Hi, i have a project to convert audio signal to digital signal, so i decided to use ADC. The signal after ADC doesn't have negative part, is there any suggestion to adjust the DC offset using op-amp. I am using digilentinc, AIO1 board.
The ADC is an AD7823. This is a unipolar part. Valid analog input voltages must be positive and be in the range of 0V to VRef. It as two input pins and they call it pseudo-differential, but it really cannot measure any negative voltages. Typical circuits have the Vin- input connected to ground.
To use this device, you must offset the audio signal with a DC voltage level to insure that in never drops below ground. There are lots of different op-amp circuits that can do this. Do an google search on the op-amp level shifter. Below is one hit.
You can try to bias the input of the ADC right in the middle of its range and then couple the audio signal through a capacitor. All you need then is make sure the cap is large enough for the lowest frequency and also make sure the signal does not exceed the range of the A/D, that is, the peak to peak amplitude of the signal must be less than the range of the A/D.