In a series of posts (most recently Measuring BitScope BS-10 input impedance), I’ve been measuring the input impedance of my various ways of measuring AC voltage:
|Radio Shack||10.87MΩ || 18.54pF|
|DT-830B||0.42MΩ || 31.59pF|
|DT-9205A||13MΩ || 22pF|
|BitScope BS-10 oscilloscope||1.025MΩ || 9.8pF|
I spent yesterday trying to add the PteroDAQ data acquisition system with the Freedom KL25Z board to that list.
One problem was that PteroDAQ was not designed to report an RMS voltage, but just a waveform, so I modified PteroDAQ to report the mean and standard deviation of a channel (not in the released version yet, as I still have some work to do on the user interface). Note that the mean of a channel is its DC bias, and the standard deviation is the RMS AC voltage. (I’ve never much cared for meters that report RMS AC+DC, which is the root-mean-square voltage without separating AC and DC components.)
A bigger problem is that PteroDAQ can only sample at fairly low frequencies, but the parallel capacitance is expected to be fairly small (pin capacitance for the pin is only about 7pF and the short wiring on the board should only add another couple of picofarads), so the RC time constants will be small. The result is that the low frequencies below the Nyquist frequency will not be much affected by the parallel capacitance, and all I would be able to estimate is the DC resistance of the inputs.
I can take advantage of a trick, however, to get effectively much higher sampling rates: aliasing. Because the input is a sine wave of stable frequency, f, I can sample it at every seconds and get a waveform that advances by phase . I can pick the integer n to be large enough to get a feasible sampling rate while still seeing the whole waveform, especially if I pick the phase advance to be about , so that I see all the 256 entries in the function generator’s table.
This trick has the further advantage of presenting the sample-and-hold with about the same value as it sampled on the previous sample, so that I don’t have to worry about the short sampling time not getting fully charged through a high-impedance input. If I don’t do the aliasing trick, then the short sample time PteroDAQ uses (4 cycles of a 6MHz clock, or 667ns) is not enough to charge the sampling capacitor to the final voltage.
At higher frequencies, even this short sampling time is too long—at 1MHz the voltage changes substantially in 667ns, and the sampling capacitor ends up averaging the value over the sampling interval, which reduces the AC RMS voltage.
I made my measurements with the hardware averaging set to 1×, since averaging multiple readings is a digital low-pass filter that would hide the analog low-pass filter I’m trying to measure. Because the measurements at 1× are so noisy, I took a large number of measurements to determine the mean and standard deviation. The results are still a bit noisy, as I did not realize the importance of having very precise sampling rates initially—if the value is too small, then I have to be careful to include an integer number of periods of the aliased waveform in the averaging to avoid bias, and if it is too large, then the short sample time is not long enough to charge fully and my waveform is not full scale. A good compromise seems to be to pick n so that the sampling rate is around 5kHz and to get about a 19.5Hz aliased waveform. Only a few of my measurements were done with these settings, so I should probably redo the whole set at some point.
The aliasing trick is not a perfect one—at high frequencies there are a lot of glitches, where it is clear that the sampling did not happen at precisely the place in the waveform desired. This is probably due to jitter in the digital phase oscillators used in FG085, as the PteroDAQ interrupts should come at precise intervals (though the intervals may not be at exactly the frequency desired). The noise is much more of a problem with a high impedance source, as it may take several samples for the sampling capacitor to get back to the correct value.
I measured PTB0 with 1× sampling both directly driven by the FG085 (with 2.9Vpp and +1.8V offset) and through a 100.1kΩ resistor.
The impedance estimate derived from these measurements is pretty solid on the DC resistance, but the parallel capacitance estimate varies depending on how much of the high-frequency measurement I use in the fitting.
Estimating the input impedance of the single-ended pins of KL25Z at 2.5MΩ || 8pF seems pretty good. I’ll have to check the differential inputs separately, as there is no reason to suppose that they have the same input impedance.
I think that the 8pF I’m seeing is mainly the pin capacitance of the PTB0 pin, with a little extra board capacitance. The sampling capacitor is not really measurable here, since it is only connected to the input for very short intervals. To measure the RC time constant of the sample-and-hold circuit, we’d have to vary the sampling time (which is possible on the KL25Z, but which PteroDAQ is not set up to do).