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INA111AP - Instrument Op Amp

Prodigy 50 points

Replies: 8

Views: 9486

Hi,

Can I get variable gain instead of fixed gain?

Thanks,

Deke

  • Deke,

    The only way that you can get a variable gain with INA111 is if you connect a MUX or a switch with different resistor values in between the RG pins to make the gain selectable.  Keep in mind that when you modulate the gain you will also modulate the bandwidth.

    Matt

  • In reply to Matt Hann:

    Thank you Matt,

    I really appreciate the very fast support from TI. 

    As you probably already noticed I'm a newbie to analog circuits, I have created a PCB with 3 components on it, a DC/DC Converter (24Vin, Dual output +15V & -15V) to power the INA111AP Opamp and a header Terminal Block.

    The application is to control a position on a hydraulic cylinder, attached to the cylinder is a Balluf linear transducer 0-10V, which is my Vin1, and from a analog output module on a PLC is my command signal 0-10V, which is my Vin2, the INA111AP then sends a signal of -10V to +10V to a servo amp board, based on the current position of cylinder and my command signal, and it works very good.

    Except I'm buring up INA111AP op amps (rate of 1 a week), my co-worker believes it's because I'm not using any capacitors on the DC/DC Converter, so I now have installed capacitors on the in and outs of the converter as the manufacturer recommends, which will be installed next monday. My coworker also believes that I need a small load of at least 150mA on the negative output of the DC Converter, which he will install as well for the next trial run on monday.  However we have the same board on another machine without caps and resistors that is not having a single problem going on 4 weeks.

    I would like to be able to put gain control at the board I make and not at the servo amp, because some machines may have on-board electronics on the valve, and not need a servo amp board.

    Did I choose the wrong type of op amp for this application?

    Any more words of wisdom to help out an old dog wanting to learn new tricks?

    Thanks,

    Deke

  • In reply to Deke:

    Deke,

    Without more details or a schematic, I can only make some general comments.  Your friend may be right about the DC to DC converter--a stable supply is a good thing; however, you might also consider the state of your inputs vs. your supply when you cycle power.  A scope capture of inputs, outputs, and supplies can be very illuminating in this regard. 

    Your inputs should have some series resistance such that at your max input voltage the input current never exceeds 10mA.  Likewise, it is always a good idea to put Zener diodes off the supplies which will provide a low impedance to ground during a power on condition where your supplies may still be establishing themselves (and therefore higher impedance) thereby shunting any transient currents that might occur during this mode.

    I don't think you have necessarily chosen the wrong device for this application; however, if want to more easily change gain you might consider a programmable amplifier such as the PGA202/203/204/205.

    Matt

  • In reply to Matt Hann:

     Thanks for all the help Matt. [:)]

    Deke

  • In reply to Deke:

    Revisting....

      Matt, could I use a potentiometer for Rg?

    Thanks,

    Deke

  • In reply to Deke:

    Deke, Do you mean a digital POT? You could do that, but keep in mind that digital potentiometers have an inherent TC that could add gain error vs. temperature. You'll have to assess what kind of errors you can live with in your application. Matt
  • In reply to Matt Hann:

    Hi Matt,

     Sorry for taking so long to reply, but I've been busy learning how to use TINA, which is pretty cool by the way![:D]

    Using TINA I setup the INA121 with the 50 gain for Rg resistor 1.02K, I then used a trimmer pot on the output of INA121 going to pin 1 of pot, pin 3 of pot to ground and pin 2 (wiper) is now my adjustable output 0 to 50 gain on a 25 turn pot.

    My next question is which Ohm size pot should I use?

    Thanks

    Deke

  • In reply to Deke:

    Deke,

    A 50k POT will get you down to a gain of ~ 2 and allow you to servo your gain (in a coarse fashion) up to about 50; however, if you want to put the INA111 in unity gain you'll have to have a switch that completely opens the gain resistor path.

    Matt

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