Current switching cell for digital to analog converter

[0002] This invention relates to a current switching cell for a multi-cell digital to analog converter and more particularly to such a cell which is locally rather than globally controlled.

[0003] Digital to analog converters (DAC's) employ a number of current switching cells to convert the digital inputs to analog outputs. Typically, one or more common or global lines operate the current circuits in each cell to provide a defined current to their respective current switches and another global line operates the driver circuit in each cell causing it, in response to a data input, to produce an analog output from the defined current from the current circuit. Operating these cells globally has some advantages but the common connection of these global control lines to all of the cells causes disturbance signals which result in distortion of the DAC analog output: the parasitic coupling, primarily capacitance, associated with the elements of the various circuits in each cell establish paths whereby disturbances in one circuit are propagated to others. At high frequency switching speeds the interference introduced by these disturbances not only propagates through other cells but extends in time to distort subsequent outputs of the DAC referred to as intersymbol interference. One approach to the problem has been to increase the current circuit output impedance but this results in reducing the “headroom”, i.e., the amount of voltage supply to maintain the circuits in saturation.

[0004] It is therefore an object of this invention to provide an improved current switching cell for a DAC.

[0005] It is a further object of this invention to provide such an improved current switching cell for a DAC which isolates transient disturbances and prevents them from propagating from one cell to another.

[0006] It is a further object of this invention to provide such an improved current switching cell for a DAC which reduces intersymbol interference.

[0007] It is a further object of this invention to provide such an improved current switching cell for a DAC which dramatically increases the impedance of the current circuit without decreasing the headroom.

[0008] This invention results from the realization that an improved current switching cell for a DAC which prevents propagation of disturbances of parasitic capacitance or other parasitic couplings which distort the DAC output and cause intersymbol interference can be achieved by isolating the parasitic couplings from the common or global control lines to the current circuit and driver circuit of each cell so that the disturbances can not pass from one cell to another.

[0009] This invention features a current switching cell for a multi-cell DAC, each cell having a data input and an analog output. There is a current switching circuit having a current node and a current definition circuit for providing current to the current switching circuit at the current node. The current definition circuit has a parasitic coupling between its input and the current node. A driver circuit responsive to a data input, actuates the current switching circuit to provide an analog output from the current definition circuit. A control circuit responds to at least one common control signal for controlling the current definition circuit and isolating the parasitic coupling between the current node and the common control signal.

[0010] In the preferred embodiment the current switching circuit may include a differential current switching circuit. The current definition circuit may include a current source and the current source may be connected to the current node. The differential switching circuit may include FET's and the current node may be a common source current node. The current definition circuit may include a cascode circuit in series with the current source and the cascode circuit may be connected to the current node. The parasitic coupling may include a parasitic capacitance. The cascode circuit may include an active cascode circuit. The control circuit may define the current supplied by the current source and the impedance of the current definition circuit. The control circuit may include a buffer amplifier. The current switching circuit, current definition circuit, driver circuit, and control circuit may be implemented with FET's. The control circuit may be responsive to two common control signals, a first common control signal which defines the current supplied by the current source, and a second common control signal which operates the cascode circuit to multiply the impedance of the current source.

[0011] The invention also features a current switching cell for a multi-cell DAC each cell of which has a digital input and analog output. There is a current switching circuit having a current node and a current definition circuit providing current to the current switching circuit at the current node. A driver circuit responds to a data input to provide a driver input to the current switching circuit to enable it to produce an analog output from the current from the current definition circuit. The driver circuit has a parasitic coupling between its driver input and the current node. A control circuit responsive to a common control signal controls the driver circuit and isolates the parasitic coupling between the current node and the driver input.

[0012] In a preferred embodiment the parasitic coupling may include a parasitic capacitance.

[0013] Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which:

[0014] FIG. 1 is a schematic block diagram showing a portion of a prior art DAC including two current switching cells;

[0015] FIG. 2 is a more detailed view of the prior art circuits of FIG. 1;

[0016] FIG. 3 is a schematic block diagram similar to FIG. 1 of DAC current switching cells according to this invention;

[0017] FIG. 4 is a view similar to FIG. 3 showing one implementation of the current definition circuit in greater detail;

[0018] FIG. 5 is a schematic block diagram of another construction of the current definition circuit of FIG. 4;

[0019] FIG. 6 is a schematic block diagram of another construction of the current definition circuit of FIG. 4;

[0020] FIG. 7 is a schematic block diagram of another construction of the current definition circuit of FIG. 4;

[0021] FIG. 8 is a schematic block diagram of another construction of the current definition circuit of FIG. 4;

[0022] FIG. 9 is a schematic block diagram similar to FIG. 3 showing the current switching circuit and driver circuit in greater detail to illustrate the isolation circuit applied to the driver circuit; and

[0023] FIG. 10 is a more detailed schematic diagram of one implementation of the isolation circuit of FIG. 9.

[0024] There is shown in FIG. 1 a portion of a DAC 10aincluding current switching cells 12aand 12aawhich are but two of a multiplicity of such cells that make up the DAC. Reference is made only to cell 12afor illustrative purposes; cell 12aauses the same numbers accompanied by two lower case a's. Current switching cell 12aincludes current circuit 14a, current switching circuit 16a, and driver circuit 18awhich has digital inputs 25aand 27a. Current circuit 14areceives control signals on global or common lines 20aand 22awhich service all of the current circuits, 14a, 14aa. . . . Current switching circuit 16ais operated by driver circuit 18aover drive lines 24aand 26ato provide analog outputs on lines 28aand 30aderived from the current supplied at current node 32afrom current circuit 14a. Driver circuit 18ais operated by a switch control signal on another global control line 34a. Control line 20amay define the current provided by current circuit 14awhile control line 22amay control the output impedance of current circuit 14a. A problem with these prior art circuits is that disturbances that occur in any one of the circuits 14a, 14aa. . . , 16a, 16aa. . . , 18a, 18aa. . . , can propagate to any of the others through the connection of the global control line because of the parasitic couplings that occur in the circuits themselves.

[0025] In FIG. 1 the parasitic couplings are shown as parasitic capacitances. For example, a parasitic capacitance 40aextends between current node 32aand the input to current circuit 14afrom control line 22a. Current switching circuit 16aexperiences two parasitic couplings capacitances 42aand 44abetween current node 32aand the driver outputs 24aand 26a. And driver circuit 18aexperiences parasitic coupling in the form of capacitances 46aand 48abetween driver outputs 24aand 26aand the global control line 34a. Thus, for example, a disturbance appearing at current node 32ais propagated through parasitic capacitance 40aand input line 50aof current circuit 14aonto the global control line 22awhere it then passes through input 50aainto current circuit 14aaand subsequent circuits in line. From input 50aait goes directly through current circuit 14aato the node 32aa.

[0026] In a similar fashion, disturbances at current node 32acan pass through parasitic capacitances 42aand 44a, depending upon the condition of current switching circuit 16a, through the associated ones of driver outputs 24aand 26aand their respective parasitic capacitances 46aand 48a, and through the input 52aof driver circuit 18aonto global control line 34a. From there the disturbances can be transmitted in the same way, through the same paths on the other cells in the system, for example, in cell 12aathe disturbance can pass through input 52aa, capacitances 46aaand 48aa, driver outputs 24aaand 26aa, and capacitances 42aaand 44aato current node 32aa.

[0027] The prior art construction of DAC 10ais shown in FIG. 2 wherein all of the transistors are implemented as field effect transistors (FET's) and current circuit 14bincludes a first FET operating as a current source 60band a second acting as a cascode circuit 62b. Current switching circuit 16bis implemented in FIG. 2 as a differential current switch employing two FET's 64band 66band driver circuit 18bis implemented in a differential mode using FET transistors 68b, 70b, 72band 74b.

[0028] In accordance with this invention, DAC 10c, FIG. 3, includes cells 12cand 12ccwhich include a current definition circuit 14c, current switching circuit 16c, and driver circuit 18c. The global control signals are no longer delivered directly to current definition circuit 14c. Rather, they are first transmitted to isolation circuit 80cin order to isolate the parasitic coupling capacitance 40cfrom the global connection line 20cand 22c. In that way disturbances which occur on one or both of those line will not be propagated to the current node 32cas explained previously. Similarly, isolation circuit 82cis interposed between global control line 34cand driver circuit 18cto prevent disturbances on control line 34cfrom passing through either parasitic capacitances 46cand 42cor 48cand 44cwhich interconnect with current node 32c.

[0029] In one embodiment in accordance with this invention, the current definition circuit 14d, FIG. 4, includes a current source 84dimplemented with an FET with its drain connected to current node 32d. In another embodiment as shown in FIG. 5, in cell 12e, current definition circuit 14eincludes current source 86eand cascode circuit 88e, both implemented with FET's. Here isolation circuit 80ereceives global input 20eand 22eand delivers two outputs, 20e′ and 22e′ to the current source 86eand the cascode circuit 88e, respectively. In this way the current supplied by current source 86eis defined and the impedance of current source 86eis mulitplied by the gain factor of cascode circuit 88e. Alternatively, current control signal 20f, FIG. 6 and impedance control signal 22fcan be separately supplied over global lines with the control signal fed directly to current source 86fwhile the impedance control signal is first delivered to isolation circuit 80fbefore it is delivered to the cascode circuit 88f. While cascode circuit 88fis shown as a conventional cascode circuit this is not a necessary limitation of the invention as it may be implemented with an active cascode circuit 88g, FIG. 7, wherein the isolator circuit 80greceives feedback on line 90gfrom the cascode circuit 88g. In this circuit the output impedance of current source 86gis increased not just by the cascode circuit but by the combination of the gain of the cascode circuit and the gain of the isolation circuit 80g. Another variation is depicted in FIG. 8 where in contrast to cell 12fin FIG. 6, cell 12hincludes an isolation circuit 80hand 80h′ associated with cascode circuit 88hand current source 86h, respectively, so that each is independently isolated from its global control signal 20hand 22h, respectively.

[0030] The use of an isolating circuit to isolate the parasitic coupling capacitances 42iand 44i, FIG. 9, associated with current switching circuit 16ican also be eliminated by the use of isolation circuit 82iwhere node 83iconnects isolation circuit 82iand driver circuit 18i. (In FIG. 9 the detail associated with current definition circuits 14ihas been eliminated for clarity.) In this way, without isolation circuit 82iany disturbance appearing on control line 34icould propagate through whichever the switches 68ior 72iis on in switch driver 18iand from there be transmitted to the gates of the respective one of transistors 64iand 66iand then through the respective ones of parasitic capacitances 42iand 44ito current node 32i. Isolation circuit 82iin one construction, FIG. 10, includes current mirror 90iincluding FET's 92iand 94iand diode connected FET transistor 96i. Any disturbance introduced by the digital input 25iand 27i, appearing at node 83iwill be isolated by current mirror 90ifrom reaching control line 34iand propagating to other cells in the DAC while diode connected transistor 96imaintains the proper voltage from node 83ito ground so that the proper operating voltage is present on switch driver 18i.

[0031] Although specific features of the invention are shown in some drawings and not in others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention. The words “including”, “comprising”, “having”, and “with” as used herein are to be interpreted broadly and comprehensively and are not limited to any physical interconnection. Moreover, any embodiments disclosed in the subject application are not to be taken as the only possible embodiments.

[0032] Other embodiments will occur to those skilled in the art and are within the following claims.