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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 5 ADC Protection
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To protect the ADCs of the OpenLab oscilloscope from too high or too low voltages, a protection circuit was designed.
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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.
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{empty} +
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image::https://es.technikum-wien.at/openlab/openlab_wiki/wikis/img/sig_proc_osci_hardware/sig_proc_osci_hardware_stage5.PNG[caption="Figure 1: ",title="Schematic of stage 5 of the OpenLab oscilloscope front-end",align="center"]
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{empty} +
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The upper limit is achieved by adding a voltage bias to the diode circuit.
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By subtracting the forward voltage of a standard silicon diode from the desired clipping value the required bias voltage can be calculated.
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{empty} +
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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.
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This offset is generated by a simple voltage divider consisting of two resistors (R2 & R5).
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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. +
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This means that if a negative voltage is outputted by stage 4, the protection circuit will let negative voltages pass.
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To prevent this, an additional offset applied to the negative cutting diode is mandatory.
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This offset voltage is set a bit higher than the forward voltage of the diode (approx. 1V), so that the clipping engages early enough.
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Finally stage 5 will protect the ADCs from damage. |
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