AVR-Chat

The benefit of the switched input rather than a full wave
rectifier is that the input now becomes "tuned" to the
frequency and phase of the generated sense signal, thereby reducing
or eliminating most other spurious signals.
It may not be necessary in this application, but it is a
very useful engineering principle to follow in general.
Ken



> --- In AVR-Chat@yahoogroups.com, Ralph Hilton <ralph@r...> wrote:
>
>> I would be inclined to use an analog switch such an a 4053 and the
> ADC. Take a resistor
>> from Vcc to an ADC pin. Connect the control of the 4053 to a high
> frequency signal. The
>> pins ADC in and GND would connect to the switch which would reverse
> the output polarity at
>> the frequency of the control signal. The 2 output lines then
> connect to the water
>> container via small caps. The ADC pin would need a smoothing cap.
> thus the ADC would see
>> an easily measured DC value but the probe would see AC.
>
> Seems unnecessarily complicated to me. Why not use a (AVR generated)
> square wave signal capacitor coupled to the probes and measure the
> current thru a sense resistor just in the halfwave with the right
> polarity? Or - if really necessary spend some cents on an opamp which
> does precision rectifying (and amplification while we're on it) of
> the voltage across the current sense resistor.
>
> Just my 2c,
> Stefan
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