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INA129 multiple gain switching

Jean Francois Pascal
Prodigy 10 points
Other Parts Discussed in Thread: INA129, PGA205, PGA103, PGA204

I'm using INA129 IN-Amp and would like to automate the switching of the RG setting gain resistor in order to better fit various input signal amplitudes.

Could you please recommend the best way for doing it (type of switching including PCB layout rules) without compromising performance.

many thanks

  • 0
    TI__Guru 62705 points

    Hello,

    I believe you will find several other references on the forum regarding this subject if you search for them where you will be able to learn and research. 

    Basically although this type of design is practical to simulate in SPICE successfully in a real PCB design the parasitic capacitance associated with whatever switch you choose to change the Rg values will likely negatively affect the CMRR and possibly the stability of the design.  To achieve optimal results from our INA's, we always suggest keeping the capacitance on the RG pins to a minimum by creating a small loop area for the Rg resistor connections and possibly removing the GND plane below the resistor to reduce parasitic capacitance to GND.  Therefore, using several different Rg resistors with switches to change them in and out goes against this basic suggestion. 

    So, in short this type of design is possible to achieve but be sure to use a switch with very small input/output capacitance, keep the additional loop area on your PCB small, and consider removing any GND plane below the resistors (assuming SMT resistors) all in an effort to reduce the parastic / stray capacitance on the RG pins.

    Another option would be to consider one of TI's PGA devices which are designed to offer the user a configurable gain.  A few basic options are the PGA103, PGA204, or PGA205 but there are many different options available which you can search from below depending on your more specific applications requirements.

    http://www.ti.com/sitesearch/docs/universalsearch.tsp?searchTerm=PGA&linkId=2

    Regards,
    Collin Wells
    Precision Linear Applications

     

  • 0 in reply to Collin Wells
    Intellectual 405 points

    Hi Jean

    You should also keep in mind that the switch will have some resistance which might be significant in your design. As Colin said you might be better off using a PGA.

    regards

    Steve

  • 0 in reply to Collin Wells
    Prodigy 10 points

    Hi Collin,

    Many thanks for your recommandations, I will take care of them.

    My problem is the particularity of my application. I have to evaluate the performance of INA129 while it is exposed to radiation.

    it means that I cannot move to PGA family or any other device.

    My intention is to multiplex RG resistor using reed relays. I try to find the more compact solution and i'm looking at the Coto B41.

    What do you think? Do you have any other suggestion?

    Regards

  • 0 in reply to Jean Francois Pascal1
    TI__Mastermind 25795 points

    Hello Jean,

    Thanks for the additional information.  It sounds like you have a very interesting application.  While we have not tried using reed relays to switch the gain of an INA129, I think this is a very intersting idea.  It looks like the B41's contact resistance is very small (125mohms) and its capacitances are also very small (all well under 1pF).  So, I think this is definitely worthy of building a prototype.

    Once you finish your PCB layout, feel free to post the portion that contains the INA129 and B41 so we can review it.  If you have any confidentiality concerns, please send me a friend request.