INA333: Strain gauge amplifier: zeroing and bridge excitation

Hi Paride,

I would like to use 5V precise reference, but I think 50mA are too much to any voltage reference. What can I choose instead?

We may be able to reduce the current through the Wheatstone bridge sensor, see the image below. There are other options, but this option may be better, since you may also reduced I2R heating within the Wheatstone bridge, though I am not sure the heating may be significant. 

I am not sure that strain gauge sensor is able to be excited at lower voltage. If not, you may have to make make some adjustment. For instance, reduce R5 and increase R6 series resistors. 

There are other options, but no excitation voltage is as precise as the reference voltage. You may also buffer the 5Vref with a precision op amp to increase 5Vref's sourcing current. You need to find an op amp that is capable to source up to 50mA current. If this is the option you want to go, please let me know. 

I know should be better to not zeroing bridge, but INA333 have only +- 0.3 V of differential input, and can be saturated by only initial unbalancing of the bridge.

Please see the following thread where Kai talks about balancing a sensor bridge. 

https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/975733/ina125-what-is-the-recommended-way-for-balancing-or-zeroing-inputs-from-a-strain-gauge-wheatstone-bridge-into-or-through-an-ina125-or-similar/3606322?tisearch=e2e-sitesearch&keymatch=zero%252525252520out%252525252520bridge%252525252520sensor#3606322

Other option is doing both balancing the bridge and zero out the output of INA333 circuit. (or calibrate  For sensitive strain gauge measurement, stain gauge calibration is necessary to calibrate out the unwanted errors, such as temperature, INA333's Vos, bridge slight imbalance, humidity etc. You may perform two  point strain gauge calibration to make sure the sensor is performing within a specification. You may then check a known force against the sensor after a system calibration, if accuracy measurement needs to be ensured.  

INA333 Strain Gauge 08052021.TSC

If you have additional questions, please let me know. 

Best,

Raymond

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Thank you Raymond,
BALANCE ISSUE
I already read the topic you linked. Unfortunatly, I have to work with already-installed strain gauges and that could result in big differences between 2 strain gauge resistence. I can't calibrate if on zero position there are 5mV, because INA333 is already saturated.
I know the best option will be to have an external shunt fixed resistor, but it requires a full kit of precision resistors and a lot of patience. What do you think about the drawn solution?

EXCITATION ISSUE
Speacking about excitation, I think the solution with series resistors will reduce not only the current but also excitation voltage across the bridge: if I'm not wrong, referring to your image  only 1/5 of Vexct is applied on the bridge.
The sensibility of my sistem is 0.38 mV/V on maximum load (delta resistence on the gauge about 0.19 ohm).
The output of amplifier using 1V across the bridge should be 2.88 V on maximum load (considering 2.5V output on balanced bridge), seems low.

The buffering solution is intresting, could it bring errors due to temperature changement?

Thank you again

Hi Paride,

The sensibility of my system is 0.38 mV/V on maximum load 

Yes, the application will need to maximize excitation voltage in order maximize the overall change, which is approx. 1.9mV or 0.1815Ω in delta of the load cell. 

In this case, I probably will not use potentiometer as Kai pointed out. You may use the potentiometer or resistor box to check out the required value and replace with the balanced circuit with a fixed resistor or balance the Wheatstone bridge circuit directly by hand without potentiometer. It will take more time, but you do not need to worry about thermal drift over temperature in potentiometer (you may place a calculated resistor value on top of 120Ω (piggyback) to null out the circuit after checking out the balance of an Wheatstone bridge).  

Typically, the type of sensitive measurement are done in a controlled temperature environment or indoor. If the ambient temperature is changed at the minimum, potentiometer may be acceptable.  Personally, I do not use potentiometer in a precision circuitry. 

With such small signal change, I will suggest a different instrumentation amplifier, such as latest INA848 or INA818, since you need high gain. INA848 is a fixed gain instrumentation amplifier with very high CMRR specification. If the analyzer is not for portable or battery operated application,  INA848 or INA818 will be a better option. INA333 will be good for low power and portable strain gauge application. 

Here is ratiometric example implemented in 3-wire RTD measurement application. 

 https://static5.arrow.com/pdfs/2013/11/24/3/19/23/744/txn_/manual/slau520.pdf

If you have additional questions, please let us know. 

Best,

Raymond

Thank you both, I will try to reply per points:

Amplifier selection
INA333 was selected due to low temp drift (better than INA818) and because of this example provided by TI. I think 1000 gain is enought, it allows to almost all 5V range (2.5V is the ideal output in zero load condition, -+1.9 V belonging from 1.9mV amplified 1000 times)

Bridge zeroing

Following your suggestions, a pad will be prepared on the PCB to sold low-drift resistor in parallel to one arm of the bridge

Bridge excitation and adc

I think the best option will be to use a ratiometric or pseudo ratiometric mesurement.
At first instance I didn't care about adc because the plan was to just amplify the strain gauge to be aquired in external sistem.
Considering that probably there will be 12 strain gauge to acquire, could be an idea to use ADS1115 analog-digital converter and arduino-based datalogger to store data?
ADS1115 is selected because of his easy set up and good documentation in arduino word.

Thanks again