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Author Topic: Reading 0-100 mV signal  (Read 2525 times)
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Texas, land of the free, home of the stupid
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Dan,
Bear in mind that NOTHING discussed here will meet air worthiness certification ( even if assembled by an air mech).
Now given that and what I have read so far, I still feel that you can get a 'mostly' functional system working using just about any single supply op-amp in the non-inverting configuration.  I suggest that you put it together, apply the input to a meter and see what your results are. If you, indeed, get readings between 0 and something less than 5V (repeatable), then apply the output to an analog input pin and start fiddling with some of the analog sketches (like the tutorial for pots, only replace the pot to supply with thw sensor/amp output) and see what you can do with it.
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OK, it's been a while since I last posted--between getting parts and traveling, I haven't had much chance to work on this in the last month.  I got a 358, an assortment of resistors and capacitors, a 7810, and a 7805, and assembled it all on a breadboard this afternoon.  I used a 56K resistor rather than a 50K, but otherwise as directed here.  I powered the 358 from the 7805, and the sensor from the 7810.  I tied the sensor input and output grounds together.

Likely as a result of tying the input and output grounds together, I got some unexpected results.  Where previously the output of the sensor was 0 V with 0 psi applied, it became 946 mV.  Instead of increasing 1 mV/psi, it increased about 0.5 mV/psi.

The output of the 358 was steady at 3.57V.  I suspected this was as close to Vcc as its output would go, so I tried powering it from the 7810 instead.  The results from that confused me.  With 0 psi and an input voltage of 946 mV, the output was 1.834V.  With 20 psi and an input voltage of 959 mV, the output was 1.826V.  At that point I stopped the test.

I understand that this wouldn't meet any kind of certification--this is for an experimental application, so that isn't a problem.

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Lacey, Washington, USA
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Schematic?
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Steve Greenfield AE7HD
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the LM358 is wired per 123Splat's reply #3 on the first page--I'll see if I can draw it up tomorrow.
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Lacey, Washington, USA
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I'm not going to try and parse all that; besides, what we need to know is how -you- parsed what he said.
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Wondering if a ADC is something that is worth considering here... John Boxall in his tutorial series gives example of a 16 bit DAC which in single-ended mode measures between 0 and 2.048V. For 100mV range you'll be looking at 1600 steps. Thats with a single piece of hardware (4 pieces of hardware if you'll decide to put in pull-up resistors and a filter capacitor).
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Narvik, Norway
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Can you determine if there is a Wheatstone bridge inside?  (measure resistance beteween all terninals)
If there is .. it can probably work with 5V as well - and scaling of output be done with an instr.amp or dual rail-to rail opamp
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Hi, I'd say that the sensor is definitely a bridge circuit, so a differential amplifier will need to be used, to measure the difference between the two outputs and amplify to the 1.0V requirement.

http://www.ti.com/lit/ds/symlink/lm158-n.pdf

This data sheet has a couple of suggestions for LM358 in diff mode.

Tom...... smiley
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a DMM measuri
OK, it's been a while since I last posted--between getting parts and traveling, I haven't had much chance to work on this in the last month.  I got a 358, an assortment of resistors and capacitors, a 7810, and a 7805, and assembled it all on a breadboard this afternoon.  I used a 56K resistor rather than a 50K, but otherwise as directed here.  I powered the 358 from the 7805, and the sensor from the 7810.  I tied the sensor input and output grounds together.

Likely as a result of tying the input and output grounds together, I got some unexpected results.  Where previously the output of the sensor was 0 V with 0 psi applied, it became 946 mV.  Instead of increasing 1 mV/psi, it increased about 0.5 mV/psi.

The output of the 358 was steady at 3.57V.  I suspected this was as close to Vcc as its output would go, so I tried powering it from the 7810 instead.  The results from that confused me.  With 0 psi and an input voltage of 946 mV, the output was 1.834V.  With 20 psi and an input voltage of 959 mV, the output was 1.826V.  At that point I stopped the test.

I understand that this wouldn't meet any kind of certification--this is for an experimental application, so that isn't a problem.


Sounds like you just have a plain Wheatstone bridge circuit with about 1V common mode voltage. How where you measuring the voltage initially, with a digital multimeter?

Disconnect the sensor from all other cables, and measure resistance across some of the pins. start with the + output pin and the positive power supply pin. Then do the - output pin and the positive supply. Do the same two reading with the negative supply instead of the positive, and report the value back here. If you get some resistance value across each pin, its likely you have a plain wheatstone bridge circuit that needs a differential or instrumentation amplifier, which are more complicated than the simple non-inverting opamp circuit you're currently using.
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Hi, have you got a part number for the sensor?

Tom....... smiley
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As polymorph requested, here's a (partial) schematic--I couldn't see a way to represent the sensor itself in Eagle without defining a whole new part.  It's connected as follows (red/black are +/- in, green/white are +/- out, according to the markings on the sensor):
  • Red to the output of the 7810
  • Green to pin 3 of the LM358
  • Black and white tied together, and to ground

I'll check resistances among the sensor terminals and report back.  I'll also try the differential amplifier circuit out of the datasheet--looks straightforward enough.


* Screen Shot 2014-01-29 at 7.57.18 AM.png (14.07 KB, 645x424 - viewed 44 times.)
« Last Edit: January 29, 2014, 08:21:46 am by danb35 » Logged

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I think we need a part number. A Wheatstone bridge sensor should be wired so one wire is ground, one wire is 10V, the other two are the differential outputs. One output will stay nearly the same,  the other output will vary around 0.5Vcc according to pressure.

Look at page 20, figure 42 of the link to the TI datasheet that TomGeorge gave you:
http://www.ti.com/lit/ds/symlink/lm158-n.pdf

The LM358 isn't the best Op Amp for the job, it is rather old. But with careful design, it can work. The LM741 has offset compensation inputs. But there are more current ICs made just for the job.


http://www.simplecircuitdiagram.com/2010/09/30/ad8221-precision-strain-gage-amplifier/

Note that a 4 wire pressure sensor -is- a strain gauge.
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Single power supply Wheatstone bridge amplifier with adjustable offset compensation.

http://circuit-diagram.hqew.net/Single-Supply-Strain-Gauge-Amplifier_8058.html
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Measured the resistance among the four wires of the sensor.  Red and black are +/- in, respectively, and green/white are +/- out.
R-WR-GB-WB-G
0 psi15.627k15.588k3.367k3.329k
50 psi15.673k15.555k3.334k3.377k
Still playing with the sample circuits.
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This setup will work (use it myself several times)
Bases on your measurement:
Feeding with 5V will change output ca. 22mV ->> ampl 210  times will do (Change in schetc from 100k to 180k)
Another change must be made as well , as the output voltage changes polarity when load is applied
This is done e.g. by applying an offset voltage to the -input (now grounded) or
by  a 120k  resistor in parallell with the one showing 3377 (under load)

OPamp needs to be rail to rail. eg MCP6002
EDIT;
with no load you should measure 5,3 mV (0psi) and 26,1 mV (50psi)
Out of ampl. this will give you 0.96V .4,70V This is well within range what the opamp can do
 
..better can be done by using R-values from E24 / or nonstatandard valus


* bridge.jpg (33.76 KB, 600x287 - viewed 44 times.)
« Last Edit: February 03, 2014, 04:00:52 pm by knut_ny » Logged

Ny

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