INA170 Help

Hello,

Does anyone have any suggestions for this thread:

http://e2e.ti.com/support/amplifiers/etc_amplifiers__other_linear/f/18/t/86349.aspx

I didn't know how to move it over to the Precision Amplifiers folder.

Basically, I have a high side current shunt on a 12V battery that should give me between +/- 5mV of voltage drop when the load current flows.  I hooked everything up as shown in Figure 1 of the data sheet.  I use 100k resistors for both Ros and RL, which should give me a gain of 100.  I have 3.3 V supply for V+, and am providing a 1.5V Vref using the wiper of a 10k variable resistor. 

The first problem is that the Vref voltage doesn't seem to have any effect on the output voltage.  In fact, the voltage at the Vref pin measures 12V (even though I have connected a 1.5 V reference as explained earlier, the chip is feeding power through the pot).  Secondly, when the shunt voltage changes even slightly, the output voltage immediately up past the 3.3V rail to around 6 V.  How is this happening?  I have tried three different chips and they all do the same thing.  Note that I have used a MSOP8 to DIP adapter and currently have everything breadboarded. 

Thanks in advance.

Mark

  • Hello Mark,

    Thank you for considering the INA170 for your application.

    I set up a similar circuit to yours but did not observe the issues you are observing.  The circuit was slightly different due to equipment limitations.  Here was my setup:

    Vcm=10V
    Vsense=100mVpp @ 100Hz
    V+=3.3V
    Vref=1.5V (adjusted down to 1.3V due to clipping...most likely due to device and equipment errors)
    Ros=RL=10k 1% discrete (Gain=10)

    Therefore I have the following recommendations:
    a)  Try replacing the potentiometer with a fixed resistor and/or apply the Vref voltage directly to the pin of the device
    b)  Try using a different location on your breadboard
    c)  Since you measured an output voltage of 6V given V+=3.3V, I would sample new devices for these modified measurements.

    Finally, I would also like to point out that the offset voltage of the INA170 (+/-1mV) can introduce up to 20% error on a +/-5mV sense voltage.  You may be interested in a lower offset voltage device such as our INA21X family.  If you use the INA214 (G=100V/V, Vos=+/-60uV) this error would be reduced to 1.2%.  

    I hope this helps.

  • In reply to Pete Semig:

    for the INA170 If I do not need an Offset Resistor on PIN 5, can I just leave that pin open?

  • In reply to Jehanzaib Karim:

    Hello Jehanzaib,

    The Ros pin is used in conjunction with Vref pin to bias the output for bi-directional operation, so I'm not sure why it's not being used.  For unidirectional operation perhaps you'd be interestd in the INA169 instead.