I need a comparator LMV7219 as a sin to CMOS converter. Generator’s output signal is 450 mV (amplitude) at the 10 K load.
I made a circuit (please, see attach "Circuit 3").
It is known, that can not apply a voltage less than minus 200 mV to the input of the LMV7219.
At the same time, the generator’s load must be within 8-11 K (otherwise frequency stability can not be guaranteed). If I set R1 equal to 5K , the Um2 voltage is 100 mV. If I set R1 more than 20K , the LMV7219 doesn’t work at all. The value of the R2 is almost no effect on the Um2. R1 and R2 were 100 K each first, but the comparator does not switched at all until I significantly reduced the R1.
I can not understand, is the input impedance of the LMV7219 at 10 MHz is only a few kiloohms (or even less)? But there must be at least 50-100 kiloohms! In the LMV7219 datasheet input impedance is not specified. Сircuit assembled on a printed circuit board, the distances between the elements are minimal.
Can I somehow estimate the value of the load of the generator, provided that I know R1 value, Um1 and Um2?
I modeled in the TINA-TI program and used the oscilloscope - TINA-TI gives the result as if the input impedance of LMV7219 is large, and all supposedly determined only by the ratio of resistors R1, R2. But the real results do not like the program. I used a broadband oscilloscope and probe with a divider 1:10. Perhaps the reason is that the signal comes into the negative region? Maybe, it makes sense to tie each input of LMV7219 to the potential of 1.65 V using a resistors dividers to 200 kilohms? Please, see attach. comparator.vsd.