PWM Waveform Capture

Posted on Jul 3, 2012

The approaches using on-chip A-to-D converters on AVR, PIC, and Cypress controllers reached sample rates of up to about 60 kHz. Not really very useful for the sort of thing I was thinking about using this for: encoded data, radio control signals, A-to-D converter waveforms, checking the dynamic range of amplifiers and capturing audio waveforms for filtering and power calculations. I realized that the comparators in AVR devices were pretty fast with a response time of several hundred nanoseconds, and that the PWM (pulse width modulation) circuit could be made fairly responsive. If there was just some way to combine these to sample analog values quickly...

PWM Waveform Capture
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To capture a waveform, the Pulse Width Modulation D-to-A converter (PWM DAC) is set to its maximum output voltage. Then, using timing loops, the microcontroller looks at the voltage comparator output to determine whether the incoming voltage is higher than the PWM voltage at regularly spaced sampling times (1 microsecond in the illustration). At each sampling time, if the incoming waveform is at a higher voltage than the PWM voltage, the PWM voltage is stored in a RAM array location corresponding to that sampling time relative to the start of the waveform. After all of the sample times have been tested against the PWM voltage , the PWM voltage is decrement and all of the sample times are compared with the PWM voltage again. This is repeated until the PWM voltage has been reduced to its minimum value, and each scan of the sample times starts by a trigger signal that is derived or in some way related to the incoming waveform. The finer the voltage resolution, the longer the waveform capture takes. As my initial use of this is with an LCD display with 64 rows, the waveform capture circuit senses 64 different levels. To capture 100 points at 64 different levels, the total capture time is: Capture time = 100 x [sample interval] x 64 (+ 64 X ([ trigger latency]) + 68 ms , where is " trigger latency" is the average time the controller waits for the trigger edge after the last sample, and the 68 milliseconds comes from...

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