To protect the ADCs of the OpenLab oscilloscope from too high or too low voltages, a protection circuit was designed. Two diodes will clip the signal if leaving the acceptable voltage range of 0 to 3.3 volts. The circuit diagram of stage 5 is shown in figure 1.
The upper limit is achieved by adding a voltage bias to the diode circuit. By subtracting the forward voltage of a standard silicon diode from the desired clipping value the required bias voltage can be calculated.
In case of the OpenLab oscilloscope, standard 0.7V forward voltage diodes were used. These leads to a required offset voltage of approximately 2.6V to cut voltages over 3.3V.
This offset is generated by a simple voltage divider consisting of two resistors (R2 & R5).
Due to the effects of the forward bias voltage drop, without an additional offset to the negative voltage cutting diode (D2), the signal will be cut off at -0.7V.
This means that if a negative voltage is outputted by stage 4, the protection circuit will let negative voltages pass. To prevent this, an additional offset applied to the negative cutting diode is mandatory. This offset voltage is set a bit higher than the forward voltage of the diode (approx. 1V), so that the clipping engages early enough. Finally stage 5 will protect the ADCs from damage.