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INA225 as current measurement

Raul Zubia
Prodigy 40 points
Other Parts Discussed in Thread: INA225, TIPD135, TIPD104, INA326

 

Circuit used:

16V DC power source with 510 Ohm Shunt Resistor inline to load.

TI INA225 powered from 5V DC source with Reference to Ground.

GS01 set to Gain of 25.

 

Application:

Need to sense current from 4uA to 200uA current (Vr = 2.04mV to 102mV).

 

Issue:

With no load connected, we discovered that there is about 160-190uA flowing through the 510 Ohm Shunt resistor. Vr = 96 mV.

From the TI INA225 specification, the internal of the INA225’s IN+ has a path to Ground through Reference. With no load connected and voltage drop across the shunt resistor, it seems there may be current flowing into IN- of the INA225. Is this current draw the INA225’s internal op-amp input bias current?

What is the Bias current? The INA225 specification shows IB at 75uA (typical) with conditions of VIN+ = 12V, Vs = 5V, and VREF = 2.5V. Our application has VIN+ = 16V, Vs = 5V, and VREF = 0V. Does TI has a specification or mathematical formula/model for different conditions to determine the Bias current?

  • 0
    TI__Genius 14825 points
    Raul,

    What you are measuring is certainly the bias current into the INA225, which is used to bias the front end circuitry (so that you can have a 16V common mode with only a 5V supply). Typically Current Shunt Monitors of this nature are intended for larger currents, in part for this reason. True Instrumentation Amplifiers have bias currents that are orders of magnitude smaller, but sometimes lack the circuitry to support higher-than-supply common mode voltages.

    Large shunt resistors and small currents do produce the same mV output as large currents and small shunt resistors, but siphoning 75uA off 1A is very different from siphoning 75uA off 4uA. In general, we suggest keeping the shunt resistors in the milliohm range.

    There is no equation for finding the bias current. That is the average of the data taken from characterization, with the min and max limits set appropriately per the data.

    Have a look at TI Designs TIPD104 (using instrumentation amp INA326) and TIPD135. These designs offer uA-mA level current measurement.

    Please let us know how else we can assist you.

    Best regards,
    Jason Bridgmon