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# INA181: Amplifying negative voltage with INA181 without using dual supply

Part Number: INA181
Other Parts Discussed in Thread: XTR111,

Hello all,

I am working on a design for a AC current transducer with 4-20mA RMS output. Output from CT secondary is converted to voltage by shunt resistor and amplified with INA181 current sense amplifier. The MCU performs RMS calculation on the signal and then send the corresponding output to DAC MCP4725, whose output is converted to 4-20mA signal by XTR111.

Schematic:

My main concern with the circuit is whether the INA181 could amplify the CT output with accuracy. Since current on CT secondary (hence voltage on shunt resistor) is AC, there will be negative voltage at the inputs on each period. Altough the INA181 datahsheet states that the lower limit of common mode range is -0.2V, but I am still uncertain if it is possible (or an good idea) to amplify negative voltage input without using dual supply.

Any general advice on the circuit would be greatly appreciated, too!

Best regards,

Zhi

• Hi Zhi,

What does the waveform on the CT secondary look like? (sinewave, magnitude, frequency?) Depending on your waveform, a different device may work better for this application.

For the INA181, as long as the Vcm is equal to or greater than -0.2V, there is no issue even with single supply. If you expect a voltage < -0.2V on the common-mode, dual-supplies can be used to shift the common-mode voltage ratings. What is the allowable voltage range input to the MCU?

Cierra

• Hello Cierra,

Waveform on the CT secondary would be sinewave.

I see that Vcm can be as low as -0.2V, but what about differential voltage? For example, -0.2V on IN+ and 0V on IN-.

Zhi

• Hi Zhi,

The absolute maximum differential voltage rating is -26V to 26V.

So if IN+ = -0.2V and IN-= 0V, differential voltage = -0.2V which is within the absolute maximum spec (though the output will swing to GND = 0.005 V in this scenario).

Cierra