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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