INA with Precision Voltage Reference

Discussion in 'Electronics Resources' started by beedees, May 7, 2015.

  1. beedees

    Thread Starter Member

    Mar 29, 2015
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    Hi everybody!
    I need use an instrumentation amplifier and it has to power the bridge. I found this.
    I don't understand how it works. What are the values to power the bridge? I would like to power the bridge with 3.3V. Is it possible using this component?
    Thanks to all!
     
  2. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    If you wish to power a wheatstone bridge using 3.3V then this chip won't be able to do it.
    What you could do is use a 3.3V reference and then use a current booster circuit to excite the bridge. After that, you could use the bridge's output to feed an instrumentation amplifier.
    The chip you've suggested is a good chip, but it only works at the 1.24V, 2.5, 5 and 10V references listed in its datasheet.
    EDIT: Why do you want to power the bridge with 3.3V? Is it an established limitation of your design?
     
  3. OBW0549

    Well-Known Member

    Mar 2, 2015
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    He won't be able to get exactly 3.3 volts, but he can get close: just short pin 16 to pin 13, and connect pin 15 to pin 4. This will program the reference amplifier for a gain of 2.714, giving a bridge drive output of 1.24V * 2.714 = 3.366 volts. Might be good enough for what he's trying to do...
     
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  4. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    Good one... I hadn't thought of that... most 3.3V circuits are tolerant to up to 4V... so it might just work for the rest of his circuit
     
  5. OBW0549

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    I think all he wants to do is power the bridge; I hope he's not planning to power the rest of his system off that voltage, because if I'm not mistaken the reference amplifier can only output a couple of milliamps (5 mA or so).
     
  6. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    The datasheet says it can deliver only up to 12mA on its output when it's short-circuited... that means he won't be able to excite most standard load cells with an impedance of 350Ω... he needs a current booster
     
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  7. OBW0549

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    Good point.
     
  8. beedees

    Thread Starter Member

    Mar 29, 2015
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    It's a good idea! The datasheet reports "Adjustment of the reference output voltage with an external resistor is not recommended because the required resistor value is uncertain". But in this case I don't use an external resistor so I think I won't have problems... right?
    I use a 12 bit ADC for data acquisition. The positive reference voltage is 3.6V max, so even if Vref of the bridge is 3.4V I think it works anyway.

    Bridge's resistances are all 120 ohm (two are strain gauges) so the equivalent resistance of the bridge is 120 ohm. The current that you say I don't think is the current flowing in the pin 4, i.e. the pin connected to the bridge, but the output current of the INA. However I'm not sure...
     
  9. OBW0549

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    Mar 2, 2015
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    I think so long as you don't use any external resistors, and rely only on the INA's internal resistors, you should get good reference voltage accuracy and stability no matter how you connect the internal resistors.

    I would think so, but I would have to see the ADC datasheet to be sure. What ADC are you using?

    Operating a 120 ohm bridge from 3.36 volts, you will be drawing 28 milliamps so you will definitely need a buffer, such as the circuit shown in Figure 4 of the INA125 datasheet.
     
  10. beedees

    Thread Starter Member

    Mar 29, 2015
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    I use this. The ADC characteristics are at page 68. V_ref+ max is equal to V_DDA and so 3.6 V (max power supply). I think it's right.

    This is due to the fact the opamp output current is very little, so I need a current boost, right? If so, the output current that cmartinez said and that is reported in the datasheet refers to all outputs of the INA...
     
  11. OBW0549

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    Mar 2, 2015
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    Looks to me it should work just fine.

    Correct.
     
  12. beedees

    Thread Starter Member

    Mar 29, 2015
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    Ok thanks! I'll try this INA!
    Another question about this. Sorry but I have never used a current boost for this kind of application. Have I to consider any particular characteristics of the BJT? I suppose I have to pay attention to max current it can draw... I have 5V power supply (V+) but I don't think could there be problems for this anyway. INA125 datasheet reports in Figure 4 a TIP29C with 100V collector-emitter voltage as you can see here. So I think it is uselessly oversized.
     
  13. OBW0549

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    Mar 2, 2015
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    I would agree regarding the TIP29. If you are going to be driving a 120 ohm Wheatstone bridge at 3.36 volts, that will require only 28 milliamps; and for that small a current, almost any general-purpose NPN transistor will do, such as a 2N3904 or 2N4401.
     
  14. beedees

    Thread Starter Member

    Mar 29, 2015
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    Ok! Now everything is clearer. Thanks a lot!
     
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