The internal supply divider of the INA165x is constructed using two 500-kΩ resistors connected in series between the VCC and VEE pins. These resistors are matched on-chip to provide a reference voltage that is exactly one half of the power supply voltage. Noise from the power supplies and thermal noise from the resistors degrades the overall audio performance of the INA165x if allowed to enter the signal path. Therefore, TI recommends a filter capacitor (CF) is connected to the VMID(IN) pin, as shown in Figure 44 The CF capacitor forms a low-pass filter with the internal 500-kΩ resistors. Noise above the corner frequency of this filter is passed to ground and is removed from the audio signal path. The corner frequency of the filter is shown in Equation 3:
When power is applied to the INA165x, the filter capacitor (CF) charges through the internal 500-kΩ resistors. If the CF capacitor has a large value, the time required for VMID(OUT) to reach the final midsupply voltage may be extensive. Adding a zener diode from the VMID(IN) pin to the positive power supply (as shown in Figure 45) reduces this time. The zener voltage must be slightly greater than one half of the power supply voltage.
Using large AC-coupling capacitors increases the start-up time of the line receiver circuit in single-supply applications. When power is applied, the AC-coupling capacitors begin to charge to the midsupply voltage applied to the COM pin through a current flowing through the input resistors as shown in Figure 46. The INA165x functions properly when the input common-mode voltage (and the capacitor voltage) is within the specified range. The time required for the input common-mode voltage to reach 98% of the final value is shown in Equation 4: