Digital-to-analog converter having enhanced current steering and associated method

A digital-to-analog converter comprising:

• digital input means for accepting a plurality of digital input signals;
• at least one output current summing bus;
• and
• a plurality of current cells connected to said digital input means and said at least one output current summing bus, at least one of said current cells comprising
• a current source,
• a pair of bipolar current switching transistors connected to said current source and said at least one output current summing bus in a current steering configuration so that one transistor is off while the other transistor is on, said pair of bipolar current switching transistors having a proportion of current steering based upon a difference in base-emitter voltages and also based upon a thermal voltage,
• a proportional to absolute temperature (PTAT) source, and
• a pair of steering transistors connected to said PTAT source and said pair of bipolar current switching transistors for controlling the difference in base-emitter voltages of said bipolar current switching transistors based upon the PTAT source to provide a predetermined proportion of current steering over a relatively wide temperature range to thereby compensate for the thermal voltage.

A current cell comprising:

• a current source;
• a pair of bipolar current switching transistors connected in a current steering configuration so that one transistor is off while the other transistor is on, said pair of bipolar current switching transistors having a proportion of current steering based upon a difference in base-emitter voltages and also based upon a thermally dependent voltage;
• and
• temperature compensated control means for controlling the difference in base-emitter voltages of said bipolar current switching transistors based upon a temperature dependent bias voltage to compensate for the thermally dependent voltage, said temperature compensated control means comprising
• a proportional to absolute temperature (PTAT) current source, and
• a pair of steering transistors and associated bias resistors connected to said PTAT current source and said pair of bipolar current switching transistors for controlling the difference in base-emitter voltages of said bipolar current switching transistors based upon absolute temperature.

A method of steering current in a current cell of a plurality of such current cells connected to an output current summing bus of a digital-to-analog converter, each current cell comprising a pair of current switching transistors connected together in a current steering configuration so that one transistor is off while the other transistor is on, the pair of current switching transistors having a proportion of current steering based upon a difference in transistor control voltages and also based upon a thermally dependent voltage, the method comprising the steps of:

• receiving at least one digital input signal;
• supplying a current to the pair of current switching transistors for steering based upon the digital input signal;
• and
• controlling the difference in transistor control voltages of the current switching transistors based upon a temperature dependent bias voltage to compensate for the thermally dependent voltage.