input resistance and capacitance

"Krishna"

** Yes.

Inputs to a great variety of devices are characterised buy a resiatance value with a paralell capacitance value.

Eg:

scope inputs are normally 1 Mohms in parallel with about 20 pF.

....... Phil

Thanks for clarifying Phil. If possible, can you please point to an online reference OR a text book which may explain this characterization.

Thanks, Krishna

It will act like that, although there may not be an actual capacitor in the device, it may be parasitic capacitance. FETs are the big area where you have to deal with capacitance, since the parasitic capacitance of the gate affects how quickly you can switch them on or off. There's no capacitor in there, it's just inherant to the nature of the device.

steve

The simple model you drew is correct for most parts, things that don't do funny stuff, like opamps and such.

The one gotcha is that some ADCs actually switch the capacitor every time a sample is taken, which complicates life. Check the datasheet. Which ADC is it?

John

John,

AD9432 @

When you say "some ADCs actually switch the capacitor every time a sample is taken" - I think you are referring to SAR ADC's? And maybe suggesting that the simple parallel RC to ground assumption may not be valid for SAR ADC's. Did I understand correctly?

Regards, Krishna

"Krishna"

** Huh ?

You need a friggin text book to tell you an R and C in parallel act like an R and C in parallel ?

Go away.

....... Phil

Yes, that is what he said. Though it doesn't apply to this. Have a look at the datasheet. Page zero shows there is an input buffer amplifier. Page 2 says input resistance

5kOhms, input capacitance 4.5pF. Page 6 shows an equivalent analog input circuit.

What more do you need ? This is it.

Rene

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Thanks all.

Krishna