Digital-to-analog converting apparatus

Apparatus for converting a digital signal to an analog signal including in combination

• a plurality of N stages arranged in series, each stage including a resistance means having a first and a second terminal;
• a source of reference voltage connected to the first terminal of the resistance means of the first stage in the series;
• an analog output connection connected to the second terminal of the resistance means of the last stage in the series;
• the second terminal of each resistance means, except for the resistance means of the last stage in the series, being connected to the first terminal of the resistance means of the next stage in the series;
• each stage including
• first constant current means for producing a constant current connected to the first terminal of the associated resistance means;
• second constant current means for producing a constant current equal to the constant current produced by the associated first constant current means connected to the second terminal of the associated resistance means;
• a signal input terminal for receiving a bit of a digital input signal, the bit being of a first or a second value;
• data storage means coupled to said signal input terminal and operable in a first condition in response to a bit of a first value and operable in a second condition in response to a bit of a second value;
• switching means coupled to the associated resistance means, the associated first and second constant current means, and the associated data storage means;
• the switching means being operable when the associated data storage means is in the first condition to provide a path for current from the first constant current means bypassing the associated resistance means, and to provide a path for current from the second constant current means flowing through the associated resistance means in the direction from the second terminal to the first terminal, whereby the voltage at the second terminal of the associated resistance means is greater than that at the first terminal by the voltage drop across the associated resistance means;
• and
• the switching means being operable when the associated data storage means is in the second condition to provide a path for current from the second constant current means bypassing the associated resistance means, and to provide a path for current from the first constant current means flowing through the associated resistance means in the direction from the first terminal to the second terminal, whereby the voltage at the second terminal of the associated resistance means is less than that at the first terminal by the voltage drop across the associated resistance means;
• whereby the voltage produced at the analog output connection is an analog output voltage of amplitude corresponding to the bits of the digital input signal at the signal input terminals.