A very simple asynchronous sigma delta modulator design for linearisation of VCO ADC's is presented. The circuit only consists of a passive feedback filter and a Schmitt Trigger. By proper sizing, the non-linearity error can be reduced to well below 0.12% for input signals that go almost rail-to-rail. The design has been manufactured in the low power version of TSMC 65 nm technology and was measured at a 1 V power supply.