In the previous design of the oscilloscope input buffer (click here), I noted that the negative part of the signal was clipped. However, there is one more problem, the value close to zero (below ~300mV) is also considered too low to be passed by the buffer. The revised design solves this problem and give better limitation on the input signal amplitude to protect your sound card.

• How does it Work?

Basically,  the 5v which is taken from the USB connector, is divided to three parts which are used as +1v, GND, -1v (Note that the GND of the USB becomes -1v when taking the reference between R3 & R2)
 Thats it! these voltages are used to power the opamp which will take the signal to the sound card. Happy scoping!