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

[Charles Steinmetz]

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