Re: Basic Signal Processing Question or Natural vs Impulse sampling

Venkat.Vijay.Kumar@xxxxxxxxx wrote:

  ...

        My confusion is what you mentioned in your reply, that is about
the clock and sampling. I agree with the sampling with impulses would
retain the input signal. (Same as Fin and not Fclk+/-Fin)
       But how does it work with a clock (which is a square wave). What
do you mean by "feeding a square wave as a clock to an ADC, the ADC
doesn't use square waves in it's actual 'sampling' of the input analog
signal."

      How does the sampling in an ADC work. I am bit confused by your
above statement. Can you eloberate??

Now you're getting into hardware design. Do you really want to go there? Working parts are not made of equations, but of bits of wire, silicon, foil, solder, and more. Circuits can be made that are insensitive to level, but respond to changes in level Such devices are called "edge triggered" In explaining an ADC, I'll ignore them; just know that they can simplify a design.

Our ADC consists of a voltage-controlled switch (you can think of it as a relay), a sample holding capacitor, a comparator (greater-than, less-than) a set of latches, and a DAC. There's also a control circuit, but for now, that's magic.)

When the clock is high, the switch connects the analog signal to the capacitor, which follows the signal. When the clock goes low, the capacitor is disconnected from the signal and its voltage stays substantially constant. The MSB of the ADC is turned on by setting the corresponding latch and the comparator is consulted. If the capacitor voltage is less than the DAC voltage, the latch is cleared; otherwise, it is left on. Then the same is done with the next-highest bit, and so on until the LSB is tested. When the process is done, the state of the latches (and sometimes the comparator output) are output as a number that represents the voltage on the capacitor. When the clock goes high again, it clears the latches and reconnects the capacitor to the analog signal.

Let's be clear: there is no magic. To make one of these work, you need to actually build the control circuit. Moreover, there are many other ways to build an ADC, some of them with more delay than the successive-approximation type I describes, some of them slower. The output can come serially or all at once. I omitted the control signals that make the information transfers possible and that control timing.

We are often warned not to mistake the map for the territory. Similarly, we need to beware of mistaking the equations for the device. Also, don't assume how a circuit works and draw conclusions from the assumptions. If you really want to know, look at a schematic. It defeats all of us when we believe that something must be a certain way because we can't conceive of any other way. "There are more things in heaven and earth, Horatio, than are dreamt of ..." There's _always_ another way.

Jerry
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Engineering is the art of making what you want from things you can get.
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