User can choose a current-sense resistor that is as large as possible to maximize the accuracy of the INA185. A large sense resistor maximizes the differential input signal for a given amount of current flow and reduces the error contribution of the offset voltage. However, there are practical limits as to how large the current-sense resistor can be in a given application. The INA185 has a typical input bias current of 75 µA for each input when operated at a 12-V common-mode voltage input. When large current-sense resistors are used, these bias currents cause increased offset error and reduced common-mode rejection. Therefore, using current-sense resistors larger than a few ohms is generally not recommended for applications that require current-monitoring accuracy. Another common restriction on the value of the current-sense resistor is the maximum allowable power dissipation that is budgeted for the resistor. Equation 2 gives the maximum value for the current sense resistor for a given power dissipation budget:

Equation 2.

where:

• PD_MAX is the maximum allowable power dissipation in R_SENSE.
• I_MAX is the maximum current that flows through R_SENSE.

An additional limitation on the size of the current-sense resistor and device gain is due to the power-supply voltage, V_S, and device swing-to-rail limitations. To make sure that the current-sense signal is properly passed to the output, both positive and negative output swing limitations must be examined. Equation 3 provides the maximum values of R_SENSE and GAIN to keep the device from hitting the positive swing limitation.

Equation 3.

where:

• I_MAX is the maximum current that flows through R_SENSE.
• GAIN is the gain of the current sense-amplifier.
• V_SP is the positive output swing specified in the data sheet.
• V_REF is the externally applied voltage on the REF pin.

To avoid positive output swing limitations when selecting the value of R_SENSE, there is always a trade-off between the value of the sense resistor and the gain of the device to consider. If the sense resistor selected for the maximum power dissipation is too large, then selecting a lower-gain device to avoid positive swing limitations is possible.

The negative swing limitation places a limit on how small of a sense resistor can be used in a given application. Equation 4 provides the limit on the minimum size of the sense resistor.

Equation 4.

where:

• I_MIN is the minimum current that flows through R_SENSE.
• GAIN is the gain of the current sense amplifier.
• V_SN is the negative output swing of the device (see Rail-to-Rail Output Swing).
• V_REF is the externally applied voltage on the REF pin.

In addition to adjusting the offset and gain, the voltage applied to the REF pin can be slightly increased to avoid negative swing limitations.