• State TI Thinks Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 2 answers
  • Subscribers 47 subscribers
  • Views 1405 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Wheatstone bridge INA326 Application

Bob_5
Prodigy 20 points
Other Parts Discussed in Thread: INA326, TIPD170, INA333, INA331, INA321

Dear Forum

I have a water level sensor which changes its resistance from 400 ohm to 1500 ohm depending on the water level. It also includes a temperature dependent resistor used to calibrate the sensor with temperature.

I am using a 0-5VDC ADC and therefore need to amplify this signal to fully utilise my ADC range.

My intention is to use a wheatstone bridge  (all resistors = 1k5 and the sensor resistor as listed above) connected to a single 5 V supply (see attached schematic).

In this configuration the common mode voltage varies between 1.7V and 2.5 V.  The differential voltage is ≈0-1.45 V.

My intention is to use an INA326 with a gain of ≈3 to obtain a 0 -4.3 VDC range at the input of te ADC.  (R1 = 56 k and R2 = 82 K)

QUESTION 1 : Is this the correct circuit  to use as the datasheet has many options and I would greatly appreicate someones input on this ?

QUESTION 2: For my application, is it necessary to level shift the output as done in Fig 6 and Fig 7 of the DS ?

QUESTION 3: For my application is it necessary to use a output series resistor of 100 ohms ?

Thanks in advance.

Bob

INA326_Design_20160619_0001.pdf

  • 0
    TI__Mastermind 25825 points
    Hello Bob,

    Please refer to the bottom half of page 9 of the INA326 data sheet. For full output swing during single-supply operation, setting R2>100kohms is recommended. Therefore I would set R2=200kohms and R1=133kohms. This should yield a gain of ~3V/V. I also recommend using 0.1% precision resistors to minimize gain error. I also recommend adding the C2 capacitor (0.5nF) and RO/CO output filter as indicated in the data sheet. These help filter the intrinsic noise generated by the device.

    If you do not level shift the output you will lose some dynamic range since the output cannot reach 0V. Therefore, I do recommend adding a resistor connected to pin 5 as shown in your drawing. Please size it such that the divider results in a reference voltage of 100mV. That way when your ADC reads 100mV, it actually means the input from the bridge is 0V. Note that this somewhat complicates the calculation of R2. This is because the gain is determined by the parallel combination of both resistors connected to pin 5. Figure 6 in the data sheet explains this.

    For this particular device I do recommend adding the RC filter on the output as recommended in the data sheet. This helps filter the intrinsic noise.

    One final note: be sure to minimize capacitance on the gain-setting pins (1 and 8) of this device. This is mentioned at the top of page 10 in the data sheet. Since this device is sensitive to parasitic capacitances, note that it will not work well if you try to prototype using a breadboard (or protoboard). Therefore I recommend developing a PCB for evaluation.
  • 0 in reply to Pete%20Semig
    Prodigy 20 points

    Dear Mr Semig

    Your response was nothing short of legendary! Thanks, this clarifies my intentions to build the circuit.

    however as per your suggestion to use values in the order of 200k for R2, this means that the voltage divider resistor R'2 must be 8 Mohm which is not that practical. any suggestions ?( am I maybe using the wrong IA for this simple application) ?

    best regards

  • 0 in reply to Bob_5
    TI__Mastermind 25825 points
    Hello Bob,

    Thanks for the compliment!

    You could reduce R2 as low as 100kohms. If that is not acceptable, perhaps you'd be interested in a different approach using a 2 or 3 op amp IA. The issue there is that you may have an output swing limitation because it depends on your common-mode voltage. For more information on this, please refer to this FAQ:

    e2e.ti.com/.../3373.understanding-instrumentation-amplifier-vcm-vs-vout-plots

    You could consider placing resistors in the bridge to adjust the common-mode such that it is closer to mid-supply which can help with output swing. TIPD170 may help you with this.

    One other potential drawback to not using the INA326 is that you will have to buffer the reference voltage with an op amp.

    If you want to use a 3 op amp IA, I recommend evaluating the INA333. For 2 op amp IAs, perhaps the INA321 or INA331 would work. The 2 op amp IAs generally have larger output swing vs. common-mode voltage, but it will have a larger offset voltage and worse CMRR.

    Hope this helps!