't that amplitude specific, you don't have to worry about offset or temp drift on the filter section.
You'll have two card inputs, and you'll have to scan assiduously at the filter/gain block input to get the small heartbeat oscillations you're looking for. You may have to throw some maths in to your program to get post-filtering to nail down the systolic and diastolic points. Residual noise is the *big* problem here.
If your results are deficient or inconsistent, once you've bashed your head on shielding, grounding and the inate orneriness of small signals, you may want to look at additional filtering, particularly for
50Hz/60Hz line frequency noise (this misery may actually be the object of this lesson). Look at the normalized graph of Fig. 2. You've got enough gain, but you may have problems with getting a clear enough signal (you should see something significant from the output of the circuit in Fig. 1 when you use the BP cup and are between systolic and diastolic points, or your sensor might be dead). The frequency of interest is in a low enough and narrow enough bandwidth that there are several things you can easily do. Feel free to post again for more advice on filtering line frequency noise and improving power supply rejection.
To give a better presentation, you might want to include a factory cal routine to provide a reality check on the pressure measurement. Using good components, it shouldn't be too difficult to get something that will pretty much correspond to the drift specs of the sensor. I would guess for class you're using a standard BP cup and teeing off the manometer fitting, so you might just want to factory calibrate to 200mm on the dial of the sphygmomanometer rather than using a tweaker pot to get it right. The gain pot will ruin your precision and your drift spec, anyway.
I spent 5-6 hours trying to get the thing workiI am sorry to the forum if you guys thought i was trying to get my homework done. Trust me that was not the case. Its just that i am frustrated with this silly little thing and i am not sure whats wrong with my circuit! ng and get rid of noise and get clean signal!
I have now decided to chuck mpx2050gp out and get ASDX015D44R, its gona cost me fortune but i wannt this thing working.
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Guys u wannt believe but in AUstralia i am not able to find mpx5050gp and neither the amplier u suggested.
Dspace card does take in negative supply but then to I will surely get rid of AD622 and get a single supply amplifier as per ur suggestion.
I have bought the LM324 as per the apllication notes and will get the filter soldered on veroboard tonight.
WIth the filter i surely agree have lot of work needs to be done! I am really confused about what filter would be best for my circuit!Ill again go through the textbooks today
I have got the pictures of th output waveform which i am getting on the scope(i am first trying to check circuit on oscilloscope) and ill try post the link for pics within next 5 hours.(lol... it will atleast prove im trying and not trying to get my homework done).
Chris i appreciate ur help and not even my best mates would have helped me so much. (sorry bout my spelling errors!)
Hi, Priyank. OK. I'm assuming you're in an electronics course of some kind, but I might be wrong. If you wouldn't mind, could you mention whether you're in a trade school or college, you are an electronics, computer programming or biology major, what's this class, and what level class this is.
Here's the deal. If you put something together on a board and plug it in, it *always* works. If flames are erupting from your board, if power is hooked up backwards, if you've got everything miswired, it's still working -- it's still doing exactly what you made it do. You may have wired it up wrong, you may have hooked up power backwards, but it's just obeying the laws of physics and the way the chips and components are designed.
If your circuit isn't doing what you want it to do, you might not understand what it's made to do. You might not have followed the correct procedure to get it to operate properly. You may not really understand what you want. But the problem is you, not the silicon or wire or metal film or electrolyte and foil, even if there's smoke pouring from your breadboard. Especially if there's smoke pouring from the breadboard.
You should be able to cobble together something that works fairly quickly. You have the sensor and the ICs right now to make something happen now. An AD622 is a great IC -- it just happens to need dual power supplies, which might not even be a problem for you. Wire up your AD622 with the gain set resistor on a small piece of perfboard, and mount it right next to the sensor (less than an inch away). Now the venerable LM741 is easily capable of doing the job of the LM324 on the other side of the 4-conductor wire (near your DAC board) if you have dual power supplies. Look here:
Your basic cut 'n' paste circuit can be logically divided into blocks. Look at block 1. It's the sensor. It gives a 40mV differential output for 0-50kPa. OK. Disconnect the outputs from everything else, power it up, and then just put a DVM across the inputs while you apply some air pressure. If you see a millivolt signal, at least you know it's kind of working. If not, you may likely have smoked it, either now or before. If these are school samples, they may have been donated from a broker who had problems with them and decided to get a tax writeoff for the lot -- it's conceivable you got the one bad one in the lot. No output? Get another one.
OK. Now look at block 2, the IA. You've chosen a resistor for a gain of 100 (0-40mV to 0-4V). Does it work? You can reconnect the sensor if you want and apply that pressure again, or you can just ground one input and apply a millivolt signal to the other (if you don't have anything handy, a "D" battery, a series resistor and a pot will make a good one). Apply a 30mV input. Do you get a 3V output? Does it do what you think it should do? If not, feel/smell for a hot IC, recheck your wiring, look at every conceivable way you could have messed it up before you conclude the IC is bad. If it is, you probably smoked it before. Get another one. Use your meter. Use your head.
Now look at the filter section. It blocks DC. It lets AC pass. If you don't have a function generator or any other lab equipment, just hook up a low voltage transformer, and use a series resistor and a pot to apply a 60Hz signal of about 2mV at the input of the filter. If you wired it up right, you should see a large amplitude 60Hz signal at the output. If you look at the app note, it says the Fig. 1 filter is amplifying the millivolt signal up to tenths of a volt, so you should see some good gain here.
Once you've gotten to this point, disconnect the sensor again, and ground one side of the IA. Apply a 30mV DC voltage with a 1/4mA AC signal riding on it (battery, transformer, series resistors will do fine -- just apply your basic DC circuit theory you must have had sometime before you got into this class). Look at the output of the IA and the output of the filter amplifier. Do they work together?
This is a basic reality check on your stuff -- is it smoked, or does it work. If I had your circuit on a board in the lab, I'd do this first. With decent lab equipment, it wouldn't take more than a few minutes to find all this out.
You've gotta walk that long road alone. And you've gotta use the tools you have, including your head.
go to a hospital, cardiology or some bloodvessel department, ask for a dispossable pressure sensor, in quantities they cost about 5 to 10$, and they just are as good as the farnell stuff.
Forwarding link to pics which shows the output im getting from filter onto scope( showed GREEN picture output to my proffesor and he is not happy with output and wannts output similar to application notes).
regarding filter have decided to try one in the application notes.
I just have seven days to go, any more suggestion?
Once again thanks for ur support guys! I will surely have few sleepless nights ahead.
OK, Priyank. If you're saying you've got a week left, you don't have time to play. Don't buy anything yet if you've got split supplies and your DAQ board can handle negative voltage inputs. You don't have time to wait for parts. Change over to the filter/amplification block shown in the Freescale appnote, using your venerable 741. Put it all together, as shown in the block diagram above. Place the AD622 right next to the sensor. (By the way, use a couple of good high frequency tantalums to bypass the AD622 power supply at the IC, otherwise you're going to have noise problems big time.) Run through the above preliminary checks to make sure the blocks are working in a gross fashion, as shown above. Replace anything you smoked before. Get it all to the point that you're ready to start.
Now that you've done that (shouldn't take more than an hour or two if you've got access to split +/-10V supplies, but +/-12V or even a +/-15V supply will work for your sensor, Freescale says +16V is abs. max.), it's time to actually start working. Start by putting the BP cup on your arm, closing the valve, pumping it up to over 200, and then cracking open the valve so it will slowly decompress (should take about
30 seconds or so). Just feel what's happening with your arm as it decompresses.
You should be able to feel the pressure of your blood pushing slightly against the BP cup as it decompresses. The BP cup tries to compress your arteries. As your heart pumps blood through your arm arteries, they push back, trying to expand to accomodate the flow of blood. If the BP cup is pressing too much, there isn't enough blood pressure being generated by the heart muscle to open the arteries even at maximum pressure. If the BP cup is pressing too little, there isn't enough pressure to close the arteries at all, even inbetween beats.
That maximum point (the point where the artery is too compressed to allow blood flow at all) is called the systolic pressure. The minimum point (where the pressure is too small to cause constriction at all) is called the diastolic point. Between those two points, the vessel is pushing open during the high pressure points in the heartbeat cycle, and closing in the lower pressure parts. This expansion and contraction of your arm as blood is trying to flow is reflected in your pressure sensor, although it's not a very reliable effect. It's a very small effect (as shown in the appnote, about 1mm Hg to 3mm Hg, see p.2). As Mr. Popelish noted above, this method isn't used in better automatic sphygmomanometers, they'll use an audio pickup. It is useful for the "free blood pressure" displays you see in local drugstores that give our senior citizens something to obsess about while they're waiting for their meds. And the "oscillometric method" is kind of indestructible, although not extremely accurate.
But the effect *is* measurable. That's your job. If you want to do this, you have to take responsibility for acquiring that signal, using the sensor and the ICs and your DAQ board to do so. If they're working properly, you do have the tools to do the job. If you have any lab equipment (an oscilloscope would be nice, a DSO nicer, a function generator would also be helpful) it will make the trip easier, but you do have what you need. I could probably get it working with a cheapie DMM, power supplies, and a transformer and handful of resistors and caps, although I'd have a heck of a time troubleshooting or tweaking it.
Again, the appnote filter alone will probably not be sufficient to do the job well. You'll need either a programming algorithm or additional filtering to get rid of power supply and line frequency noise. Depending on the acquisition frequency of your board and your software skillz, you may also want to add a peak detector to simplify the data acquisition software to a minimum.
But you can work on that later. Get to the starting line first by taking ownership of your project, really understanding what you need to do, and putting together the tools you need to do the job.
Everything I've described here up to this point could easily be prototyped by an engineering technician in an hour or two. It's not that hard.
By the way, if you want more help, I need to know something about your background to be able to answer usefully. Are you in a trade school or university? What year are you? If university/college, is this the senior project?
Chris ur explaniation was really helpfull and inspirational. I followed the steps u told me.
1stly started with the amplifier. Got a signal from Func. Generator and supplied 100mv-pk pk signal to amplifer @ frequency range from 1 khz-
1HZ. Amplifer works perfectly. Only thing was at frquency below 50 HZ output signal on the scope was flickering.
Further tested the filter. With the filter again supplied the same 100 mx-pk pk signal. The pin config in application notes is wrong. Pin 11 needs to be grounded and supply should be at pin 4. At 1khz signal i had no luck. WHen i went down to freqauency below 100 hz, output amplitude increased and gain was around 100-150. Onlt issue was the poutput i got was like triangular wave. If i pull out 330 microfarad cap output turns to be a squarewave.
Further tried combination of AMp and Filter. Agan had no luck at 1 khz input. When i decreased frequency below 100 HZ, ouput i got was like combination of sinewave and the triangular wave. Gain was around 200.
For noise at input i have put 22 microfarad electrolytic cap. EVen my professor said that noise is gonna be a big issue. I will try varioud stuff to get rid of noise.
WIth teh sensor i coudnt get hold of the professor. Today i will get a new one.
I have a feeling that something is wrong with my sensors. I belive that output signal from sensor is not being transfered to the input of amplifier properly. There is no rocket SCIENCE in this project. I am making some minor mistake.
No matter the cost i am gonna get this thing working!
Sometimes words are not enough to convey the thoughts and feelings, but thanks a lot chris.
Chris ur explaniation was really helpfull and inspirational. I followed the steps u told me.
1stly started with the amplifier. Got a signal from Func. Generator and supplied 100mv-pk pk signal to amplifer @ frequency range from 1 khz-
1HZ. Amplifer works perfectly. Only thing was at frquency below 50 HZ output signal on the scope was flickering.
Further tested the filter. With the filter again supplied the same 100 mx-pk pk signal. The pin config in application notes is wrong. Pin 11 needs to be grounded and supply should be at pin 4. At 1khz signal i had no luck. WHen i went down to freqauency below 100 hz, output amplitude increased and gain was around 100-150. Onlt issue was the poutput i got was like triangular wave. If i pull out 330 microfarad cap output turns to be a squarewave.
Further tried combination of AMp and Filter. Agan had no luck at 1 khz input. When i decreased frequency below 100 HZ, ouput i got was like combination of sinewave and the triangular wave. Gain was around 200.
For noise at input i have put 22 microfarad electrolytic cap. EVen my professor said that noise is gonna be a big issue. I will try varioud stuff to get rid of noise.
WIth teh sensor i coudnt get hold of the professor. Today i will get a new one.
I have a feeling that something is wrong with my sensors. I belive that output signal from sensor is not being transfered to the input of amplifier properly. There is no rocket SCIENCE in this project. I am making some minor mistake.
No matter the cost i am gonna get this thing working!
Sometimes words are not enough to convey the thoughts and feelings, but thanks a lot chris.
Hi, Priyank. I'd still like a bit of information on the class you're in -- it's hard to give practical advice without knowing where you're coming from. From the message string I'd guess you're enrolled at VU, but I'm not clear if you're in a TAFE sequence, the standard EE program, or a biomed undergrad.
Again, this isn't hard -- at least to get a start on things. You've got a sensor and two ICs, one mounted in the pressure cuff (IA) and the other mounted close to your DAQ board (filter/amplifier op amp). You've got the standard IA configuration for a gain of 100, and you can cut&paste the filter from Freescale AN-1571, Fig. 1 (V(offset) = GND if you're using a split supply and an LM741)
The question of electrical noise is always a tough one. There are many ways to work on this problem, which is central to a lot of biomed applications. From the way I read your post, it sounds like you've just hung a cap at the DAQ and filter/amp input -- I'd like to offer the suggestion that having the instrumentation amp drive a cap directly is just so wrong.
Let's count the ways this setup is asking for problems. To start with, while the AD622 is current limited and can handle an indefinite short circuit, it's not made to directly drive a capacitive load. You're almost guaranteed to have the IA oscillate like heck. It will also get hot with the current spikes while it's oscillating, which is going to degrade its performance pretty severely if it lives. Mercy on the poor Analog Devices chip? Sadly, no.
Now the power supply current is going to be bypassed at the IA by two tantalum caps. This, again, is a good thing. But you're also going to have the power supply current variations impressed on the GND line coming back to the DAQ board. If you're looking for a millivolt-level signal, this is a prescription for problems. Having the bypass caps there will just tend to even out the spikes, and also lower their frequencies, bringing them closer to your frequency of interest. This becomes a bad thing. It gets worse when you start looking at the poles in the feedback loop of your power supply ripple. This also increases the noise, and the chances of oscillation.
If I were doing this, the first thing I'd say is that you have to separate the power supply lines from the signal. That would mean a separate 3-conductor for the split supply, and a two-conductor twisted pair shielded for the signal. I would connect the two-conductor at the IA itself (pin 6 +, pin 5 -), and have that go directly to the DAQ and filter/amp board. At the filter board, I'd just use a load resistor at the input, and use a diff amp to separate out the signal itself from the noise generated by the power supply lines, having that output go to the Fig. 1 filter.
This should at least be a start on getting where you want to go:
Note that the twisted pair negative wire should be connected as close as possible to pin 5 of the AD622 IA. Of secondary importance would be keeping the trace between pin 5 and the power supply GND as low impedance as possible.
The diff amp prior to the filter will do the most to knock out noise (any antenna pickup should be impressed equally on both lines, and will be cancelled by the diff amp). The voltage drop caused by the 2.2K resistor is a linear function of the current, and while it will tend to attenuate the signal a little, it shouldnt affect the basic signal fidelity.
In the "real world", of course, this circuit has the deficiency of being pitifully unprotected from ESD and the other challenges of putting electronics in the hands of people. Op amp inputs and outputs floating in the breeze, indeed. But the main difference between this circuit and anything you'd make for real would be the utterly essential requirement of providing galvanic isolation between the patient and your electronics. That changes a lot.
Just for yuks, a biomed engineer would probably be looking at something like using a galvanically isolated DC-to-DC converter at the box end, and then use a precision optoisolated V-F conversion to read the gross pressure (DC level), and a linear isolation amplifier to cull the millivolt signal from the DC level. Added expense and complexity -- you can see how doing this stuff in a medical environment can add costs. Also, you would probably want a dedicated DSP to do post-filtering on the oscillometric information, which makes this a real project and a half. But if this were a free-standing box, the DSP could also do the pump control, user interface and display, too -- there's not much to do while the conversion is taking place, and many DSPs have ability to spare. And if that isn't enough, you'd have to look at radiated emissions (stringent medical requirements to avoid interfering with other kewl stuff).
Feel free to post again if you've got problems, but I'd guess that the WISER CHOICE setup above (cost: 1 additional op amp for the diff amp, and a second 2-conductor twisted pair shielded wire) should be pretty much what the doctor ordered, at least for your classroom setting.
I didn't mention that the easiest way to set up a diff amp is by using an instrumentation amp. If you have a spare AD622, you might want to do this (view in fixed font or M$ Notepad):
Use an LM385BZ2.5 for the reference voltage (helps keep the 33uF cap from being reverse-biased -- good tantalums will act funny when they've got AC across them). Or cobble together a 2.5V reference from whatever you have handy.
Now you literally have the whole thing. Cut & Paste this and WISER CHOICE above to Notepad, assemble it, and watch it run. I'll bet your noise issues will, at the least, be the best in the class. The programming is up to you (or at least, get somebody to give you the coding answers on another newsgroup).
I am enrolled in Victoria UNi, Melbourne in EE course!Majoring in Telecom, Control and EHV power!
When it comes to software application(matlab, controldesk, CAD, simview, programming -C++,java,VHDL ) i can beat any student in my class.I must confess i am not good with handling of hardware. Specially when it comes to the noise issue.
I hardly have any options left now--i surely do have spare AD622 and will try ur suggestion.
I now feel i have taken a wrong project, should have taken a software based project, Its basically a do or die situation now for me.
Actually, Priyank, considering you're doing a biomed project, it's
"For DUTY and HUMANITY!" -- Moe Howard, Larry Fine and Curly Howard, "Men in Black (1934)
Some hardware background is invaluable for a programmer.
Consider it like our Russian friends, who go into a steaming sauna, switch themselves with birch branches, then run out into the snow and jump in the frozen river.
"Comrade, why do you do such a crazy thing?"
"Because, comrade, it feels so good when it's over!"
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