Sensing change in ambient pressure using honeywell TPB gage sensor

Discussion in 'General Electronics Chat' started by John Duddy, May 9, 2015.

  1. John Duddy

    Thread Starter New Member

    May 9, 2015
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    I bought a TBPDPNS150PGUCV sensor so I can detect when pressure is rising or falling and react to it via an arduino program. This is for a buoyancy control system, so all I need is to track the changing pressure with reasonable accuracy.

    It's unamplified, and the data sheet http://sensing.honeywell.com/basic-output-signal-adjustment-temp-comp-008245-2-en-final-22jan142.pdf shows an amplifier circuit. However, before I buy the specific parts listed, I want to do it myself, if I can, with op-amps I have.

    My problem? I hook it up to 5-9V power on pins 1 & 3, but no matter how much PSI I put it under, I see NOTHING on my scope measuring pins 2 & 4, even set at 50mV scale. I hooked it up to an LM386 op-amp I use for ultrasonic transducers (capacitor for 200x amplification), and still nothing. If I put something else in its place, like an ultrasonic transducer, I see the expected signal and all works as I expect.

    I'm not an electronics guy, so I have no idea how to troubleshoot something that provides no response. I assume it's something I'm doing wrong, but I have no idea what to change. Input power is good, output pins are dead.

    I bought 2, and they both behave identically, so it's not likely a bad part. I also noticed that this data sheet (http://media.digikey.com/pdf/Data Sheets/Honeywell Sensing & Control PDFs/TBP_NBP_Install_Instr~.pdf) shows pin1 taking Vsupply, but the above one shows pin1 as ground and pin3 as "VsExcitation", which has left me thoroughly confused.

    All I want is 0-V volts corresponding to the pressure. Is this possible to get with this sensor?

    Thanks!
     
  2. pwdixon

    Member

    Oct 11, 2012
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    Which circuit from the app note have you tried to implement?
    This bridge sensor requires a difference amplifier so a reading on one of the output pins will not show you much.
     
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    The LM386 is an audio amp; not an instrumentation op-amp.
     
  4. John Duddy

    Thread Starter New Member

    May 9, 2015
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    I assumed that the LM386, being an opmp and having V+/V- inputs, would amplify the voltage difference.

    That said, I don't understand op-amp theory and I am not an EE. Wouldn't be posting here if I was ;-}
     
  5. John Duddy

    Thread Starter New Member

    May 9, 2015
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    I'm sorry, but I do not understand op-amp theory - I'm a hobbyist, just starting. I figured anything with +/- inputs and a way to set gain would at least let me see an amplified signal on my scope. Obviously that was too simplistic a world-view ;-}

    Is this just one of those things where I have to buy an instrumentation amplifier and an opamp and build the exact circuit in figure 8 of the first link, or else it will just not do anything I can see on my scope?

    If so, do you have recommendations for specific parts to buy? I'm afraid I don't have that knowledge, and they did not specify parts for the sample circuit. They did mention AD623, but their diagram shows 2 amplifiers, so I'm a little hesitant to buy parts I don't completely know how to hook up.

    Thanks!
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    I suggest you read up about instrumentation amps. Your sensor has a full-scale (150psi) output of only ~40mV when a 5V energising source is used, and I guess you will be trying to track changes of less than 1mV. 'Ordinary' op-amps, and audio amps, typically have input offset voltages and drift greater than that, so would not give accurate measurements; hence the need for an instrumentation amp (e.g AD623).
    The second (lower) op-amp is not as crtitical, but since it is only providing a reference voltage it could advantageously be replaced by an accurate adjustable reference such as a TL431.
     
  7. John Duddy

    Thread Starter New Member

    May 9, 2015
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    Reading about op-amps (off and on over months) hasn't helped - too much EE-lingo to bootstrap myself. But if I may summarize, to make sure I understand what you are saying:
    - instrumentation op-amps are specially designed to have less noise/drift/whatever it is that makes things inaccurate, so I need to buy an AD623
    - The reference voltage is not as critical, so I could substitute something like TL431.
     
  8. BR-549

    Well-Known Member

    Sep 22, 2013
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    Whistle across the sensor.
     
  9. Alec_t

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    Sep 17, 2013
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    Yes, instrumentation amps (IA) are specially designed for the sort of task you're proposing. There are many to choose from, but they're not cheap. See, for example, these. Some have a fixed gain. Depending on the range of voltage you'll be measuring, you may need to follow the IA with an 'ordinary' opamp to scale and offset the output of the IA, so the circuit you linked to may not be appropriate.
     
  10. John Duddy

    Thread Starter New Member

    May 9, 2015
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    As an update, I managed to get a response using an AD623 set to 100x gain. However, I seem to just get response on pressure deltas - the signal swings positive when I apply pressure, and swings negative when I remove it, and returns to 0v rapidly after each swing.

    Full disclosure - I did not use the voltage regulator for Vref, but hooked Vs- and Vref to ground. The datasheet seems like the pressure sensor should be maintaining an output proportional to the pressure. What am I missing?
     
  11. Alec_t

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    The 'returns to 0V' is odd. Post the schematic of your circuit using the AD623.
     
  12. John Duddy

    Thread Starter New Member

    May 9, 2015
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    I implemented the circuit in figure 8, http://sensing.honeywell.com/basic-output-signal-adjustment-temp-comp-008245-2-en-final-22jan142.pdf, with 2 changes:
    Vref and -Vs to ground (negative pole of battery or PS)
    Swapped the polarity of the sensor ( 1<-3, <2-> 4) since the pinouts in that figure 8 are incorrectly labeled with respect to the documented pinouts here: http://media.digikey.com/pdf/Data Sheets/Honeywell Sensing & Control PDFs/TBP_NBP_Install_Instr~.pdf
     
  13. Alec_t

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    I see nothing in that Fig 8 circuit which would give the 'return to 0V' effect. It's an effect you'd expect if the sensor were capacitively coupled to the amp, or if the sensor were a piezoelectric type instead of a piezoresistive type.
     
  14. John Duddy

    Thread Starter New Member

    May 9, 2015
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    That's what I thought. I'll keep snooping. Thanks!
     
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