[time-nuts] SR620/PM66xx/CNT-90 input stages
bruce.griffiths at xtra.co.nz
Tue Jun 20 17:35:22 EDT 2017
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.
> On 21 June 2017 at 09:11 Charles Steinmetz <csteinmetz at yandex.com> 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,
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
More information about the time-nuts