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== https://es.technikum-wien.at/openlab/openlab_wiki/wikis/home[Home] | https://es.technikum-wien.at/openlab/openlab_wiki/wikis/sig_proc_hardware[<Signal Processing Hardware] | https://es.technikum-wien.at/openlab/openlab_wiki/wikis/ETS_theory[Equivalent Time Sampling - Theory>]
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= Stage 4 Adding a Constant Offset to the Signal
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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.
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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.
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Finally, this will create a “virtual” zero point which enables measuring negative and positive voltages.
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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.
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{empty} +
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This offset addition is done by using an inverting amplifier configured with an constant offset of 1.75 volts.
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The necessary voltage is generated at the positive terminal of the op-amp by a voltage divider.
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Configuring the gain level of this stage to 2, the voltage at the positive terminal, and therefore also the offset, gets doubled.
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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.
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Those 1.75 volts offset is called the "virtual" zero point.
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{empty} +
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Figure 1 shows the schematic of stage 4.
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