Sample multiple channels 'simultaneously' with a single ADC: Page 5 of 7

September 16, 2011 //By Kendall Castor-Perry
Sample multiple channels 'simultaneously' with a single ADC
Kendall Castor-Perry (aka The Filter Wizard) explains that despite its title, this is indeed an article about a filtering technique. Using it, you can sample a number of analog input channels sequentially with a single ADC in such a way that the data appears to have been acquired at the same instant in time on every channel. It sounds like magic – but it's just Wizardry!
our impulse response into four sets, each ‘carries’ one-quarter of the signal. Figure 2 showed a gain of 4x to compensate this right from the start. You could also start with a unity gain filter and just multiply all the final coefficients by four at the end, with the same result. Figure 4 shows the frequency and phase responses of the four subfilters; they now have unity gain. The amplitude responses are identical, but the phase responses are skewed apart as you’d expect from four filters that have slightly different delay. Figure 5 shows the group delay values of the four filters directly, along with an expanded passband plot. The simulations were taken for an Fs of 3000 Hz, which was the sample rate we used in the metering work.

If you’ll pardon the cliché, the proof of the pudding is in the eating. Figures 6 and 7 are “before and after” plots for a segment of data from a four-channel .wav file acquired from a multiplexed-ADC project built on a PSoC3 development board. The source signal consists of equal amounts of 50 Hz and its first 21 harmonics, phased randomly. It was generated by playing out a synthesized .wav file through a PC’s line output, and applied to the four multiplexed inputs (through carefully matched input AC coupling networks). We took a lot of care to ensure that everything on one channel settled really well before switching over to the next multiplexer channel (otherwise you get crosstalk, which can cause significant accuracy problems in a meter). Because the four channels aren’t sampled at the same time, the data points for the four streams plotted in Figure 6 aren’t identical. This extreme waveform was chosen to make a visual point, even though it would be a rather unusual signal to apply to an actual electricity meter.

Figure 5:
 Closer detail of the passband gain and the different group delay values
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