IMPROVING THE PERFORMANCE OF COMPARATORS The objective of this section is: To improve the performance of continuous-time, open-loop comparators. Two areas in which the performance of an OL HG comparators can be improved Input Offset Voltage (most serious effect) - can be solved by Auto-zeroing Single transition of the comparator in a noisy environment – can be solved by the introduction of Hysteresis using a bi-stable circuit Outline • Auto-zeroing techniques • Comparators using hysteresis

Input Offset Voltage: It is a voltage that must be applied between the two terminal of an op- amp to null the o/p. Ideally, when the I/P terminals of the DA are connected together, the O/P voltage is at a desired quiescent point. In real DAs, the O/P offset voltage is the difference between the actual O/P voltage & the ideal O/P voltage when the I/P terminals are connected together. If this offset voltage is divided by the differential voltage gain of the DA, then it is called the I/P offset voltage (VOS). For CMOS DA, VOS is typically is 5- 20mV. I/P Offset Voltage can be a particularly difficult problem in comparator design. In precision applications, such as High-resolution ADCs large VOS cannot be tolerated.

Auto-zeroing Techniques Infinite input resistance of MOS transistors allows long term storage of voltages on the TX’s gate. As a result, offset voltages can be measured, stored on capacitors and summed with the input so as to cancel the offset. Use the comparator as an op amp to sample the dc input offset voltage and cancel the offset during operation. A model of comparator with an input-offset voltage is shown in figure. A known polarity is given to VOS for convenience. Neither the value nor the polarity can be predicted.

Auto-zeroing Techniques Comparator is connected in unity-gain configuration so that the input offset is available at the output. The comparator must be stable in the unity-gain mode (means self-compensating comparators are suitable for auto-zeroing, the two-stage op comparator would require compensation to be switched in during the auto-zero cycle.)

• Complete offset cancellation is limited by charge injection