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Topic: Problem with amplifying a pressure sensor for measuring blood pressure (Read 6 times) previous topic - next topic

Papa G



Don't trimpots usually have much higher tolerances and produce noisier signals?


Trim pots of the 10 and 20 turn variety are often used in precision high quality instrumentation circuits for calibration purposes. One trick is to not let the pot control a larger range then it needs to be, so if you need a +/- 5 percent range of adjustments you might use say a 100 ohm 20 turn trim pot that has it's end terminals wired to 1% 10K fixed resistors. That way any drift in the pot due to tempco would have a much smaller effect in the overall circuit compared to just using a 20K ohm trim pot.

Lefty


Quite true. Also as microcomputers and SOC's get cheaper and cheaper, nowadays it is becoming quite common to use a fixed resistor and do the calibration in software during final test of the instrument. When it's time for a periodic instrument calibration, if anything changes, the calibration values are simply updated in EEPROM or whatever.

retrolefty

Quote
Is that more clear?


I always got it. Calibration/tweaking can be done in software or externally via hardware adjustments. Depends on the application and users requirements. If it's for one's own project then software is cheaper as one has the freedom of recompiling the sketch at any time when external things are changed. If however the project is designed to support changes in sensors or other external changes by users of the project then external calibration might be more desired. At the refinery I worked at we used thousands of sensors wired into central control systems, and we always required that any sensors used had the means to calibrate them externally so that we did not have to have 'custom' loop calibrations for every sensor wired to the central systems.

Different strokes for different folks, there is no single "best method", just what works best for you.

Lefty

Papa G


Quote
Is that more clear?


I always got it. Calibration/tweaking can be done in software or externally via hardware adjustments. Depends on the application and users requirements. If it's for one's own project then software is cheaper as one has the freedom of recompiling the sketch at any time when external things are changed. If however the project is designed to support changes in sensors or other external changes by users of the project then external calibration might be more desired. At the refinery I worked at we used thousands of sensors wired into central control systems, and we always required that any sensors used had the means to calibrate them externally so that we did not have to have 'custom' loop calibrations for every sensor wired to the central systems.

Different strokes for different folks, there is no single "best method", just what works best for you.

Lefty


When I first read this thread, I wondered if I had the means to calibrate a 0 - 37 kPa transducer. It turns out my smaller Chandler deadweight tester will actually go down that low. I wouldn't be able to match the exact full scale, but 1 psi increments would probably give a good enough calibration.

oscarcar

Didn't realize the thread was continuing. I have been busy reading some books and trying to understand electrical circuits and how to do the current source.

I'm working with an LM317T, but I'm having trouble with the basics. I thought I understood what was going on, but I don't.
V=IR, so since the IC is maintaining 1.25 volts across the "output" and "adjust" pins, I can use a 12.5kOhm resistor.

Oddly, I never get a 1.25V reading across the output and adjust pins.  Obviously I'm missing something.
If anyone wants to take a look, this is my understanding of the circuit diagram that I'm trying to duplicate in the breadboard. Attached pdf.

But I am having many difficulties with the basics. The voltages at the output and adjust seem to be zero, with both being about a 0.5 volts less than the input with reference to ground.

I am also having difficulty measuring current. I setup an LED so that I make sure that the current was flowing thru the multimeter, but when I connect the multimeter in series the LED goes off.
My multimeter is very confusing in regard to units of current (uA/mA/A), and seems unstable. But that's probably cause I'm doing something wrong if I can't get current to go through it.

And when I plug the circuit into a higher voltage power source (2.5V, 3.3V) the LED gets brighter, which would seem to indicate more current and thus not working as a constant current source. As I think LEDs get brighter on current and not voltage. When I plug in a 5V power source it starts fading and getting erratic so I guess that voltage is nearly out of range for the LED.

I did manage to get IC fairly warm when I plugged stuff together in certain ways so I assume it does work.

Oh, and yes at some point I can calibrate the sensor. But I still likely need a constant current source so that the calibrations don't change over time.

dc42

If you look on page 40 of the sensor datasheet, you will see the arrangement they suggest for the constant current supply, using an op amp and three resistors.

Forget the LM317T, its adjustment pin current is around 50uA, so it's completely useless as a 100uA constant current source.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

oscarcar

OK, I'll do that. But I'd like to figure out why this circuit is not behaving like I expect. It bugs me that I can't even get 1.25 volts that this IC is supposed to maintain across those 2 pins. I'd like to see if I could get something like 1 mA or something from it.
If someone can tell me that the breadboard looks OK that would be very helpful. At least I would know if I'm understanding the circuit diagrams. Thanks for any help. I think I'll go read some of the general electronics post and see if I can feel comfortable with other circuits and their discussions.

emailcausey

I am using a similar setup. I have a voltage regulator (TI LP2950) going to a resistor (33kohms) and then to the Omron SMPP-O2 sensor pin #6. I set the connections exactly as was posted in this forum. I plan to send Vout(+) and Vout(-) to an opamp, but first I need to get the pressure sensor to work!

I measure Vout(+) and Vout(-) using a voltmeter and I always get a very small reading (~0 mV). This concerns me since oscarcar said that he got -4mv with a null signal. I also have the pressure valve attached to a pressure cuff and when it is inflated the value that I measure with the voltmeter Vout(+) and Vout(-) does not change. I have replaced the first pressure sensor with another. That did not correct the problem. Any advice would be appreciated.

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