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USB DSO: The Front End Design

Posted Sun, 18th Mar '12 at 11:45pm by Blackthorne TA

So, what does the front end of a DSO do?

It takes the analog signal to be measured and manipulates it via buffering, attenuation, and amplification to the level range accepted by the ADC on the back end. And since I wanted to be able to use a standard x1, x10 oscilloscope probe, it needed to have an input resistance of 1M ohm.

If you remember from the back end design, the ADC I chose had an input range between 2 and 3 volts; in other words a 1 volt peak to peak signal centered at 2.5 volts. A useful oscilloscope needs a much wider range of measurement than that; something from a few millivolts to a few tens of volts would be useful. So small input signals have to be able to be amplified to bring them up to the ADC range, and large input signals need to be attenuated to bring them down to the range of the ADC in order to make useful measurements.

So with no futher ado I present the front end design schematic for my DSO:

On the far left is the BNC connector to which a standard oscilloscope probe would be attached to bring in the analog singal to be measured.

The first amplification stage is provided by one half of the Analog Devices AD8039 dual voltage feedback amplifier. Labelled U12.2 on the schematic, it is configured as a non-inverting amplifier to buffer the input signal before the set of voltage dividers provided by resistors R14 through R23.

The voltage dividers provide 8 different attenuations that feed to the Analog Devices ADG608 8-channel analog multiplexer labelled U13. This allows the SoC on the back end to select which of the 8 levels to pass forward to the rest of the amplifier chain.

The second amplification stage is provided by the second half of the AD8039 (labelled 12.1), again configured as a non-inverting amplifier, and its purpose is to shift the signal level up by 2.5 volts (remember the ADC needs a signal centered at 2.5 volts).

The final amplification stages are provided by an Analog Devices AD8008 dual current feedback amplifier.

The first half of the AD8008, labelled 11.2, is configured again as a non-inverting amplifier and is used to buffer the signal now centered at 2.5 volts.

The second half of the AD8008, labelled 11.1, is configured as an inverting amplifier that amplifies the incoming signal by 10 to be then provided to the ADC for digitization.

The extra circuitry in the lower left is an Analog Devices ADM8828 charge pump voltage inverter to supply the -5 volts needed by the AD8039 and ADG608.

So there you have it: The front end design of my DSO.

Coming up: Bringing the designed circuitry into reality.
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  1. Old Comment
    Dice's Avatar
    This is very cool that you are designing this piece of technology. I was wondering if you have a "Dummies" version of your blog available because I look at it and just get confused.
    Posted Wed, 21st Mar '12 at 3:25pm by Dice Dice is offline
  2. Old Comment
    Blackthorne TA's Avatar
    Hehe. I would be happy to answer any questions if the above is too opaque.

    As I mentioned in one of the other blog comments, it's difficult for me to judge what level would be appropriate for those who might be interested.

    I tried to make the opening brief descriptions simple enough for the layperson to get the general idea at least... was that not successful?

    Honestly, feel free to ask anything you like
    Posted Wed, 21st Mar '12 at 4:18pm by Blackthorne TA Blackthorne TA is offline

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