If the source impendance at dc is 10k at dc a bias current of a few microamp produces a dc offset of tens of millivolts which may be an issue if the signal amplitude is low and has a low slew rate.
Bruce > > On 21 June 2017 at 09:11 Charles Steinmetz <[email protected]> wrote: > > Attila wrote: > > > > > > Now, if the downstream circuit would be low > > impedance, I could understand that, but the sink is a comparator > > with a > > high impedance input (only a few µA input current). I am sure the > > engineers > > had a good reason to add those amplifiers, but I cannot guess why. > > Would > > someone be so kind and enlighten me? > > > > > > I was going tp post the Smith chart for the input of the AD96885/7 ECL > comparator they use, but was surprised to find that AD did not include > it on the datasheet. However, just from the spec table we can see why a > buffer is necessary: > > Input Resistance: 200k ohms > Input Capacitance: 2pF > > For starters, even at low frequencies the input impedance is only 200k > ohms, much less than the 1Mohm rated impedance. Further, the > capacitance seriously degrades bandwidth to as low as 400kHz (this > depends on the source impedance, but even with a 10kohm source the BW is > only 8MHz). > > (Note that the input current spec is not necessarily a good proxy for > input impedance -- consider a FET-input amplifier with an input current > of 1uA and a 100k gate resistor.) > > Best regards, > > Charles > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
