There were a few basic requirements for whatever circuit I came up to justify the “practically usable” label: There are quite a few simple “sort of an oscilloscope” type circuits out on the web, but all I know fall short in one or more respects:
• Only one input channel – very often one must display one signal in relation to another (e.g. clock and data, or input vs. output) or trigger on a signal different from the one to be displayed, and this needs at least two channels.
• No reasonable protection against overvoltage at the input.
• Very limited range of input voltages (e.g. just 0…5V) and no adjustable input amplification or attenuation.
• Only usable for very slow signals because of limited sample rate (a few kSa/sc) and/or low bandwidth (a few kHz).
• Non-standard input impedance – standard scope probes need 1 MOhm. Input stage not frequency compensated (limits bandwidth to a few kHz at best).
• Use of exotic, obsolete or unnecessarily expensive components.
• Unnecessarily complex circuit especially given the limited performance.
With the DPScope SE I tried to address all these issues. Of course the end result can never compete with a professional Tektronix or Agilent scope costing hundred times more, but it is still good for a large number of applications.
To minimize cost some preliminary considerations set the general direction:
• Try to integrate as much of the scope hardware into a single chip.
• The instrument should use a PC for control and display – saves the cost of a dedicated LCD display (while providing unmatched resolution and processing power) and front panel control knobs and buttons.
• Connection should be through USB (fast and today’s standard – legacy serial port connection require USB converters which again adds cost since most new PCs don’t even have RS-232 ports anymore).
• Power to be provided through USB connection – saves the cost of a wall-wart power supply.
• Be very careful about adding components and be diligent looking for low-cost, easily available components.
• Don’t use hard-to-solder, fine pitch components so the scope can be distributed as a self-assembly kit anybody can build.
In the end I was able to squeeze almost everything onto a single microcontroller (a Microchip PIC18F14K50) – acquisition control, capture memory, USB interface, digital-to-analog converters (ADC), trigger circuitry and trigger threshold generation. The only major block outside is the analog frontend – and even this is just one quad op-amp chip and a number of resistors and capacitors.
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