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INA117KU ratings

Other Parts Discussed in Thread: INA117, TINA-TI, INA149

Hello,

I am using INA117KU to sense a current going to my DC motor on the high side.  The max. voltage across the motor is +/-36vdc (travels in both directions).  The sense resistor is 0.1 Ohm. I have no other resistors in line with the input pins of INA117.  I have experienced failures of INA117 on three occasions.  One of the things to mention is that I am powering INA117 with +/-12Vdc.  I do know that the specs on CM will not be +/-200V and it will decrease but not sure by how much.  I am wondering if the inputs could be protected somehow.  I have seen some samples with isolated pre-op amps but that I can’t do due to the space requirement.

Any ideas?

Thanks

Peter V.

  • Hello Peter,

    Given +/-12V supplies, the common-mode range is nearly +/-200V. Please refer to the plots at the bottom of page 4 in the data sheet. To fully understand what's going on, however, please provide the schematic and oscilloscope captures of each input pin.

  • Pete,

    Thanks for your reply.  I am attaching one part of the circuit that involves INA117KU setup as current sensor.  The FETs are being switched at 100kHz. The motor drive voltage is +/-32V.

    One of the things that I should also mention is we did very simple experiment with INA117 on the desk being powered up with +/-12V and found that signal polarity that produces a negative output at pin 6 works past 10 and even past 11 V.  However, on a positive signal, the output is saturating and starts folding back at around 9.4 V input.  When the input is 10.5 V, the output is down to 8.67 V. Unity gain.

    This is the reason I am mentioning CM with +/-12V bus.

    Peter V.

     7271.INA117.pdf

  • Hello Peter,

    Thanks for the reply and schematic.

    Please be aware that at 100kHz, the common-mode rejection ratio (CMRR) is just 50dB. So, when you refer to 'failure', do you mean complete functional failure? Or, does 'failure' mean that you don't think it's operating within specification?

    I would probe the inputs of the INA to fully understand the signals that it is subjected to. I suspect that they are very large transients and may be much larger than you think due to the inductor. If so, I recommend protecting the INA with TVS diodes (differential and common-mode).

    Concerning your bench test, did you use a damaged or known good device?

  • Hi Pete,

    Thanks for the information.

    Yes, the failure was that INA117 was outputting positive rail voltage not depending on the inputs. Complete failure on 2 or 3 occasions.

    Currently I am looking into installing TVS as a common mode but I could install an extra one as differential. Having PC board done already there is no place for me to install TVS right at the input pins so it will have to be further down the lines.  I need to prove that TVS are going to do the protection I need first and then I could redo the board accordingly. 

    I used known good device when I was doing bench test but I can't confirm that.  I grabbed INA117 from our inventory.  Yesterday I bought brand new INA117P (easier to connect on the proto board) and I will try it again. One thing to mention is that I am using 0.1uF decoupling caps on +/-12V rail.  I see in the datasheet it is 1uF tantalum. I doubt it will make difference but may as well ask?

    Thanks

    Peter V.

  • Hello Peter,

    The output range of the INA117 is +/-9V (typical) or +/-7V (minimum) given +/-12V supplies. These were obtained by looking at the output swing limits given +/-15V supplies from the data sheet table.

    In addition, please be careful with how you apply your input signal for your bench test. For example, if you're grounding pin 2 and applying a signal to pin 3 (+In) the output will likely not be what you expect because you don't have the transfer function you think you do. Please see the attached TINA-TI files. TINA-TI can be downloaded from www.ti.com/tina-ti.

    INA117Xfer.zip
  • Hi Pete,

    I downloaded Tina and put the circuit together and Tina was giving me the same output that I saw on the bench.  I did not have pin 2 grounded but used it as differential input. Circuit attached.  0841.INA117 characteristics.TSC

    Here are things that I tried:


    1.  If you have V2 and V1 set to 15V  and VG1 and VG2 to 5V I get 10V output.

    2.  If you have V2 and V1 set to 15V  and VG1 and VG2 to -5V I get -10V output.

    3.  If you have V2 and V1 set to 12V and VG1 and VG2 to 5V I get 8.13V output.

    4.  If you have V2 and V1 set to 12V and VG1 and VG2 to -5V I get -10V output.

    Datasheet states the output is +/-10V @ 15V rail which is correct looking at the above results.  Am I doing something wrong with the software?  If I am getting the same results on the bench I would say that 12V rail voltage does degrade output of INA117.  I can't use this as is so can you recommend another op amp with high CM that gives me correct output being supplied by 12V rail?

    Thanks

    Peter V.

  • Hello Peter,

    I recommend evaluating the INA149. It was release a couple years ago and has a more thorough data sheet and more robust simulation model.

  • Thanks Pete.

    TINA does not have INA149 in its library.  I guess I need a full version?

    Peter V.

  • Hi Peter,

    It is available on the INA149 product page.  Click on the link below:

    http://www.ti.com/product/ina149

    Then click on "Tools & Software".

  • Ok I got the reference design. It looks like INA149 works the way it is suppose to.  Did the same setup that I used for testing INA117 and the results were what I expected from unity gain amplifier.

    I do have one question though.

    I see that the REF A and REF B resistors are not matched.  What is the reason for that?

    Peter 

  • Hello Peter,

    Here is a forum post that should answer your question:

    http://e2e.ti.com/support/amplifiers/etc_amplifiers__other_linear/f/18/t/244237.aspx