1. SPS Accounts:
    Do you find yourself coming back time after time? Do you appreciate the ongoing hard work to keep this community focused and successful in its mission? Please consider supporting us by upgrading to an SPS Account. Besides the warm and fuzzy feeling that comes from supporting a good cause, you'll also get a significant number of ever-expanding perks and benefits on the site and the forums. Click here to find out more.
    Dismiss Notice
Dismiss Notice
You are currently viewing Boards o' Magick as a guest, but you can register an account here. Registration is fast, easy and free. Once registered you will have access to search the forums, create and respond to threads, PM other members, upload screenshots and access many other features unavailable to guests.

BoM cultivates a friendly and welcoming atmosphere. We have been aiming for quality over quantity with our forums from their inception, and believe that this distinction is truly tangible and valued by our members. We'd love to have you join us today!

(If you have any problems with the registration process or your account login, please contact us. If you've forgotten your username or password, click here.)

USB DSO: The Front End Design

Discussion in 'BoM Blogs' started by Blackthorne TA, Mar 18, 2012.

  1. Blackthorne TA

    Blackthorne TA Master in his Own Mind Staff Member ★ SPS Account Holder Adored Veteran Pillars of Eternity SP Immortalizer (for helping immortalize Sorcerer's Place in the game!) New Server Contributor [2012] (for helping Sorcerer's Place lease a new, more powerful server!) Torment: Tides of Numenera SP Immortalizer (for helping immortalize Sorcerer's Place in the game!)

    Joined:
    Oct 19, 2000
    Messages:
    10,415
    Media:
    40
    Likes Received:
    232
    Gender:
    Male
    Remove all ads!
    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: [PDF]http://dl.dropbox.com/u/66799035/oscilloscopeschematic2.pdf[/PDF]

    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. :)
     
    Blackthorne TA, Mar 18, 2012
    #1
  2. Dice

    Dice ★ SPS Account Holder Adored Veteran

    Joined:
    Jun 23, 2002
    Messages:
    5,125
    Media:
    24
    Likes Received:
    149
    Gender:
    Female
    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.
     
    Dice, Mar 21, 2012
    #2
  3. Blackthorne TA

    Blackthorne TA Master in his Own Mind Staff Member ★ SPS Account Holder Adored Veteran Pillars of Eternity SP Immortalizer (for helping immortalize Sorcerer's Place in the game!) New Server Contributor [2012] (for helping Sorcerer's Place lease a new, more powerful server!) Torment: Tides of Numenera SP Immortalizer (for helping immortalize Sorcerer's Place in the game!)

    Joined:
    Oct 19, 2000
    Messages:
    10,415
    Media:
    40
    Likes Received:
    232
    Gender:
    Male
    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 :)
     
    Blackthorne TA, Mar 21, 2012
    #3
Sorcerer's Place is a project run entirely by fans and for fans. Maintaining Sorcerer's Place and a stable environment for all our hosted sites requires a substantial amount of our time and funds on a regular basis, so please consider supporting us to keep the site up & running smoothly. Thank you!

Sorcerers.net is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to products on amazon.com, amazon.ca and amazon.co.uk. Amazon and the Amazon logo are trademarks of Amazon.com, Inc. or its affiliates.