1. Field of the Invention
This invention relates generally to electronics and more specifically to a resistor ladder network.
2. Description of the Prior Art
The most critical element of a successive approximation type of analog-to-digital converter is usually a resistor ladder network. The resistor ladder network may additionally be used as a feedback element of an operational amplifier to construct an amplifier with a selectable gain.
Most often, the resistor ladder network is what is known as an R-2R ladder network. The R-2R ladder is typically comprised of a plurality of cascaded prototype resistance divider networks terminated by a termination resistor.
The prototype divider is comprised of two resistors. A first resistor of a known value is connected between the input and the output of the prototype divider. A second resistor of twice the known value is connected from the output of the prototype divider to a common terminal-typically ground.
The termination resistor is twice the known value. When, for example, one volt is applied to the input of an R-2R network that includes four of the dividers terminated by the termination resistor, the dividers respectively provide 1/2 volt, 1/4 volt, 1/8 volt and 1/16 volt.
The R-2R ladder has desired characteristics when fabricated from resistors of one temperature coefficient. Moreover, the R-2R ladder is preferably made from resistors in a precise one-to-one and two-to-one ratio; resistance values are of second order importance.
Typically, the R-2R network is packaged as an integrated circuit of either thick or thin film, or monolithic construction. Moreover, the packaged circuit is fabricated from resistors all of substantially the known value and one temperature coefficient. When a resistance of twice the known value is desired, two resistors of the known value are connected in series. Resistors of substantially one value and temperature coefficient are readily provided through well known fabrication techniques of integrated circuits. Moreover, the fabrication techniques are advantageously used by fabricating the packaged network from resistors of substantially one value.
The R-2R network is one of few known resistor ladder networks that is practical to fabricate from resistors of substantially one value. Because of the fabrication techniques of integrated circuits, there is a need for the other resistance ladder networks that are practical to fabricate from resistors of one value.