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OPA2180: Designing a strain gauge amplifier circuit using OPA2180IDR

Shibin Varghese
Intellectual 835 points
Part Number: OPA2180
Other Parts Discussed in Thread: INA823, INA125, TINA-TI

Hi team,
I want to design a Single-supply strain gauge bridge amplifier circuit using the op-amp OPA2180IDR.
The application note link is really helpful.

Attaching the specifications of the strain gauge.

The excitation voltage is 12V.
The output voltage is 2mV/V
1). Does this means the output voltage swing will be 2mV only regardless of the excitation voltage range 10 - 15V?.
The Bridge resistance is 350 Ohms.
2). Does bridge resistance means the total voltage of the bridge or individual arms?.
When I connected to a 12V supply, a current of 34mA flows through the strain gauge.
3). Will this high current affect the performance of the strain gauge?.
Also when I limited the current through the strain gauge using a resister my output voltage swing is decreased.

As per the circuit diagram below,



The opamps are powered from a 12V source.
My Vref is now set to 2.5V.
When Vref is set mid-supply (6V) my output voltage will be about 7V which is very higher than my input voltage range of the ADC - ADS1220IRVAT.
Under the balanced condition of the bridge, my output is about 2.9V, when strain is applied the voltage will be increased to about 4V.
4). Is there any chance the voltage at the output of the second opamp goes above a 5V range?.
5). Do you believe an instrumentation amplifier is better than this type of op-amp configuration?.
If yes please suggest an instrumentation amplifier suitable for my needs.





  • 0
    Guru 140200 points

    Hi Shibin,

    I think the first three questions should be addressed to the manufcaturer of strain gauge.

    Kai

  • 0
    TI__Guru* 93105 points

    Hi Shibin,

    Please do follow Kai's suggestion and contact the bridge manufacturer for answers to you questions about the bridge:

    4). Is there any chance the voltage at the output of the second opamp goes above a 5V range?.

    The output voltage Vo is a function of the common-mode voltage (Vcm) applied and the gained up differential voltage (Vi2-Vi1). If there sum exceeds 5 V, then Vo can be above that level when the supply voltage is 12 V. Should that be the case the differential gain can be reduced and/or the Vcm could be reduced.


    5). Do you believe an instrumentation amplifier is better than this type of op-amp configuration?.
    If yes please suggest an instrumentation amplifier suitable for my needs.

    The two Op amp instrumentation amplifier (INA) presented in the guide can provide high performance, but much is highly dependent on the precision resistors required by the circuit. If they are not precision and accurate the electrical performances such as gain error and common-mode rejection can suffer. The alternate 3 Op amp configuration eliminates most of the resistor inaccuracy by incorporating precision resistors internal to the INA. Then, the main resistor concern is the one external RG resistor. 

    An example of an easily applied 3 Op amp INA well suited for a bridge application is the INA823:

    https://www.ti.com/lit/ds/symlink/ina823.pdf

    There are additional Precision Amplifier INA products that you may want to review:

    https://www.ti.com/amplifier-circuit/instrumentation/products.html#p358min=1.8;5.5

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Intellectual 835 points

    Hi Team,
    Thank you for your reply.
    I tried reducing the Vcm by increasing the resistance R9, but when I do so, my voltage swing gets reduced.
    Are there any other methods to reduce the Vcm?.
    I calculated Vcm as 
         Vcm={[(Rbridge/2)+Resisnce of one arm of the bridge]/(Rbridge+R8+R9)}*Vex
    In my case, Rbridge = 350 Ohms
                       R8 = R9 = 0 Ohms
                       Vex = 12V
    Resistance of one arm of the bridge is taken as 350 Ohms (not given in the datasheet of the strain gauge, I hope the guess is correct)
                 Vcm = ((350/2)+350](350+0+0)}*12 = 18.
    Is my calculation correct?.
    If yes, how to lower the differential gain of the system?.

  • 0 in reply to Shibin Varghese
    Guru 140200 points

    Hi Shibin,

    the bridge resistance and the leg resistance are the same. Just calculate the equivalent resistance of a bridge containing four legs of 350R each. You will get 350R as well.

    Looks like the bridge shall be connected to an excitation voltage of 10VDC. You should ask the manufacturer of bridge whether you are allowed to decrease this voltage. Some bridges contain linearizing and temperature drift compensating elements causing an additional voltage drop and by this demanding a certain minimum excitation voltage.

    Increasing the excitation voltage too much above the nominal voltage isn't a good idea either, because then the bridge can heat up too much and show considerable temperature drift.

    Kai

  • 0 in reply to kai klaas69
    TI__Guru* 93105 points

    Hi Shibin,

    Please consider Kai's advice for the two Op amp IA bridge application. If you think you want to change the design to use a three Op amp IA we can provide support as needed.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to kai klaas69
    TI__Guru* 93105 points

    Hello Shibin,

    Do you have all the information you need for your bridge amplifier application?

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • +1 in reply to Thomas Kuehl
    Intellectual 835 points

    Hi Thomas,
    Thank you for your reply.
    In our case,
    The bridge resistance is 350 Ohms.
    The excitation voltage is 12V.
    When the bridge is balanced (no load), the output voltage at each output is 6V with respect to the ground.
    The output voltage is 2mV/V, ie, 24mV for 12V.
    The Vref is set to 2.5V.

    With the 2 op-amp model, I am getting a voltage of around 2.9V at the output of the systems when the bridge is balanced.
    When strain is applied to the sensor, the voltage will reach up to 4V.
    If by any chance the output voltage goes above 5V, it will kill the ADC.
    If the output voltage Vo is a function of the common-mode voltage (Vcm) applied and the gained up differential voltage (Vi2-Vi1).
    I calculated Vcm as 
         Vcm={[(Rbridge/2)+Resisnce of one arm of the bridge]/(Rbridge+R8+R9)}*Vex
    In my case, Rbridge = 350 Ohms
                       R8 = R9 = 0 Ohms
                       Vex = 12V
    Resistance of one arm of the bridge is taken as 350 Ohms (not given in the datasheet of the strain gauge, I hope the guess is correct)
                 Vcm = ((350/2)+350](350+0+0)}*12 = 18.
    1). Is my calculation correct?.
    I tried reducing the Vcm by increasing the resistance R9, but when I do so, my voltage swing gets reduced.
    2). Are there any other methods to reduce the Vcm?.
    3). How to lower the differential gain of the system?.
    4). Is there any method I can follow to get an output voltage of 0  to 5V for the +/-24mV variation of the bridge by using the 2 opamp model?.

  • +1 in reply to Shibin Varghese
    TI__Guru* 93105 points

    Hi Shibin,

    1). Is my calculation correct?.

    Applying the Vcm equation in SBOA247 Vcm = 18 V. Your calculation is correct.


    I tried reducing the Vcm by increasing the resistance R9, but when I do so, my voltage swing gets reduced.

    R9 should only affect the Vcm voltage. It is not a factor in the differential voltage equation. Which Op amps are you using in the circuit?

    2). Are there any other methods to reduce the Vcm?.

    The Two Op amp INA circuit provides the instrumentation amplifier function with the minimum number of Op amps, and it has its limitations such as the Vcm range. The Vcm Equation 2 in SBOA247 sets the variables that can be applied. 


    3). How to lower the differential gain of the system?.

    Equation 5 in SBOA247 provides information how to establish the R11 value for a particular differential gain.


    4). Is there any method I can follow to get an output voltage of 0  to 5V for the +/-24mV variation of the bridge by using the 2 opamp model?

    I have put together a TINA-TI model for the Two Op amp INA so that you can apply the SBOA247 equations and see how Vo changes with Vcm and the differential gain. Since you are in control of your design I believe it is in your best interest to make the circuit do what you need it to do.

    OPA192_2_OPA_Bridge_01.TSC

    Regards, Thomas

    Precision Amplifiers Applications Engineering