Hello!
I need to design a zero crossing detector using ADCMP561. The input is 144 MHz sinewave. The comparator needs output a square wave. The high to low and low to high transitions happen at each zero crossing. The result should therefore be a 144MHz 50% duty cycle square waveform at the output.
The problem I have is that the outputs of the comparator (QA and ~QA) are a sinewave instead of a squarewave. If I turn down the frequency into 5-10MHz range, the output is a squarewave as expected. In other words, the output is fine up until 10MHz input and then starts deteriorating with higher frequencies until it finally looks like a single sinewave.
Voltages:
Vcc = 5V
Vdd = 5V
Vee = -5.2V
Outputs QA and ~QA are terminated into Vdd-2 = 3V over a 50ohm resistor each. I use only QA output for the next stage in the system. Other than 50ohm into Vdd-2, the output sees a 5kohm pulldown to ground. So, the ADCMP561 should be loaded primariliy by the 50ohm termination. Trace length from QA pin to the next input stage is around 1cm. The second comparator unit is not used. Here is the schematic:
What do you think might be the problem?
