Yes, Thank you for the clarification. I did misrepresent my point. I was thinking that the appearance of aliasing could occur because the sampling rate, of course, is not affected by the impedance factors. It is good that you have clarified it so that no one is actually misled. I have experimented with A/D converters in order to design a test fixture for evaluation. I was speaking from that experience. ----- Original Message ----- From: "Forrest Christian" To: "Microcontroller discussion list - Public." Sent: Tuesday, August 05, 2008 11:52 AM Subject: Re: [EE] ADC Input Buffer Circuit > Aliasing has to do with sampling a high-frequency signal at a too-low > rate.. Generally you need to sample at two times the highest > frequency component of the input signal. For instance, if you sample a > 10khz signal at 5khz, you will not have collected enough data to > determine the frequency of the signal.. In fact, a 10khz signal sampled > at exactly 5khz will appear to be DC - since you are sampling it at the > same point in the cycle and only getting the one sample. Instead, you > need to sample at at least 20khz so you get a representative sample of > the input signal. > > About the only function that the input circuitry can have on aliasing is > whether or not it blocks or creates a high-frequency signal. In the > DSP world it is pretty common to add a low-pass filter which blocks at > least everything over 50% of the sample rate to avoid aliasing. > > I can see how a too-high impedance can create a symptom that *looks* > like aliasing, but is not. Aliasing has to do with the signal being > accurately delivered to the ADC, and then the interaction between the > ADC's sample rate and the input signal causes "aliased" frequencies > below the too-high frequency. About the only thing that a too-high > impedance on an ADC will cause is for the reading to be off (sampling > error) - which may cause a signal with artifacts related to the sampling > error, but won't be aliasing in the true meaning of the word. > > -forrest > > Rich wrote: >> I disagree! Think it through. What happens if the source impedance is >> to >> high and cannot source the required current or even sinks current from >> the >> converter? Analog Devices has some good articles on this. True, they >> are >> older articles, but they can still apply depending on the choice of >> devices >> used. >> >> >> ----- Original Message ----- >> From: "Olin Lathrop" >> To: "Microcontroller discussion list - Public." >> Sent: Tuesday, August 05, 2008 8:03 AM >> Subject: Re: [EE] ADC Input Buffer Circuit >> >> >> >>> Rich wrote: >>> >>>> I have not followed this thread but the title caught my eye. One very >>>> important thing to remember when driving the input to an A/D is that >>>> the output impedance of the driver should be kept as low as possible >>>> in order to avoid aliasing and conversion error. >>>> >>> Conversion error, yes. But A/D source voltage impedence has nothing to >>> do >>> with aliasing. >>> >>> >>> ******************************************************************** >>> Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products >>> (978) 742-9014. Gold level PIC consultants since 2000. >>> -- >>> http://www.piclist.com PIC/SX FAQ & list archive >>> View/change your membership options at >>> http://mailman.mit.edu/mailman/listinfo/piclist >>> >> >> > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist