Low-output capacitance, current mode digital-to-analog...

Details Low-output capacitance, current mode digital-to-analog... Low-output capacitance, current mode digital-to-analog...

2001-09-26

2003-04-01

Young, Brian (Department: 2819)

Coded data generation or conversion

Analog to or from digital conversion

Digital to analog conversion

C345S111000, C345S182000, C345S182000, C345S182000

Reexamination Certificate

active

06542098

ABSTRACT:

FIELD
Embodiments of the present invention relate to circuits, and more particularly, to digital-to-analog converters.
BACKGROUND
Digitally controlled current sources, or current sinks, are used in many types of circuits. An example of a digitally controlled current sink,
101
, is shown in
FIG. 1
, comprising port (or node)
102
for receiving a bias voltage V
bias
, port (or node)
104
for receiving a digital signal d, and port (or node)
106
in which a current I
out
, is sunk. When port
104
is HIGH, nMOSFET (n Metal Oxide Semiconductor Field Effect Transistor)
112
is OFF and nMOSFET
108
is ON, so that the gate of nMOSFET
110
is at the bias voltage V
bias
in order to sink current I
out
. When port
104
is LOW, nMOSFET
108
is OFF and nMOSFET
112
is ON so that nMOSFET
110
is OFF and no current is sunk.
For convenience, throughout these letters patent, the term “current source” is meant to include either a circuit that sources a current, a circuit that sinks a current, or both. Similarly, although a current source may source current and a current sink may sink current, for convenience both functions will be referred to as sourcing a current. It will be clear from context, such as a circuit drawing, whether a current is sourced or sunk. Consequently, the circuit of
FIG. 1
may be referred to as a current source, where current I
out
is sourced at port or node
106
.
A current mode digital-to-analog (D/A) converter may employ a plurality of digitally controlled current sources to convert a digital signal to an analog signal. These current sources may be connected in parallel. For example, in
FIG. 2
n digitally controlled current sources are connected in parallel to node
202
, which is connected to network
204
. For each i=0, 1, . . . , n−1, the digitally controlled current source indexed by i sources a current I
i
. An output signal may be taken at node
202
. For example, if network
204
is a simple resistor connected to a voltage source, the voltage at node
202
is an analog signal indicative of the digital signals controlling the digitally controlled current sources.
Other circuit functions may be realized by the high level functional diagram of
FIG. 2
depending upon network
204
. In general, network
204
represents a sub-circuit, and may comprise active elements as well as passive elements. For example, network
204
may be a differential amplifier in which the current sourced at node
202
provides biasing current to adjust the amplifier gain, where the gain is controlled by the digital signals controlling the digitally controlled current sources.
Some applications may require a relatively large number of parallel connected digitally controlled current sources. For example, a current mode D/A according to the circuit of
FIG. 2
converter with a resolution of N bits uses
2
N
digitally controlled current sources. A large number of current sources connected in parallel to a node may lead to the node having a large capacitance, which may slow down the speed of the circuit.


REFERENCES:
patent: 5252910 (1993-10-01), Agaesse
patent: 5475339 (1995-12-01), Maida
patent: 5512815 (1996-04-01), Schrader
patent: 5870049 (1999-02-01), Huang et al.
patent: 6006169 (1999-12-01), Sandhu et al.
patent: 6249236 (2001-06-01), Lee et al.
patent: 6295233 (2001-09-01), Kim et al.
patent: 6297685 (2001-10-01), Ewen et al.
patent: 2002/0033729 (2002-03-01), Lee et al.

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