• State
  • Locked Locked
  • Replies 8 replies
  • Subscribers 48 subscribers
  • Views 556 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

INA220

bj4069
Prodigy 70 points
Other Parts Discussed in Thread: INA220, INA282

I have an INA220 designed into a product that now needs to accommodate a voltage of 28V at the VBUS, VIN+ and VIN- pins (high side current sense). I have unused component locations for resistive dividers on all of those inputs. I tried using pairs of 40.2K 0.1% resistors to divide down the differential voltages from the current sense resistor. With a small current (20 mA) through a 0.01 Ohm resistor the voltage differential at the VIN+ and VIN1 inputs was about 0.4V - much larger than expected.  The original circuit (without resistive dividers) used 10 Ohm resistors in series with the VIN+ and VIN- pins of the INA220 and worked just fine.  The input circuitry to those pins is symmetrical and within 0.4" of the input pins of the INA220.

In an effort to understand what is causing the large differential voltage I removed the halves of the resistive dividers going to ground, so the circuit is the same as my original circuit, but with 40.2K Ohm resistors instead of the original 10 Ohm resistors in series with the VIN+ and VIN- pins of the INA220.  I reduced the applied voltage to 24V.  The large differential voltage is still across the VIN+ and VIN- pins of the INA220.  This seems to imply the input impedance of the VIN+ pin is different than that of the VIN- pin.  Is that the case or am I overlooking something?

I consulted the data sheet and it specifies the same input impedance for both the VIN+ and VIN- pins: 20 uA,  Can anyone explain why the input impedance is listed in units of uA?

Thanks.

 

  • 0
    TI__Mastermind 25815 points

    Hello bj,

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

  • 0 in reply to Pete%20Semig
    Prodigy 70 points

    Hi Pete,

    Have you had a chance to work on this?

  • 0 in reply to bj4069
    TI__Intellectual 2940 points

    Hello BJ,

    Pete is out of the office on a personal matter.  Let me take a look at this and I will get back to you later this morning.

  • 0 in reply to Scott Hill
    TI__Intellectual 2940 points

    Hello BJ,

    The INA220 is not designed to allow for more than 26V to be applied to the device's input pins.  The voltage divider that you are creating would drop the voltage down to the allowable common-mode voltage range but, as you have noticed, with this device doing so creates an additional error term at the inputs. 

    This is due to a mismatch in input currents.  Under normal operating conditions, this mismatch has no effect on the system.  However, when external resistance is placed in series with the inputs, an additional voltage drop is created that either adds to or subtracts from the drop across the shunt resistor depending upon the direction of the load current.  The 10 ohm input filter resistors shown in the datasheet were chosen to be in this range to limit this effect on the system. 

    Is the 28V level that you say you need to accommodate a point that you are trying to measure or are you just trying to make sure the device can withstand that voltage?  What is the maximum level that you are actually trying to measure the device at?  What is your full current load range that you are monitoring?

  • 0 in reply to Scott Hill
    Prodigy 70 points

    Hi Scott,

    The maximum voltage that my system will see is 28V (i.e. there will be 28V on the high side of the current sense resistor).  The current sense resistor is 0.010 Ohms and the current through it can vary from 100mA to 25A.

     

  • 0 in reply to bj4069
    TI__Intellectual 2940 points

    Hi BJ,

    My question about the 28V that you see in your system is to determine if you are trying to monitor the 28V or if we can put a clamp on the line so that the INA220 doesn't see this high of a voltage.  If you are trying to actually measure the differential voltage while the common-mode voltage is at 28V, the INA220 is not the appropriate device to use. 

    We will be releasing a comparable device later this year that will extend this common-mode range some to include your 28V level but we don't have another device with the digital interface available today.  You may want to look at the INA282 for a wider common-mode voltage range.  This device is an analog output current shunt monitor with a common-mode range up to 80V.

  • 0 in reply to Scott Hill
    Prodigy 70 points

    Hi Scott,

    Yes, the common mode voltage for the differential inputs is 28V.

    Thanks for the explanation on the differential inputs.  For my future reference it sounds like I should interpret the input impedance for the differential inputs really as the input current.

    Thanks for your help.

  • 0 in reply to bj4069
    Prodigy 70 points

    Hi Scott,

    Do you know if the comparable device you mentioned in your previous email that is due to be released later this year will be pin-compatible with the INA220?