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Instrumentation Amplifier with Offset Amplifier

San Ban
Prodigy 30 points

Other Parts Discussed in Thread: INA122, INA321, INA333, INA326

Hi, 

I'm new to create circuits. I want to make a strain gage conditioner. I searched Internet and found that I need to use instrumentation amplifier. I chose my IC as INA122 but lately I saw that people use offset amplifiers after instrumentation amplifiers. What is the reason for this? 

Another thing that I really want to learn is rail to rail opamps. What is the difference between rail to rail opamp and instrumentation opamp? 

Any help will be appreciated. 

Thanks in advance.

  • 0
    TI__Mastermind 25865 points

    Hello San,

    The use of an op amp at the output of an instrumentation amplifier depends on the application. For example, the instrumentation amplifier may not have sufficient bandwidth or capacitive load drive for the ADC, which would necessitate the use of an external op amp. Another possibility is if you're trying to drive a large capacitive load and require access to the feedback path to compensate the amplifier properly. Please take a look at the INA321. It is essentially a 2-op amp instrumentation amplifier (with a fixed gain) and an output amplifier that is used to adjust the gain.

    A rail-to-rail input/output (RRI/O) amplifier is a term that describes amplifiers whose linear input range extends to within a couple hundred millivolts of the supply (or sometimes even beyond the supply) and whose output can swing to within a couple hundred millivolts of the supply. Instrumentation amplifiers (IAs) should not be directly compared to an op amp because an IA is made of op amps (a traditional IA is made of 3 op amps). 3 op amp IAs have a more complex relationship between the input common mode voltage and output swing. Please take a look at Figures 20-23 in the INA333 data sheet to see an example of an IA's common-mode vs. output voltage capabilities. There is one instrumentation amplifier that is RRI/O...please take a look at the INA326. The section entitled "Inside the INA326" in the data sheet describes how it achieves RRI/O.

    I suggest reading "The Signal", which is a series of blog posts by Bruce Trump. Here is the link: http://e2e.ti.com/blogs_/b/thesignal/default.aspx

    Hope this helps!