Due to the fact that the OpenLab oscilloscope uses non-differential ADCs for both input channels, the voltage at the ADCs should only be at positive levels. In order to measure negative voltages, the signal has to be shifted into positive levels during signal conditioning. This will add an offset to the zero point of the measured signal. Finally, this will create a "virtual" zero point which enables measuring negative and positive voltages. The exact location of this "virtual" zero point is known by the Graphical User Interface (GUI). The GUI will than reverse the effect of stage 4 in order to display the measured signal correctly.
This offset addition is done by using an inverting amplifier configured with an constant offset of 1.75 volts. The necessary voltage is generated at the positive terminal of the op-amp by a voltage divider. Configuring the gain level of this stage to 2, the voltage at the positive terminal, and therefore also the offset, gets doubled. As a result, the input signal of stage 4 gets shifted by 1.75 volts. This will enable measuring negative voltages with an non-inverting ADC. Those 1.75 volts offset is called the "virtual" zero point.
Figure 1 shows the schematic of stage 4.
Figure 2 shows the input and output of the fourth stage. To demonstrate the behavior of this front-end stage, a test signal was applied. This waveform is a sinusoidal signal with a frequency of 1kHz and a amplitude of 5V.
The input of stage 4, which is applied by stage 3, is represented by the red waveform. The output of stage 4 is shown in blue. As mentioned before, the third amplification stage was selected in order to correctly amplify the input signal. As shown in figure 2, the output of stage 4 is centered at 1.75V. This is the previously mentioned virtual zero point of the signal. It enables the measurement of negative signals with non-differential ADCs.