[time-nuts] SR620/PM66xx/CNT-90 input stages

Bruce Griffiths 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,
>     Charles
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