Sample multiple channels 'simultaneously' with a single ADC: Page 3 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!
the audio world. In our example here, N=4, of course.

Decimation involves reducing the sample rate, and we know that we run the risk of aliasing, unless we take steps to remove those components from the signal whose frequency is higher than 0.5Fs. In other words, a decimation filter is really just a digital antialiasing filter. There are several types of filter topology we could choose to do the job in our case. Here I’ll use an FIR lowpass filter, and the benefits of that choice will become obvious later on.

We’re going to filter sequences {3} in such a way that we can then decimate them from 4Fs down to Fs. So we need a lowpass filter with a good stopband that begins at 0.5Fs when it’s operating on 4Fs data. The frequency response of a suitable 128-tap FIR filter is shown in Figure 2. It’s a linear phase filter designed with the ever-so-easy “windowed sinc” method, using nothing more complicated than a spreadsheet. The stopband is never poorer than 75 dB down, which limits potential aliasing errors to a reasonable level for this application. You might spot that the passband gain is 12 dB, i.e. 4x. The reason for that choice of scaling factor will also become clear later.

Figure 2:
Magnitude and phase response of our starting 128-tap FIR lowpass

So let’s take four of these 128-tap filters, one for each of the sequences {3}. Samples go into, and come out of, each filter at 4Fs, to make new sequences {4}, none of whose samples are now zero:

And because we’ve filtered off the high frequency stuff, we can now decimate each stream by a factor of four, meaning that we only take every fourth output sample, for example to give sequences {5} at Fs.

So far, we could have used any form of digital filter, but using an FIR offers a great simplification. Look at

Design category: 

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