AVR Freaks

The Shannon-Nyquist Theory deals with digital sampling of signals.

It is important with sampling repetitive signals with the intention of being able to "reproduce" the original signal.

If you were sampling a 1 KHz sin wave and want to reproduce it with a DAC you would want to sample the input sin wave at > twice the highest frequency component within the input signal of interest, (i.e. 2 * 1 KHz, or sample at > 2 K samples / second).

That said, the above doesn't really apply, (much / directly) to what you are doing.

The battery is a steady state DC voltage.

The resistor divider and cap is perfect for that.

The frequency of interest for the current sense is how fast the current is changing, is it pretty steady state, is it varying a lot, (quickly).

The sampling rate impacts your ability to measure rapid changes in the current an not miss them.

So, generally, motors are mechanical systems can change "slowly", from an electronics perspective.

You might think of it as switching state fro running / stalled / running / stalled / running / stalled, etc., as a periodic wave form for the two states the system can be in.

Not to useful, but the point is your basically looking for the transition between to states, and not a periodic waveform.

And for that the Shannon-Nyquist theorem isn't really pertinent.

You might decide to set a Timer/Counter to interrupt once every 1 mSec and measure the motor's current.

Actually, you might have the micro read the last reading, and trigger the next reading, so you are always 1 mSec behind...

Given this, you want a filter on the input to remove the higher frequency spikes and noise on the current sense signal.

But you would want to be sampling at twice the highest frequency in the input.

So, following the above, you could have a LPF of 500 Hz and sample at 1 K samples / second.

Obviously the lower the LPF cutoff, the slower is the response of the input signal to sudden changes in current.

So, you might deviate from the above an go with a higher cutoff frequency, aimed at just eliminating the higher freq noise and spikes on the input signal.

What yo do depends a lot upon how quickly the system changes state, (i.e. running normally to stalled).

How fast your monitoring is.

How fast your control loop is running.

etc.

Putting an O'scope on the signal will tell you a lot about the noise present that you want to filter out.

Lastly, nothing wrong with using an LC filter.

The AVcc pin should have an LC filter powering the pin from the Vcc rail.

But, for filtering the input signals I would have probably investigated an RC LPF first.

Finally, the impedance of the source driving the ADC input on the micro should be less than 10K ohms.

Therefore, for real systems, one might want an op-amp filter driving the ADC input.

The op-amp filter has a low impedance output, good for driving the ADC.

A carefully designed input filter might not require the op-amp.

JC

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