An apparatus includes a first voltage reference circuit, a second voltage
reference circuit and a third circuit that is coupled to the second
voltage reference circuit. The first voltage reference circuit provides a
first reference voltage between a terminal of the first voltage reference
circuit and a first power line. The second voltage reference circuit
provides a second reference voltage between a terminal of the second
voltage reference circuit and a second power line that is separate from
the first power line. The third circuit is coupled to the second voltage
reference circuit to establish a magnitude of the second reference
voltage in response to a potential difference between the terminal of the
first voltage reference circuit and the second power line.