Waveform Capture and Control
Posted on Jun 2, 2012 8664
Under: Meters Circuits
The result was a project that includes a waveform capture circuit, a graphic LCD (Liquid Crystal Display) and a control panel with LED indicators. As described, the display plugs directly into the waveform capture and control unit, which is what the what the rest of this page is dedicated to. The control panel controller appears as an ASCII terminal to the waveform capture controller and the two chips communicate using their on-chip hardware UARTs at 9600 baud. Both the controllers are Atmel AT90S2313. The control controller could have been implemented using an AT90S1200 but it was a little easier with the AT90S2313 because it has a hardware UART and a luxurious RAM stack, and by the time I came that part of the project I was much more anxious to see waveforms than to see if I could use the lower cost processor.
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The board contains all of the analog circuit including the +5 volt power supply as well as the waveform sampling circuit and control panel interface with LED display. A clock display picked up surplus is used to display the sampling rate and it is driven by the control panel controller. A separate LED mounted just below the clock display indicates that the sampling is being triggered and is driven directly by the analog circuitry. AC coupling through the two 0.33 uf capacitors in series gives a time constant on the input of about 2 seconds -may be a little long for some applications. The easiest way to shorten it is to replace the series capacitors (there are two of them because the ones in this range that I have on hand are tantalum) with a nonpolarized capacitor with a suitable voltage rating. The 10: 1 divider on the input circuit is frequency compensated with a single 5 pf capacitor to ground. I used parts I had on hand. If you are picky, you can use metal oxide resistors to get a more precise and stable divider, but remember this is only a 6 bit waveform monitor. The ideal case when the divider switch is in the "X1/10" position is for the capacitance across the 10 Meg Ohm resistor to be 1/10 the capacitance from the top of the 1 meg Ohm resistor to ground. A good practice is to place a relatively large resistor from the top of the 1 meg resistor to ground, say 10 or 15 pf, and then select or adjust a trimmer...