SAR ADC No Output Code Issue

Symptom:

• ADC output code is fixed to 8b'100000; 
• when supply voltage is increased, ADC starts to work properly;
• when clock frequency is lowered, ADC starts to work properly;

Bug:
Driving of ADC clock is weak.
At low voltage, the clock signal at the invertor gates is like a triangle wave, and the tipping point of the invertor is a little higher than half of supply voltage, so clock buffer is not working at all.

When we crank up the supply voltage, the driving capability of driver (is also an invertor) is increased, although the tipping point is also increased with supply voltage, the driver now have enough strength to drive the clock input buffer.