SELF-TESTABLE FULL RANGE WINDOW
COMPARATOR
2.1. Window comparator circuit
For completeness, the circuit structure of the full
range window comparator (FRWC) is described
below. Refer to the circuit within the solid line in
Fig. 1 the FRWC consists of two simple comparators,
three inverter gates and one 2-input NOR gate.
Each comparator is constructed using a high gain
operational amplifier (Op Amp); detailed
specification of the CMOS Op Amp circuit is given
in [5]. The comparator circuit has 3 inputs (Vi, Vrefh
and Vrefl) and 1 output (Vo). The window width is set
by the Vrefh and Vrefl, such that a logic ‘1’ will be
outputted at Vo if the voltage value of Vi is within the
window range else Vo will be set to logic ‘0’.
The FRWC employed in this study is an
improved version of that presented in [1]. Note the
use of an inverter gate together with a 2-input NOR
gate to replace the 2-input EXOR gate as proposed in
[1]. This leads to a small saving of the number of
transistors used.