低压压缩扩展器

　　The SA575 is a precision dual gain control circuit designed for low voltage applications. The SA575’s channel 1 is an expandor， while channel 2 can be configured either for expandor， compressor， or automatic level controller （ALC） application.

　　Expandor The typical expandor configuration is shown in Figure 2. The variable gain cell and the rectifier cell are in the signal input path. The VREF is always 1/2 VCC to provide the maximum headroom without clipping. The 0 dB ref is 100 mVRMS. The input is AC coupled through C5， and the output is AC coupled through C3. If in a system the inputs and outputs are AC coupled， then C3 and C5 can be eliminated， thus requiring only one external component， C4. The variable gain cell and rectifier cell are DC coupled so any offset voltage between Pins 4 and 9 will cause small offset error current in the rectifier cell. This will affect the accuracy of the gain cell. This can be improved by using an extra capacitor from the input to Pin 4 and eliminating the DC connection between Pins 4 and 9. The expandor gain expression and the attack and release time constant is given by Equation 1 and Equation 2， respectively.