Current steering circuits are the basic building blocks of certain types of digital-to-analog converters (DACs) used in a wide range of applications, such as digital radio transmit signal chains, test equipment stimulus synthesis, wire-line data transmission, and so on. In these applications, a critical DAC specification is the spurious free dynamic range (SFDR), which is directly determined by the amount of undesired distortion unavoidably added to the generated output signal.
FIGS. 1A and 1B illustrate an example of a prior art current steering (CS) circuit 100. A fixed current source 130 is selectively steered towards the output node A or output node B as directed by complementary control signals VCA and VCB.
FIG. 1A shows a first state of the CS circuit 100. A current steering device 110 couples a current source 130 to output node A when signal Von is applied to node 112 through switching element 118, as directed by control signal VCA present at control terminal 111. The current steering device 110 may be any type of suitable device. In some embodiments, the device 110 may be a transistor, including n and p-channel enhancement and depletion MOSFETs, JFETs, MESFETs, heterojunction devices, NPN or PNP bipolar transistors, etc. The switching element 118 may be any type of signal generator that, in response to control signal VCA, provides a suitable bias to the current steering device. At the same time, current steering device 120 decouples current source 130 from output node B when signal Voff is applied to node 122 through the switching element 128, as directed by control signal VCB present at control terminal 121. In this first state of the CS circuit 100, steering device 110 functions in a conductive state while steering device 120 functions in an isolating state.
A second state of the CS circuit 100 is shown in FIG. 1B. The current steering device 110 decouples current source 130 from output node A when signal Voff is applied to node 112 through switching element 118, as directed by control signal VCA. At the same time, current steering device 120 couples current source 130 to output node B when signal Von is applied to node 122 through switching element 128 as directed by control signal VCB. In this second state of the CS circuit, steering device 110 functions in an isolating state while steering device 120 functions in a conductive state.
The relatively low output impedance of steering device 110 in its conductive state is represented by resistor 115 in FIG. 1A. The relatively low output impedance of steering device 120 in its conductive state is illustrated as resistor 125 in FIG. 1B. Both current steering devices (110 and 120) in their isolating state have sufficiently high output impedance such that it can be practically ignored for the purpose of this description.
Because of physical implementation constraints, a relatively substantial parasitic capacitance is present at common node 132 and is illustrated by capacitor 135.
In the first state illustrated by FIG. 1A, a signal present at output node A may change the voltage stored on capacitor 135 through resistor 115. Similarly, in the second state illustrated by FIG. 1B, a signal present at output node B may change the voltage stored on capacitor 135 through resistor 125. Such may be the case where the CS circuit 100 is one of many CS circuits in a DAC applying currents to a differential output bus connected to output nodes A and B.
When the CS circuit 100 transitions from the first state to the second state, the desired steering of current Io from output node A to output node B is accompanied by the undesired transfer to B of charge stored on common node 132 through resistor 115 by the signal present at node A during the first state. Similarly, when the CS circuit 100 transitions from the second state to the first state, the desired steering of current Io from output node B to output node A is accompanied by the undesired transfer to A of charge stored on common node 132 through resistor 125 by the signal present at node B during the second state. This transfer of charge creates distortion, limiting the spurious free dynamic range (SFDR) of the DAC.
FIG. 2 illustrates a DAC 200 constructed from a plurality of current steering circuits CS1, CS2 . . . CS(n−1), CSn coupled to differential output nodes A and B. In one embodiment, these CS circuits are scaled (i.e., current sources increase for each bit position according to 20, 21, 22, 23, etc.) and are selectively controlled by complementary pairs of control signals VCA1, VCB1, . . . VCAn, VCBn to transition between first and second states at such time and in such sequence as to produce a desired differential output signal at nodes A and B. The differential signal thus produced at the output nodes A and B is an analog equivalent of the incoming control signals. In other embodiments, some or all of the CS circuits may be equal weighted or non-binary weighted or any combination thereof.
Due to the relatively low output impedance (represented by resistors 115 and 125) of the current steering devices 110 and 120 (FIGS. 1A and 1B) in their conductive state, the desired output signal is accompanied by undesired signal-dependent charge transfers, resulting in output signal distortion on nodes A and B.
What is needed is a current steering circuit, such as for use in a DAC, that generates less distortion of the signal at its output nodes.