9.3.5 Input Impedance

The TPA3221 input stage is a fully differential input stage and the input impedance changes with the gain setting from 7.7 kΩ at 34 dB gain to 47 kΩ at 18 dB gain. Table 1 lists the values from min to max gain. The tolerance of the input resistor value is ±20 % so the minimum value will be higher than 6.2 kΩ. The inputs need to be AC-coupled to minimize the output DC-offset and ensure correct ramping of the output voltages during power-ON and power-OFF. The input ac-coupling capacitor together with the input impedance forms a high-pass filter with the following cut-off frequency:

If a flat bass response is required down to 20 Hz the recommended cut-off frequency is a tenth of that, 2 Hz. Table 3 lists the recommended ac-couplings capacitors for each gain step. If a -3 dB is accepted at 20 Hz 10 times lower capacitors can used – for example, a 1 μF can be used.

The input capacitors used should be a type with low leakage, like quality electrolytic, tantalum, film or ceramic. If a polarized type is used the positive connection should face such that the capacitor has a positive DC bias.