For this application, the INA254 measures current flowing through the speaker from the CLASS-D amplifier. The integrated shunt of 400 µΩ with an inductance of only 2 nH is an excellent choice for current sensing in speaker applications where low inductance is required. The low-inductive shunt enables accurate current sensing across frequencies over the audio range of 20 Hz to 20 kHz.

The INA254 is setup to support bidirectional currents with the reference set to mid-supply as shown in Figure 8-9. When the power supply to the INA254 is set at 3.3 V and there is no current flowing in the speaker, the output of INA254 is at 1.65 V. When operating with a gain of 75 mV/A with peak-to-peak current of ±15 A, the output of the INA254 will swing from 0.525 V to 2.775 V. In this application the output can be directly connected to an ADC input that has a full scale range of 3.3 V. The INA254 has a low THD+N of 0.1% at 1 kHz that enables distortion measurement of speaker. The INA254 can measure the impedance of the speaker and accurately measure the resonance frequency and peak impedance at resonance frequency. The INA254 can accurately track changes in the impedance in real-time.