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

oscarcar

I have a few 0.1uF caps. Will that work?

So where there is voltage output of the sensor, I bridge it to ground with a capacitor?
And where there is voltage output coming out of the opamp, I bridge that to ground too with a capacitor?

Docedison

If you are looking for a 2 terminal current source try a CC100 diode. It's a specially constructed Jfet that is a 2 terminal constant current diode (@ 100uA). It can be done with a Jfet and a resistor but I suspect that this is a little beyond your current grasp of theory. There are extensive design notes available in the App note but I doubt they are available any longer... Siliconix was one Mfr of CC diodes...
The app notes I refer to are from the National Semiconductor analog design app notes, Vol 2.

Bob
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

oscarcar

I'm sure you're right that I won't comprehend it, but I can get some assistance from others who do. So I'll look into it.

I do see this which may be what you were talking about:
http://www.vishay.com/docs/70596/70596.pdf

oscarcar

I think part of my noise problem is the resistor I'm using now for the instrumentation amplifier.

I don't know the tolerances on the one I have but it's probably not good. The one I was expecting to use, until it was apparent the output of the sensor was 10x what the datasheet said, was very good. Now I'll just need to find one with better tolerances.

I hooked up the resistor with lower tolerances and the signal looks pretty clean with me just blowing pressure in the tube to the sensor.

GoForSmoke

Words can be such a pain. Lower tolerance is the opposite of +/- smaller percent. Did you really mean that using a lower tolerance resistor made it work when the higher tolerance resistor did not? If so then the resistance you really want is not the one calculated but closer to what the LT resistor actually is.

A 5% tolerance resistor is higher tolerance than a 10% tolerance resistor. The higher the tolerance, the higher the accuracy must be to pass. It's intuitive if you make things to tolerances but otherwise not.

I think that they test the things every so many and if within 10% then that part of the run goes in the 10% bin even as the process is tweaked for higher accuracy. Still there's nothing saying that +/- 10% can't be within 1%, just that's not likely given consistent manufacture.

Nick Gammon on multitasking Arduinos:
1) http://gammon.com.au/blink
2) http://gammon.com.au/serial
3) http://gammon.com.au/interrupts

oscarcar

You're right, I worded it wrong.

I meant the opposite. The good resistor I have is 0.05%, and the other one I'm using I don't know. But I'm guessing probably 5 or 10%.

GoForSmoke

Your context pointed that way but hey, your thinking counts more than your nomenclature.

Nick Gammon on multitasking Arduinos:
1) http://gammon.com.au/blink
2) http://gammon.com.au/serial
3) http://gammon.com.au/interrupts

oscarcar

The signal is looking better with a better resistor for the instrumentation amplifier. I used a 10.2kOhm/0.1% tolerance. I attached a pdf of the pressure signals again.

The pictures are from 2 different trials.
Oddly, it does seem the device is altering the pressure a bit when it is detecting the systolic & diastolic blood pressure. Cause some noise-like signal appears where those pressures are in each trial.

GoForSmoke

I should have thought to mention, add a trim pot to the resistor and you have a chance to tweak the ohms.


Nick Gammon on multitasking Arduinos:
1) http://gammon.com.au/blink
2) http://gammon.com.au/serial
3) http://gammon.com.au/interrupts

oscarcar

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

GoForSmoke

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


Tolerance? They adjust. Noisy? I don't know, mine don't seem to be. Maybe they are -while- they're being turned. Otherwise get audio pots. The whole idea is to be able to tweak the resistance instead of picking the resistor closest to best, since they only make so many values of resistor.

I guess you could also put some fixed-value resistors in parallel to get a between value too. It'd be cheaper and noise-free.


Nick Gammon on multitasking Arduinos:
1) http://gammon.com.au/blink
2) http://gammon.com.au/serial
3) http://gammon.com.au/interrupts

retrolefty


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

Papa G

Is it really necessary for you to be able to tweek the gain of your instrumentation amplifier? It seems to me you just need to select a precision resistor that gives you the gain you need for the maximum signal level that doesn't exceed your ADC input range. Use the instrumentation amp's gain equation to get the actual gain that results from the value of the gain resistor you actually use.

I imagine you are balancing the transducer at zero pressure in software. I don't think you've talked about how you intend to calibrate it. Maybe the data sheet span voltage is good enough for your application, or maybe the transducer comes with a calibration sheet.

GoForSmoke

I
Quote
t seems to me you just need to select a precision resistor that gives you the gain you need for the maximum signal level that doesn't exceed your ADC input range.


So you can get non-standard value resistors?

Nick Gammon on multitasking Arduinos:
1) http://gammon.com.au/blink
2) http://gammon.com.au/serial
3) http://gammon.com.au/interrupts

Papa G


I
Quote
t seems to me you just need to select a precision resistor that gives you the gain you need for the maximum signal level that doesn't exceed your ADC input range.


So you can get non-standard value resistors?




Quote
It seems to me you just need to select a precision resistor that gives you the gain you need for the maximum signal level that doesn't exceed your ADC input range. Use the instrumentation amp's gain equation to get the actual gain that results from the value of the gain resistor you actually use.


Let me state it more simply. Decide what maximum gain you need, based on not exceeding your ADC's input spec. Solve the data sheet equation for the resistor value that will give you that gain. Select the standard value of the precision (1%, 0.1%, 0.05%, etc) resistor that is closest to the calculated value without causing the amplifier to exceed the maximum gain. Then, if you need the actual amplifier gain in your calculations, solve the data sheet equation for gain based on the resistor value you actually use. Is that more clear?




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