9.2.1.2.5.4 Using the Line Driver Amplifier in the TAS5760LD as a Second-Order Filter

Several audio DACs used today require an external low-pass filter to remove out-of-band noise. This is possible with the headphone amplifier / line driver inside the TAS5760LD, as it can be used like a standard operational amplifier. Several filter topologies can be implemented, both single-ended and differential. In Figure 59, multi-feedback (MFB) with differential input and single-ended input are shown.

An ac-coupling capacitor to remove dc content from the source is shown; it serves to block any dc content from the source and lowers the dc gain to 1, helping to reduce the output dc offset to a minimum.

The component values can be calculated with the help of the TI FilterPro™ program available on the TI Web site at: http://focus.ti.com/docs/toolsw/folders/print/filterpro.html.

Figure 59. Second-Order Active Low-Pass Filter

The resistor values should have a low value for obtaining low noise, but should also have a high enough value to get a small-size ac-coupling capacitor. With the proposed values of R1 = 15 kΩ, R2 = 30 kΩ, and R3 = 43 kΩ, a dynamic range (DYR) of 106 dB can be achieved with a 1-mF input ac-coupling capacitor.