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mV signal amplifier

Discussion in 'General Electronics Discussion' started by jf11235813, Mar 21, 2015.

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  1. davenn

    davenn Moderator

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    would you like to clarify that please its very vague ??
     
  2. jf11235813

    jf11235813

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    Mar 21, 2015
    I have mV signals for pressure, temperature, moisture, and vacuum. I have analog inputs and a labview program built. I need to use these modules to amplify these signals.
     
  3. hevans1944

    hevans1944 Hop - AC8NS

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    Well, good luck with using undocumented Chinese parts for that. The information on the site you linked says these have a gain of 3, which I find highly unlikely but possible. Are the two potentiometers for adjusting gain and offset? Your guess is as good as mine. Why not just hook them up and see what comes out versus what you put in? What! No instructions?

    And where do you get replacements a few years (or months) down the road? Millivolts to volts for an Arduino Omega... sounds like an op-amp and a few resistors plus a roll-off capacitor across the feedback resistor might do it... maybe a pair of TL072 dual op-amps configured as a classical three op-amp differential input instrumentation amplifier, depending on your source impedance, gain requirement, and offset, will do the job. Are the sensor sources bi-polar millivolt signals, or uni-polar? Last time I looked, the Arduino Mega wants to see uni-polar analog inputs in the range of 0 to +5 VDC. And how does LabView enter into this picture? You have LabView drivers for an Arduino Mega?
     
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  4. jf11235813

    jf11235813

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    Mar 21, 2015
    I am really just looking for a guess at the diagram for these modules. I didn't pick them, they were pushed on me.
    I made my own labview drivers for the mega. I was simply saying that I was outputting into labview.
    I'll probably just hook them up and see what I get, but I really don't know. I'm wondering why there are two VCC. Things like that.
    If there is a pre-built alternative, I would be happy to submit it for revision. Anything would be helpful.
     
  5. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Looks like you might be caught between a rock and a hard place. I hated it when things were pushed on me to "make work," especially when the pusher knew less about it than I did. But you know the old saying, "The Boss may not always be right, but they are always the Boss." So I tried to make do, if that were possible, or worked around the problem by coming up with a workable solution. This requires serious diplomacy because no one wants to be held up to ridicule, which can surely occur if the thing being pushed at you is a serious POS that has no hope of working as the Boss expects it to. The trick often is to say, "Well, Boss, it almost works but I needed to make a few changes to get it to do exactly what we need." You present a fait accompli that does the job and does it well. Then no one remembers the POS you discarded along the way.

    So get out your multimeter and make some intelligent measurements. Umm. You sound like a software kind of guy... they do let you have a multimeter and electrical stuff, right? Anyhoo, there are two Vcc terminals... are they connected together on the circuit board? Use the ohmmeter function of the multimeter. Maybe you can also follow their circuit board traces to find out. There appear to also be three GND terminals... are these connected together on the circuit board? And there is one gazinta (labeled In) and one gazouta (labeled Out) terminal. Maybe you can rig up a millivolt source consisting of a large-value fixed resistor in series with a low-value potentiometer and a dry-cell. Apply that to the gazinta terminal, while measuring with your multimeter whatever appears on the gazouta terminal. Maybe play with the adjustments of the two potentiometers on the board to see what those do.

    Fine business using the Arduino Mega as a LabView instrument. Was it difficult to write the driver?
     
  6. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    There are plenty of pre-built alternatives, but first you have to define the problem before looking for a solution. Answer my previous questions about the sensors. What is the source impedance? What is the range of signal amplitudes? Are the sensor outputs bi-polar or uni-polar? How much offset and drift do you expect from the sensors? What provisions will you make to calibrate the sensors?
     
  7. jf11235813

    jf11235813

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    Mar 21, 2015
    Thanks for the advice. The driver wasn't difficult, I'm basically just using it as a UI and running pre-built loops in C.
    The sensors I'm trying to measure are these archaic vacuum sensors and from further reading it appears they are basically just a TC. Maybe I'll just use a type K as a source and see what I get. I am an automation engineer by trade, but I am more comfortable with newer technology. I was basically told that I would need to use these old 4 pin Russian vac sensors and a couple of 100-100 shunts, and when I asked for mV amplification, this is what I was given. Now they're already purchased and I'm stuck. It's a contract though, which means if they cease to function in a year, it's on them.
    My personal dmm is a 189 so I'll probably just poke around till I get something intelligible.
    I'm not sure what kind of impedance I'm dealing with, haven't had alot of time to go over the system first hand. I guess I was hoping, with the seeming proliferation of these modules on the web, that somebody had already figured them out. My range shouldn't be more than 1-30 mV. I'm not sure if they are bipolar our uni, not having much experience with the distinction. I will most likely calibrate them myself. Got a couple of buddies at NIST who owe my some favors.
     
  8. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Yikes! Russian 4-pin thermocouple vacuum gauges! These are not archaic so much as old and reliable. You can find modern versions at Kurt J. Lesker. Pin-outs vary all over the place. They were usually excited on two leads with a low-voltage AC, which heated the thermocouple. The TC lost heat via conduction to whatever gas remained in the vacuum, eventually reaching an equilibrium temperature. The TC voltage was millivolts and generally was applied to a D'Arsonval meter movement. They needed a well-regulated AC excitation for any semblance of accuracy and did not work at pressures below about 3 milliTorr or so, which is considered by most folks to be a rough vacuum. I've never tried to use one without a TC gauge controller, but the theory of operation is simple enough that you should be able to cobble up some sort of power supply to excite them.

    Measuring temperature with Type K (chromel-alumel) thermocouples is usually done with a TC signal conditioner that provides an electronic cold-reference junction, amplification and scaling. If the temperature being sensed is below the cold-reference junction temperature (usually 0C), the output will go negative, hence the terminology bi-polar.

    Many pressure sensors also provide bi-polar outputs, especially if they measure below atmospheric pressure. And there are two types: absolute and gauge pressure sensors. Some modern versions only require that you apply power. You do need to determine if your pressure transducer provides a 4 to 20 mA current loop output or a voltage output. The current loop transducers require an external power supply to provide their compliance voltage.

    Humidity sensors are also available in a wide range of technologies. Some are available with integrated electronics requiring only that you provide power. Calibration is difficult and accuracy is generally poor unless you purchase a cold-mirror dew-point humidity sensor.

    My go-to source for industrial automation sensors is Omega Engineering. Try to get them to send you their entire set of hard-bound catalogs, although just about everything you ever wanted to know about their products is available online.

    I don't know what a 100-100 shunt is. Most high-current shunts I have used produced 100 mV at their rated current. We used two of these connected in series to measure electromagnet analyzer DC current that varied from a few amperes to 150 amperes.

    I like the Fluke 189 multimeter.

    You will probably be able to use the Chinese amplifiers, but since they use a single positive supply the output will be a positive voltage. Play with them on the bench to discover their limitations. Their input impedance is very high, at least according to the site blurb on that, so you shouldn't have to worry about source impedance of your transducers/sensors.

    Please let us know what you find out about these amplifiers. I couldn't even begin to assemble a PCB for what they are selling for.
     
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