Integrated circuit having controlled impedance

An integrated circuit having a controllable impedance, said impedance comprising a first resistor formed of a given material and having a nominal resistance value greater than the upper limit of a desired tolerance range of resistance for said first resistor, and a second resistor switchably coupled in parallel with said first resistor;

• and further having a tracking resistor formed of'said given material in the same fabrication process that forms said first resistor, and control circuitry that causes said second resistor to be switched in parallel with said first resistor if said tracking resistor has an actual resistance value greater than a the upper limit of an expected resistance range for said tracking resistor,
• wherein said control circuitry comprises a first comparator (309) for comparing a comparison voltage (V.sub.COMP) developed across said tracking resistor (308) to a first to reference voltage (V.sub.1), and for generating a first control signal (T0) that causes said second resistor to be switched in parallel with said first resistor when said comparison voltage is greater than said first reference voltage,
• wherein said control circuitry includes a current source that causes a current (I2) to flow through said tracking resistor, with said current being relatively constant with respect to changes in integrated circuit operating temperature and variations in the fabrication process used to form the integrated circuit, and
• wherein said current source comprises an operational amplifier (304) having an inverting input coupled to receive a reference voltage (V.sub.REF), an output coupled to the control terminal of a current source transistors (305) having a first controlled terminal coupled to a first power supply voltage (V.sub.DD) and having a second controlled terminal coupled to the non-inverting input of said operational amplifier.