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Posted: 11/27/2013 2:36:53 PM EDT
Hey guys! I have a project going, I'm trying to use a pressure transducer as something of a pitot tube to measure air speed. I'm using a PXCPC 0-4 inH20 gage pressure transducer. Data sheet can be found here

The output of said transducer is 31.25 mV at full scale. I'm using +15V excitation. In order to boost the signal for data acquisition, I'm using an LM358 op amp, with 50 ohm and 15kohm resistors to get a 300 gain.

Here's my quandary. I've verified that my amplification circuits work. (The LM358 is a dual amplifier, one on each side data sheet here) I used a function generator to output a signal to boost, and then observed through the scope that the signal was boosted by the expected gain of 300.

The pressure transducer works. If I hook it straight up to the board (to analog in 0+, analog in 0- to ground), and look at the scope, I can get some response by blowing in it. Very small, but about what I expect. When I try to hook these things together, nothing happens .

My setup is as follows. The pressure transducer has 4 pins:

-EXC - hooked to ground
+OUT - hooked to non-inverting IN on near side amplifier
+EXC - Sharing +15V with LM358 op amp
-OUT - hooked to non-inverting IN on far side amplifier


Amplifier VCC shares +15 with pressure xducer
Other side hooked to ground
Out 1 hooked to Analog In 0+
Out 2 (far side) hooked to Analog in 0-

I get nothing on the scope, no response to blowing, just about a 15 mV straight line.

I've tried different configurations, like just using +EXC and +OUT, grounding everything esle, etc, moving wires every which way trying to get SOMETHING to happen. Can you help me make this work? Thanks!

Click thumbnails for large images,




Edited for clarity
Link Posted: 11/27/2013 3:08:50 PM EDT
[#1]
The output impedance of the sensor is rated at 3K.

You are hooking it up to an op amp through a 50 ohm resistor. The other side of the resistor is at virtual ground (isn't it?).

So wouldn't that mean that you are hooking up a high-impedance sensor output to a low-impedance load, effectively short-circuiting your sensor's output to ground?

(but maybe I need to look at your op amp wiring more closely - I thought that I read that you were powering it from + and - 15 volts, but maybe it's powered from +15 v and ground?)
Link Posted: 11/27/2013 3:21:45 PM EDT
[#2]
I am powering the op amp from +15 and ground.

The output is not passing through a resistor before going in to the input of the amplifier. (Should it be?) Admittedly, I'm not real familiar with impedance and what it means for this circuit. The 50 ohm resistor is between ground and the - amp terminal. The 15kohm is between out terminal and - terminal.

I'm not sure I understand how my sensor output is shorting to ground. Could you explain?
Link Posted: 11/27/2013 3:33:34 PM EDT
[#3]
I'm trying to understand your hookup by looking at your photos, but it's a bit confusing. I thought that you were using an inverting amplifier where the input impedance would be effectively the 50 ohm input resistor, but instead it looks like you are maybe using a different configuration.

Can you post a rough schematic diagram of what you are trying to hook up?

Link Posted: 11/27/2013 4:15:49 PM EDT
[#4]


Please excuse the schematic. It is, indeed, rough

The resistors are in place strictly to set the gain of the amplifiers, and the configuration is identical between the two amplifiers. There is a 15k between out and the negative amp terminal, and a 50 between the negative amp terminal and ground, on both amps. My thinking is that I have two signals coming out of the sensor (OUT+, OUT-), and both need to be amplified. I'm not sure this is correct, but this is where I am. The connections between the outs of the sensor, and the (non-inverting) input of the amplifiers have no resistance. Just a wire.

I've tried just amplifying only one part of the signal (OUT+, hooked up to one amp, OUT- hooked to ground), and still get the same result.
Link Posted: 11/27/2013 5:05:04 PM EDT
[#5]
Sorry, that I do not have a bunch of time to explain in detail, but here is a try.  Look up how to design a difference op amp.  You want to input both outputs from the transducer into a single op amp.  Also for the difference op amp you are going to need a + and --- voltage supply for the op amp.  I think that will do what you are trying to do.   Off hand I do not know the number of op amp to use....

link for some info:
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar7.html#c1


----------------------------------------------------------------

Member, Friday the 13th
"Nothing tastes as good as skinny feels"  K. Moss
You can NEVER be too rich or too thin...
Life is not a journey, but a series of unplanned detours....
Perfection: is not a goal---it's a demanded expectation.
Link Posted: 11/27/2013 5:14:21 PM EDT
[#6]
The Omega datasheet is pretty worthless. I guess that the excitation inputs are expecting DC on the + input and ground on the - terminal.

I also guess that the device is designed to provide a differential output in the mV range, measured between OUT+ and OUT-.

So I don't think that you can amplify it with two independent amplifiers (the way that you show in your schematic).

Since the op amp is using a single polarity supply voltage, what is the DC voltage at the amplifier inputs? Is it half of Vcc? If so, then if you hook up the sensor's OUT+ to the amplifier input and OUT- to ground, then are you drawing current through the sensor? Do you need to block the DC with a capacitor? What is the DC voltage at the output of the amplifier when hooked up that way? Is it pegged in one direction or the other?




Link Posted: 11/27/2013 5:16:40 PM EDT
[#7]
Quoted:
Sorry, that I do not have a bunch of time to explain in detail, but here is a try.  Look up how to design a difference op amp.  You want to input both outputs from the transducer into a single op amp.  Also for the difference op amp you are going to need a + and --- voltage supply for the op amp.  I think that will do what you are trying to do.   Off hand I do not know the number of op amp to use....

link for some info:
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar7.html#c1


----------------------------------------------------------------

Member, Friday the 13th
"Nothing tastes as good as skinny feels"  K. Moss
You can NEVER be too rich or too thin...
Life is not a journey, but a series of unplanned detours....
Perfection: is not a goal---it's a demanded expectation.
View Quote


Ok, so I see a differential op amp can amplify the difference between the outputs, correct? I can wire that up without too much issue, but my main problem is that I don't know WHY the pressure transducer has two outs... Would one not suffice? I'm getting confused as to what each signal will be, and exactly what I need to do with them.
If I should be amplifying the difference between them, then that's what I'll do, although I'm not sure why I'm doing that.
Link Posted: 11/27/2013 5:22:07 PM EDT
[#8]
Discussion ForumsJump to Quoted PostQuote History
Quoted:
The Omega datasheet is pretty worthless. I guess that the excitation inputs are expecting DC on the + input and ground on the - terminal.

I also guess that the device is designed to provide a differential output in the mV range, measured between OUT+ and OUT-.

So I don't think that you can amplify it with two independent amplifiers (the way that you show in your schematic).

Since the op amp is using a single polarity supply voltage, what is the DC voltage at the amplifier inputs? Is it half of Vcc? If so, then if you hook up the sensor's OUT+ to the amplifier input and OUT- to ground, then are you drawing current through the sensor? Do you need to block the DC with a capacitor? What is the DC voltage at the output of the amplifier when hooked up that way? Is it pegged in one direction or the other?
View Quote


Ok, a differential voltage between OUT+ and OUT- makes sense. The poster above suggested a differential amp, which, now that I think about it, is making quite a bit of sense. I'm going to try that, and see what happens. It shouldn't take too long to wire up. If not, I'll get back to your suggestions.
Link Posted: 11/27/2013 5:27:16 PM EDT
[#9]
I think that opus1776 and I are on the same subject, namely that the output from the sensor is a differential signal.

You should be able to handle that with, for example, a differential input instrumentation amplifier or maybe just using the op amp inputs in a differential manner.

I would expect that you could also use a single-ended amplifier input, with one end of the sensor's output grounded, but you would have to pay attention to the DC voltage considerations that I was alluding to in my post.


Link Posted: 11/27/2013 5:37:13 PM EDT
[#10]
Discussion ForumsJump to Quoted PostQuote History
Quoted:
I think that opus1776 and I are on the same subject, namely that the output from the sensor is a differential signal.

You should be able to handle that with, for example, a differential input instrumentation amplifier or maybe just using the op amp inputs in a differential manner.

I would expect that you could also use a single-ended amplifier input, with one end of the sensor's output grounded, but you would have to pay attention to the DC voltage considerations that I was alluding to in my post.


View Quote


I see what you mean. I had one out the outs grounded, the other in to one of the amps, and tried that several hours ago, but to no avail. I didn't check to see if I was drawing current, so I might have to revisit that as well. I really appreciate the help, I've been screwing with this all day. I'm tired, and will get back to it tomorrow before the family gets here. Thanks!
Link Posted: 11/29/2013 11:32:04 AM EDT
[#11]
Thank you guys very much

That hyperphysics link led me directly to what I needed to know, and likely wouldn't have found that by myself. You guys were right about it being a differential output. I used an amplified difference hookup, and all works well now
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