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Problem in INA188 voltage measurement

Part Number: INA188
Other Parts Discussed in Thread: TINA-TI, INA326

  • At rest, INA188 output voltage is less than the input voltage by 50-60mv.
  • During charging as the voltage of battery increases the difference between the input and output of INA188 IC also increases respectively.This behavior remains the same for different current and temperatures.
  • During discharing the ouput of the INA188 is less than the input by 102±2mv irrespective of current and temperature conditions.
  • Offset is more in INA188(50-60mv) than simple differential amplifier circuit(7-12mv).
  • A simple differential amplifies exhibits less difference than INA188.

  • Roobini,

    You do not provide any information about circuit configuration.  In order to assist you, we need to see your circuit schematic.

    You can download a free Tina-TI simulation tool under following link: 

  • Hi Roobini,

    The issue is that your input cannot simply float  with respect to the INA ground. There is nothing to set the common-mode, and there is no path for bias current to flow. Populating R102 and grounding the negative input doesn't really solve the problem either, because one of the internal amplifiers will still saturate because it cannot drive below ground.  You could ground one terminal if you used a device like the INA326, but at that point you could also use a standard op amp. If you want  to use this circuit in single supply configuration, you will have to provide a path to ground on the input and bias the common-mode (ideally to mid supply) as shown below

  • Hi Zak,
    That was good explaination. Could you please elaborate on how does this common mode affect the phenomenon of voltage variations seen during charging (voltage difference a function of battery voltage) and discharging (constant offset), as seen in the graphs attached.

    Thanks
    Sushant
  • Sushant,

    If you float the inputs, there is no path for the IB flow to system ground, which will cause the input common-mode voltage to collapse against one of the rail; this will violate the input common-mode voltage specified range (see below) resulting in the non-linear operation of the input differential pair and thus leading to a large increase in Vos and otherwise unpredictable behavior you see.

  • Would like to have clarification on these: (with reference to explanatory circuit commented before)

    1. How do you select the mid-voltage V4? Given in my application I am measuring the voltage of a li-ion cell, so the voltage source shown V3 will be a cell in my case with voltage range of 0 to 5V.

    2. How stable shall this voltage V4 be, can I give this voltage using potential divider circuit from Vcc supply ? 

  • Hi,
    Would like to have clarification on these: (with reference to explanatory circuit commented before)

    1. How do you select the mid-voltage V4? Given in my application I am measuring the voltage of a li-ion cell, so the voltage source shown V3 will be a cell in my case with voltage range of 0 to 5V.

    2. How stable shall this voltage V4 be, can I give this voltage using potential divider circuit from Vcc supply ?
  • Hi Sushant,

    The mid voltage is based on the common-mode limitations of the amplifier you are using. Setting the common-mode to mid-supply always maximizes your input/output range. You may find this tool useful for determining the limitations in your given application: www.ti.com/.../INA-CMV-CALC

    Assuming your input is truly floating and the only path to ground is that you are providing is through V4, then yes you could use a resistive divider from the supply to generate this rail. You can verify all of this in simulation using TINA-TI: http://www.ti.com/tool/TINA-TI
  • Sushant

    We haven't heard back from you so we assume this resolved your issue. If not, just post another reply below.

    Thanks
    Dennis