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INA169: Non linear output and general oddities

Jens Johansson76
Prodigy 30 points
Part Number: INA169
Other Parts Discussed in Thread: ADS1015,

Hi,

We are using the INA169 as a part of a design and have run into a problem. The INA 169 is configured in a pretty straightforward manner: It runs on a 5 V input which is separate from the Measured Bus. There is a 100nF capacitor between V+ and GND. Rs is 0.047 Ohm and Rl is 8.2 K. It outputs into a plain 358 Op-amp buffer which in turn feeds into and ADS1015 ADC (and from there to the MCU via I2C). The measured current is in the range from 0.5 to 10.37 A, hence the small Rs.

Now, when measuring this, it behaves very odd. All Vo measurements are at the output of the 358 out of convenience, I have verified this seems to be the same as the direct output from the INA169 down to a few microvolts. From 0.1 to 1A the measurements are:

Is Vo
0.1A 0.0045V
0.2A 0.0275V
0.3A 0.0652V
0.4A 0.0824V
0.5A 0.0897V
0.6A 0.1921V
0.7A 0.4006V
0.8A 0.5125V
0.9A 0.6178V
1.0A 0.7156V

Now, the first values I suppose are off since the Vs diff being to small for it to measure correctly, however it seems fairly linear from 0.3 to 0.5A, then it more than doubles it output at 0.6A and then again at 0.7A. Again it's linear up to 1.0A and after that things went to weird to continue organized measurement: At 1.1A Vo is 0.8121V which seems in line. However, after that it starts to drop sharply, and at Is 1.2A Vo is back down to about 0.5 V. This continues, and at 3.5A Vo is -0.4582 V(!

I'm sure I'm missing something very obvious here since the behavior is very weird. I can verify that the voltage drop over Rs is as expected for whatever Current applied. I've tried three different INA169 chips with the same results.

Cheers

//Jens

  • 0
    TI__Mastermind 21805 points
    Hello Jens,

    Thanks for considering to use Texas Instruments in your design. The measurements you are getting do deviate from what we would expect as normal device behavior. Would it be possible for you to remove the buffer and probe directly at Vout of the INA169? We first would like to eleminate any error contributions from the buffer, before we attempt to debug and pinpoint what is wrong with the INA169 setup.
  • 0 in reply to Patrick Simmons
    Prodigy 30 points

    Sure. Like I said, I already measured this and confirmed it is within a very small deviation from the buffered values:

    Is Vo
    0.1A 0.0045V
    0.2A 0.0264V
    0.3A 0.0638V
    0.4A 0.8070V
    0.5A 0.0880V
    0.6A 0.1919V
    0.7A 0.3982V
    0.8A 0.5115V
    0.9A 0.1664V
    1.0A 0.7098V

    Further, I realize I made a mistake in my quick measurements above this range earlier: Slightly above 1.0A, Vo begins to drop rapidly, down to 0.1972V at 1.15A and at 1.2A it is down to 0.001V, where it stays no matter how far further I push it, at least up to 5.0A which is as far as I've tried.

    Cheers

  • 0 in reply to Jens Johansson76
    TI__Mastermind 21805 points
    Hello Jens,

    That is peculiar. I will run some tests on the INA169 in our lab and see what kind of results I get. However, in the meantime could you provide me with a basic schematic of your INA169 and 358 setup including how you are providing your load current and your power supply voltages?
  • 0 in reply to Jens Johansson76
    TI__Mastermind 21805 points

    Jens,

    I believe I have one possible explanation for what you are seeing.  While in the lab, I took a few approaches of generating the current conditions you have posted.  In my first test, I did a high-side current measurement configuration with a 1 ohm resistor in series with 0.042 ohm shunt.  (In reality there were also some parasitic resistance from my leads and board traces.) In this test, I used a voltage supply to slowly ramp up my current going through these resistors and I managed to generate results similar to yours (As shown in Table 1 below).  One thing to note with this approach is that it violates the minimum common mode specification of 2.7 volts.  So naturally I did another test to account for this.

    Table 1

    In the second test, I replaced my 1 ohm resistor with an electronic load.  In this particular setup I set the common mode to 5V.  Now with this setup, I get values that you would expect as shown below in table 2.

    Table 2

  • 0 in reply to Patrick Simmons
    TI__Mastermind 21805 points
    Hello Jens,

    I have not heard from you in awhile so I presume you have resolved your issue and I will close this thread. However, if you do have further questions, feel free to reach out in a new thread.