Typically your board would have a ground plane. Since you are using the audio I/O pins, you should have 2 ground planes - one for digital and one for analog. All of your analog components should use the analog ground plane, and all of your digital components should use the digital ground plane. The 2 ground planes should be connected at only one point. Typically this is at the SIM300's analog ground pin. By seperating the grounds you keep the digital an analog signal return paths seperate, and therefore keep digital noise from getting into the analog ground.drbizzarow said:there is only one ground how i seprate digital and analog ground????