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Multiple Inputs to INA126 (or equivalent)

Mobile Wave
Prodigy 130 points

Other Parts Discussed in Thread: INA126, LM4040, INA322, INA332, INA321

Hi,

I am working on a cost sensitive system where data from 3  transducer bridges is added and then sent to an ADC. If b1, b2 and b3 is the analog output of the three bridges, then the signal of concern is b1 - x*b2 - x*b3, where x<1.

One way of implementing this is to have three INA126 (with equal gains) amplify the data from the 3 bridges and send it to 3 channels of an ADC for implementing this formula. My question regarding this approach is:

Do I need 3 different voltage reference (LM4040) to set the reference level for the three INA126. The system will have Vdd of 5V and the low side is a ground. The reference voltage is 2.5V for all three INA126s. Do you think I can get by with one reference voltage.

A second approach is to scale the voltages of bridge b2 and b3 using resistors and inputting b1-x*b2 - x*b3 across the differential input of a single INA126. This will be much cheaper than the previous approach and I was wondering if there is a major drawback in the signal quality of this approach. 

Finally, is there a better choice for an In-Amp than the INA126. I have chosen INA126 it as a tradeoff between cost and input offset current.

  • 0
    TI__Mastermind 25875 points
    Hello,

    I don't see an issue with using an LM4040 to drive the reference pins provided that the LM4040's output impedance is low so that it does not degrade the INA126's CMRR performance (please see applications section of data sheet). Please refer to the impedance vs. frequency graph in the LM4040 data sheet. Just in case, though, I recommend adding a footprint to your PCB for an op amp that would buffer the output of the LM4040.

    I recommend using 3 instrumentation amps (again, you can use 1 dual and 1 single channel) and performing the mathematics in the microcontroller.

    I looked at a number of alternate, cost sensitive options (INA332, INA322, and INA321 in particular) that come in dual packages. You may want to look at them as well, but they tend to have greater input offset voltage than the INA126, which is probably not desirable. However, they tend to have pA's of input offset current as opposed to the INA126, which has nA's of input offset current.

    I hope this helps.