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INA129 Issue

Mark Moriarty
Prodigy 20 points
Other Parts Discussed in Thread: INA129, INA826, TINA-TI

Hi,

I'm attempting to amplify a signal from a Hall effect Current Sensor with an INA129. Over the range 0-30A the sensor outputs 2.5 - 3.7V. I have this output connected to the Vin+ input of the amp and a 2.5V connected to the Vin- input. The 2.5V comes from a regulated 5V voltage divided by 2. The amp is single supply powered with 24V.

I want to have a 0-10 V output across the 0-30A range. I'm currently using a 7K68 Ohm resistor for Rg which would give a Vout of 9V at 30A.

When tested at 25A I am only getting a Vout of 4.1V. I have switched Rg for a 800 Ohm resistor (Gain = 73) and at 25 A I'm getting 4.5V.

I understand that I won't be able to get Vout right down to 0V but is this also affecting the Vout max? If not could you provide some further suggestion including a more appropriate part for the application?

I look forward to your reply.  

  • 0
    TI__Mastermind 25865 points

    Hello Mark,

    I am working on this now and will get back with you shortly.

  • 0
    TI__Mastermind 25865 points

    Hello Mark,

    The output swing of the INA129 is dependent on the input common-mode voltage.  Graphs of input common-mode voltage vs. output voltage swing are shown on page 5 of the PDS.  Unfortunately we do not have such graphs for single-supply operation of the INA129.  With a single supply and given your common-mode voltage, the output of the device is unable to reach the levels you require.

    The most straightforward approach to solving the issue is to power the INA129 with dual supplies (e.g. +/-15V).  The TPA65130 is a power device that can provide such rails given an input voltage of 2.7V-5.5V.  You can then ground the reference pin.  A gain of ~8V/V yields ~10V at the output given a 1.2V differential input.  

    Alternately you may be interested in the INA826.  The INA826 has a few figures that show its single-supply behavior.  Pedestaling the input with ~12V allows the output to swing appropriately.

    I have attached a zip file with TINA SPICE simulations (which can be downloaded from www.ti.com/tina-ti) for both the INA129 and INA826 suggestions.  I hope this helps.

    Mark.zip
  • 0 in reply to Pete%20Semig
    Prodigy 20 points

    Hi Pete,

    Thanks for your help. Before you replied I had simulated my setup in TINA and and could replicate the voltage I am seeing here. I then simulated using dual supplies and could see the correct results. Thanks for the circuits. I'll look to implementing the INA129 circuit suggested. The power device you mention doesn't seem to return any results. Is is the correct part number?

    On the INA129 datasheet the common mode voltage range is defined as (V-)+2 & (V+)-2. Does this not mean, that for a single supply, the common mode requirement is met when both input voltages are above 2V? 

  • 0 in reply to Mark Moriarty
    TI__Mastermind 25865 points

    Hello Mark,

    Sorry about the typo...it's TPS65130.

    The specification for input common-mode that you mention is given for dual-supplies, listed at the top of the page.  The output for a common-mode voltage within that range (with dual supplies) is subsequently shown in the figures at the bottom of page 5, per note (1).  We do not have a graph in the INA129 PDS that shows the input common-mode voltage vs. output voltage range given a single supply, but the INA826 does.

    I hope this helps!