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How it this analog front end circuit working?

Sudeep AJ

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This circuit has been taken from a "DIY Oscilloscope" project. This circuit is changing the input wave swing to 0V to 3.3V. The circled voltage source is Vsys/2, which is the voltage at which the zero level of the input wave will be shifted to. Vsys is 3.3V here. 

The output is as follows:

image

I wanted to know how is this working and can I use it for my project?

The original circuit with Raspberry PI PICO is as follows.

image

Thank you.

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  •  As  indicates, this circuit attenuates the input by a factor of 10, but it would work over a wider range of input voltages if it used rail-to-rail output opamps.

    U1-1 might be problematic as a clamping circuit with this particular op-amp, but it can't exceed the rail unless VSYS is a high voltage, so it shouldn't even be needed in this circuit.

  • in reply to dougw

       . Thank you for your valuable inputs. If I really need rail-to-rail op-amp and input range is only 1.8V. How in simulation it is working then? In simulation it seems like it is serving the purpose, basically taking the input sine of large swing to 0V - 3.3V swing centered around 1.65V so that my Microcontroller can take it properly. What I also observed in the simulation is, if I change the Vsys to 5V(if I want to connect it to Arduino UNO) and make the reference voltage as 2.5V, it serves the purpose too. I am also not understanding the purpose the U2 op-amp and 3 Diodes.

  • in reply to Sudeep AJ

    All 3 diodes are clamping voltages to protect the opamp input from over voltage as well as the MCU. The value of D3 and its opamp buffer, U1:1 is questionable, if the opamp is on a 3.3V rail and the MCU input can go up to 3.3V.

    I agree the LM358 is a bad choice, primarily b/c the low bandwidth and the non linearity it has in the crossover region. The input voltage range is not limited by the output swing, and the inverting input always maintains 1/2Vcc. It would be nice to use the full range up to full Vcc, however this benefit comes if the MCU’s ADC input tolerates up to full Vcc. Not all does. 

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  • in reply to Sudeep AJ

    All 3 diodes are clamping voltages to protect the opamp input from over voltage as well as the MCU. The value of D3 and its opamp buffer, U1:1 is questionable, if the opamp is on a 3.3V rail and the MCU input can go up to 3.3V.

    I agree the LM358 is a bad choice, primarily b/c the low bandwidth and the non linearity it has in the crossover region. The input voltage range is not limited by the output swing, and the inverting input always maintains 1/2Vcc. It would be nice to use the full range up to full Vcc, however this benefit comes if the MCU’s ADC input tolerates up to full Vcc. Not all does. 

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