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    EEVblog #1022 – Finding A $50 Oscilloscope On Ebay

    Another look at finding a $50 analog oscilloscope on ebay. Forum HERE Podcast: DownloadBe Sociable, ...

    • Simon

      Hi Dave, if you are still keen on trying to track down the reference source, try measuring the voltage across the thru-hole mounted glass packaged diode close to the CPLD. If it comes out at circa +6.2V that’s almost certainly a temperature compensated reference diode (where the breakdown voltage is determined by a complimentary combination of Zener and Avalanche breakdown effects yielding a low tempco). A (previously) well known UK manufacturer of DVMs etc. used these diodes in their equipment and they were far better than any IC bandgap references available at the time (mid 80s to 90s). The diodes were selected from a large batch and burned in for a year or so, such processes come at a price though and I recollect these diodes (not strictly Zeners, but usually referred to as such) came in at a unit price of around 70 UK pounds for the top end parts (and that was in the 80s/90s!)

    • Mmarks3141

      The deduction on the ADC is correct – the Altera part controls it. Don’t much more than that since I was on the software and user interface side of the project.

    • Simon

      Sorry it’s me again, but the components in the region (lower left of the CPLD) have PCB legend annotation prefixes VR. I’m sure you noticed that too, but I just thought I’d reiterate it. I don’t know whether these relate to “coordinate”s in a circuit diagram or perhaps just reference a prefix to Voltage Reference. In view of my reference diode comments (which incidentally is annotated as VR600) I just thought that I’d mention that this diode is close to a few other linear parts / op-amps that might set up the constant (recommended bias) current for such a part.

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