inverting input impedance of AD8000


I'm using AD8000 to build schmitt trigger. I connect the input signal to the inverting input. I would like to have 1M ohm input impedance while now I wonder how much the inverting input impedance of AD8000 is. How can I make sure my input impedance is 1M. AD8000 is supplied between 0 and 5V.


  • Hello Yijun,

    Though in some cases op-amps can be used satisfactorily as comparators, it's not a good idea to do this as a general rule.  There are a number of reasons for this, most all relating to the fact that op-amps are designed for closed-loop linear operation and comparators are designed for open-loop operation with logic-level outputs.  The AD8000 is a current-feedback op-amp (CFA), which further complicates things.  The inverting input of a CFA is actually the output of an emitter-follower, and has a very low resistance, typically ranging from a few ohms to a few tens of ohms. The best solution would be to use a product that is specifically designed to be a comparator.

    Best regards.


  • Hello Jonathan,

    Thanks for your reply.

    Yes, I am using op-amps as comparators. The reason Is that I can’t find the right comparators. I need a comparator which can handel 100MHz signal with rise/fall time smaller than 1ns. There are +5V and -1.2V available in the schematic. The ref voltage could vary from 0.05V to 3.3V. VIH of output should be higher than 3.5V, VIL lower than 1.5V. Do you have any suggested comparator or solution?



  • I would also suggest looking harder at comparators. The AD8000 (or any other opamp I can think of) will struggle to slew in 1 nsec. There are some very fast comparators around with clocked or latched outputs that may be of interest, as the digital system you are interfacing with will probably by clocked.