8.3.2 Input Impedance

The TPA3126D2 input stage is a fully differential input stage and the input impedance changes with the gain setting from 7.3 kΩ at 36 dB gain to 50 kΩ at 20 dB gain. Table 1 lists the values from min to max gain. The tolerance of the input resistor value is ±20% so that the minimum value will be higher than 5.9 kΩ. The inputs must 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 along with the input impedance forms a high-pass filter with the following cut-off frequency:

Equation 1.

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

Figure 26. Input Impedance

The input capacitors should be a type of low leakage, such as quality electrolytic, tantalum, or ceramic capacitors. If a polarized type is used the positive connection should face the input pins which are biased to 3 Vdc.