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quick question. What happens (in general) to my entry impedance when I add an ADC into my circuit? Is this independent of the rest of my circuit? If no, this is my circuit

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  • \$\begingroup\$ @hilmar definitely not "nothing" happens. Something happens but how much and is it significant, it depends on ADC type and sampling rate, and of course components Rs, R and C too. Unless that information is available, the question cannot be answered. Hester please add more information. \$\endgroup\$ – Justme Apr 14 '20 at 16:27
  • \$\begingroup\$ In addition to Justme's comment - ADC inputs vary very widely depending on the device so the answer currently is that something will change but we simply cannot know by how much. \$\endgroup\$ – Peter Smith Apr 18 '20 at 12:55
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Do you mean - does the impedance that the source sees changes due too the connection of the ADC?

Usually, an ADC has a high input impedance - several mega ohms. So, in that case, if the input impedance of the ADC is much higher than the output impedance of the circuit (without the ADC), the connection of the ADC will merely affect the total impedance that the source will see.

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The ADC is affected by the impedance, not usually the other way around.

Most cheap ADCs will have a sampling capacitor on their input. This connects to the outside world for a short period of time, charges up, then disconnects and the conversion begins inside this device.

Every time the ADC takes a sample, it has to charge up this capacitor for enough time constants that the difference in voltage with what you're measuring is ideally less than 1 step of the output. As such when there is more and more "impedance" between the ADC and the "to be measured" voltage, you start losing meaningful resolution in your output.

Most ADCs in their datasheet will quote what the maximum input impedance can be for a given sampling speed, with the general rule being lowering the ADC clock speed by half will allow for more impedance because the sampling switch is open for longer. But if you get too slow, the sampling caps leakage may start affecting the measurements.

Below is a schematic of what is usually inside an actual microcontroller ADC:

ATmega ADC internals

Image source: Open Music Labs - ATmega ADC tutorial

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  • \$\begingroup\$ I will add that if the source DC impedances Rs and R are too high for the ADC , the capacitor C has to provide low enough AC impedance to take one single sample reliably. If the ADC samples too often, the C does not have time to charge fully, so continuously sampling gulps of current is equivalent to continuous DC load. \$\endgroup\$ – Justme Apr 18 '20 at 13:59

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