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INA149: reader in a 4–20 mA loop

Hielke Bakker
Prodigy 60 points
Part Number: INA149
Other Parts Discussed in Thread: INA849

We are using the INA149 as the reader in a 4–20 mA loop. The INA is powered with a +15V and -15V supply. REFa and Refb are connected to ground, and the measurement resistor for the 4–20 mA loop is 91 Ohms.

When we arrange the loop so that the INA149 is the 'last' item in the loop, we are measuring precisely the current that flows through the 4–20 mA loop (Circuit A in the picture below). However, when we switch out the sensor and the INA149 (Circuit B in the picture below), the reading of the INA149 is off by about 0.02 mA

What could be the reason for this behavior? Could it have to do with the common-mode range, as mentioned in the datasheet below? We have observed that when we lower the loop voltage, the reading error also decreases.

  • 0
    TI__Guru 51766 points

    Hi Hielke,

    In the example above, is the input common-mode voltage change 24V between A and B seen by the INA149 inputs?

    I am assuming that the change in measurement is 0.02mA out of the 20mA full-scale. The typical INA849 CMRR is ~100dB typical, or ~10uV/V.  Assuming a 24V common-mode change, this corresponds to a delta change of the INA149 input offset of ~240uV over the common-mode voltage change.  The shunt resistor is 91Ω, hence the voltage across the shunt is ideally @ 20mA is 1.82V.  There is an amount of error due to the common-mode rejection ratio of the device, but the CMRR does not account for the dominant error.

    The common-mode input impedance of the INA149 is ~200kΩ typical, resulting on an input current change that flows to the INA849 input terminals, as the common-mode voltage changes. The change in the difference amplifier input current is probably the dominant source of error. See simulation below.

    Thank you and Regards,

    Luis