SKEWLESS DIFFERENTIAL SWITCH AND DAC EMPLOYING THE SAME

Abstract: A differential switch (30) accepts a binary control signal (MSB) and its complement (which may be skewed with respect to the control signal) and latches both signals simultaneously. The latched output signals drive the control terminals of a differential switch pair (38) which connects one of two terminals to a third terminal, depending upon the state of the control terminals. The differential switch may optionally include an inverter (INV3) which complements the binary control signal, thus eliminating the need for external inversion of the control signal. The switch is particularly applicable for use in a digital to analog converter.

Description: SKEWLESS DIFFERENTIAL SWITCH AND DAC EMPLOYING THE SAME BACKGROUND OF THF, INVENTION Field of the Invention The invention relates to the field of differential switches and, in particular, to differential switches oper- ated under control of a single binary signal, as in the case of digital to analog converters (DACs) . Description of the Related Art Binary-control differential switches operate as single-pole double-throw switches and are employed in many applications such as DACs. Within such a switch, a single control terminal effects contact between a first conducting terminal and a second conducting terminal while breaking contact between the first conducting terminal and a third conducting terminal. Although the utility of such switches will be described in reference to their application within DACs, they may be used for many other applications. There are a number of conventional DAC architectures which employ differential switches. Some are current out- put, some are voltage output. For examples of both see, Analog-Digital Conversion Handbook. Daniel H.Sheingold ed.,Prentice Hall, Englewood Cliffs, NJ, 1986, pages 191- 206. A current output DAC is illustrated in FIG.l, but the new switches are applicable to voltage output DACs as well . Generally, " a DAC is a device which converts a quantity specified as a binary number (this includes BCD, two's com¬ plement and other binary codes) into a current or voltage which is proportional to the value of the digital input. The digital input is typically held in a digital section li which may receive the digital input either serially or in parallel through a digital interface 13. The binary number thus stored typically controls an analog output section 15 which comprises a set of differential switches Sl-Sn, with each bit of the binary number controlling which one of two terminals within an associated differential switch Sl-Sn is connected to a third terminal within the switch. In some cases a single switch is employed to "build up" an analog signal over some period of time. More specifically, in the design of FIG.l, an array of differential switches Sl-Sn connect binarily-weighted cur¬ rent sources II-In to either a return, or reference, termi- nal 10 or output terminal 12 under control, respectively, of binary control inputs from the least significant bit LSB 14 to the most significant bit MSB 18. Current for the sources Il-In is provided through terminal 9. The sum of the currents appearing at the output terminal 12 provide a "stair step" approximation to the continuous signal repre¬ sented by the binary control inputs MSB-LSB. Alterna¬ tively, the reference terminal 10 and output terminal 12 could be connected to high and low voltage references and the terminal 9 would then provide a voltage output. In this case, the taps of a resistor ladder are connected in place of the current sources Il-In. As the binary values at the control inputs LSB-MSB vary, the switches Sl-Sn route currents corresponding to the varying binary values of the control inputs to the out- put terminal, where the total current may be converted into a voltage. The switches are generally switched "simulta¬ neously" to provide valid current levels at the output terminal 12. However, as will be explained in greater de¬ tail in relation to FIG.2, there is often a delay intro- duced between the "make" and "break" actions of the switches Sl-Sn, causing spurious signals, or "glitches", to appear at the output terminal 12. The mid-scale glitch, produced by the transition of the control codes appearing at terminals 18-14 between 1000...0 and 0111...1, is usually the worst glitch because all the switches Sl-Sn are switching at this transition. Glitches will also occur at other transition points, but they will generally be of lesser magnitude. Glitches are particularly onerous in waveform reconstruction applica¬ tions such as direct digital synthesis systems. Code-dependent glitches, such as those just discussed, will produce both out-of-band and in-band harmonics of the desired signal. For example, in reconstructing a sine wave, the midscale glitch occurs twice during each sine wave period, at each mid-scale crossing. In this manner the midscale glitches produce a second harmonic of the sinewave. Although filtering may eliminate or reduce to a tolerable level the contribution from some of the glitches, higher order harmonics, which alias back into the Nyquist bandwidth, cannot be filtered. To avoid filtering and to eliminate spurious signals that cannot be filtered, it would therefore be desirable to avoid introducing the glitches whenever possible. The block diagram of FIG.2A provides a more detailed view of a typical conventional switch, which may be em- ployed as one of switches Sl-Sn of FIG.l. As an example, switch Si includes a differential switch pair 20, compris¬ ing switches swa and swb, which connect either terminal a or b to terminal c. Control terminals T and I are con¬ nected to receive complementary control signals developed within a latch Ll. The latch LI accepts the binary control signal MSB and converts it into a complementary pair of control signals for use with the differential pair swa and swb. The utility of the latch Ll derives from the fact that, at a system level, whatever device is driving, or controlling, the DAC, in all probability has other duties to perform and may address those other duties only if it stores its required digital patterns within the switches Sl-Sn, and then proceeds to other operations. The control input MSB provides a digital signal path for control inputs to the switch SI. An MSB signal enters the latch Ll and, under control of enable signals ck and ckb, is transferred through an analog switch ASWl. It is then inverted, or complemented, by an inverter INVl to pro¬ duce a control signal INVERTED which is applied to the con¬ trol terminal of the switch swb. Analog switches are known in the art. A description of them may be found in, Paul Horowitz, and Winfield Hill, The Art of Electronics, Cam¬ bridge University Press, New York, 1989, pages 142-143. The output of the inverter INVl is connected to the input of a second inverter INV2 which inverts the signal INVERTED to produce a control signal TRUE which is applied to the control input 24 of the switch swb, the other switch of the differential pair. Note that inversion of the INVERTED sig¬ nal by inverter INV2 produces a delay between the control signals applied to the differential pair. That is, the INVERTED signal will arrive at the control terminal 22 of switch swb one inverter's delay before the TRUE signal ar¬ rives at the control terminal 24 of switch swa. Conse¬ quently, a glitch impulse will be created at the SI output terminal c. Returning to the operation of the latch Ll, the enable signals ck and ckb are assumed to be complementary and non- overlapping. That is, more circuitry than a simple in¬ verter is required to produce ckb from ck. During a first phase of the enable signals ck and ckb, the input signal from MSB is "clocked" through the analog switch ASWl. At the same time, because the control inputs to analog switch ASW2 are connected opposite to the connection of ASWl, ASW2 will be "off", thus isolating the output of ASWl from the output of the inverter INV2. However, during the second phase of the enable sig¬ nals, analog switch ASWl is off and ASW2 is on. With ASWl off, the MSB terminal is isolated from the circuit beyond the analog switch ASWl. With analog switch ASW2 on, in¬ verters INVl and INV2 are "cross-coupled". That is, the output of INVl feeds the input of INV2 and the output of INV2 feeds the input of INVl. In this conventional config¬ uration, there are two stable states which the inverters may assume, i.e., INV1=1, INV2=0 or INV1=0, INV2=1 and, by feedback, they will remain in whichever state to which they are forced. In this way the cross-coupled inverters, cou- pled through the switches ASWl and ASW2, form a latch which provides TRUE(delayed) and INVERTED control signals for a differential switch pair from a single binary control sig¬ nal i.e., that from the MSB terminal. The switches swa and swb which comprise the differen- tial pair may be any type of switch, including p-channel or n-channel MOSFETs, NPN or PNP bipolar transistor or analog switches. Employing analog switches for switches swa and swb provides some flexibility in choosing between current output or voltage output DACs. An implementation which employs PNP transistors as switches swa and swb is illus¬ trated in FIG. 2B, for example. The emitters of two PNP transistors are connected to a current source such as II in FIG.l A single control signal, e.g. MSB, is converted to a differential pair of control signals and applied to the respective control terminals 22 and 24, i.e. the bases, of the transistors. ith complementary control signals, only one of the transistors will conduct at a given time (ideally) , switching current, for example, from the current source 11 into either a return 10 or output 12 path. An analog switch implementation of the differential switch pair, illustrated in FIG.2C, operates substantially the same as the PNP transistor pair of FIG.2B. Differen¬ tial control signals derived from a control input such as MSB are applied, cross-coupled, to the inverting and non- inverting control inputs of two analog switches. One in- put, or switch contact, of each analog switch is connected to a current source, the other input is connected to the return terminal 10, the other input of swb is connected to output terminal 12. Ideally, the differential control sig- nals place only one of the switches in the conduction mode at a time, thereby switching current from the current source either into the return path or into the output 12 path. However, as described above, the delay between con¬ trol signals TRUE and INVERTED sometimes place both switches swa and swb into conduction at the same time. The advantage of employing analog switches for switches swa and swb lies in the fact that they conduct bidirectionally; therefore a voltage output may be produced by substituting voltage references at the return 10 and output 12 terminals and taking the output from the terminal 14 which, in the current output configuration, provides the reference current II. Another latch may be added "in front of" Ll to produce a conventional master/slave latch which provides added iso- lation between input and output. This additional level of isolation may be used, for example, to update a binary in¬ put value by shifting a desired binary value into position at the inputs to a set of master latches, keeping the slave latches isolated, then shifting the updated value into the slave latches simultaneously. As just described, conventional switches require a somewhat elaborate scheme to produce non-overlapping com¬ plementary enable signals to drive the control inputs of analog switches which, along with a pair of inverters, form a binary to differential control latch. Not only is an elaborate enable signal required, glitches, which may cre¬ ate unfilterable spurious signals, are produced by the de¬ lay between the generation of TRUE and INVERTED control signals for the differential switch pair. SUMMARY OF THE INVENTION The invention is directed to a differential switch that minimizes the complexity of a switch controller's clock generation circuitry and reduces spurious switching, thereby reducing the occurrence and duration of undesirable switch outputs, or glitches. These goals are achieved by a latched differential switch which inverts the control input and then simultaneously transfers the control input and its complement through transfer switches into storage elements. Although inverting the control signal introduces a delay between the control signal and its complement (re- ferred to as the "TRUE" and "INVERTED" signals hereinaf¬ ter) , simultaneously transferring them into storage ele¬ ments eliminates this skew. The storage elements' outputs are connected to the control inputs of a differential switch pair, thus providing "de-skewed" control for a dif- ferential switch pair from a single binary control input. In one implementation, the novel switch includes a n intermediate set of transfer switches, operated from the same "enable" signal as the first set of transfer switches, and an intermediate set of storage elements. The interme- diate sets of switches and storage elements are interposed between the TRUE and INVERTED inputs and the first set of transfer switches. During the first cycle of the enable signal, the TRUE and INVERTED signals are simultaneously transferred into the intermediate set of storage elements . As described above, this simultaneous transfer eliminates the skew between the TRUE and INVERTED signals. The TRUE and INVERTED signals are then transferred into the first storage element, which is isolated from the intermediate storage element, during the second cycle of the enable sig- nal. The isolation between the first and second storage elements prevents transitions at the input to the second storage element from appearing at the first storage ele¬ ment. The TRUE and INVERTED signals are therefore avail- able from the second storage element without the skew be¬ tween them that had been introduced by inverting the con- trol signal. The outputs of the first storage element are connected, as described above, to the control inputs of a differential switch pair, thus providing "de-skewed" con¬ trol for the switch pair. The novel switch may be used, for example, within a DAC to reduce the DACs glitch energy output . These and other features, aspects and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG.l is a block diagram, described above, of the intercon¬ nections between control bits, differential switches and current sources within a conventional DAC. FIG.2A, described above, is a schematic diagram of one of the differential switches of FIG.l. FIG.2B, described above, is a schematic diagram of an PNP transistor implementation of the differential switch block of FIG.2A. FIG.2C, described above, is a schematic diagram of an ana¬ log switch implementation of the differential switch block of FIG.2A. FIG.3 is a block diagram of a preferred embodiment of the new differential switch provided by the invention, illus- tra ing the switch's latch and the latch's inter-connection with a differential switch pair. FIG.4 is a schematic diagram of one implementation of the novel differential switch which employs cross-coupled in¬ verters as storage elements. FIG.5A is a schematic diagram illustrating "switched" cross-coupled inverters, which may be used as the novel switch's storage elements. FIG.5B is a schematic diagram illustrating a gated inverter employed as one of a cross-coupled inverter pair, which may be used as one of the novel switch's storage elements. FIG.5C is a detailed schematic diagram of a gated inverter such as illustrated in FIG.5B. FIG.5D is a schematic diagram illustrating the use of a capacitor as a storage element within the novel switch. FIG.6 is a schematic diagram of a preferred master/slave embodiment of the novel switch. DETAILED DESCRIPTION OF THE INVENTION The new switch 30 of FIG.3 may be employed within a DAC, for example, as one of the differential switches Sl-Sn illustrated in FIG.l. The new switch 30 comprises a latch 32 with a binary input MSB and enable input ck. The latch 32 accepts a single binary signal at an MSB input, and pro¬ duces de-skewed TRUE and INVERTED control signals at like- named outputs. These outputs are connected to the control terminals 34 and 36 of a conventional differential switch pair 38 which may be compose, for example, as described in the background section, of NPN or PNP bipolar transistors, N-channel of P-channel MOSFETs or CMOS analog switches. The latch 32 accepts a binary signal at the input MSB, inverts the signal with an inverter INV3 and, under control of transfer switches TSWl and TSW2, transfers the TRUE and INVERTED signals thus produced into storage elements 40 and 42. Outputs from the storage elements 40 and 42 are con¬ nected to the control inputs 34 and 36 of a differential switch pair comprising switches swa and swb. The enable signal ck controls the simultaneous transfer of TRUE and INVERTED control signals into storage elements 40 and 42, respectively. Although the inverter INV3 introduces a skew between the TRUE and INVERTED signals as it produces the INVERTED signal from the TRUE signal, the simultaneous transfer of these signals into the storage elements elimi¬ nates this skew. In some applications, TRUE and INVERTED signals may be available as inputs to the novel switch(with an inverter's delay between them) . In those cases, the inverter INV3 could be eliminated from the switch 30. As indicated in FIG.3, the control signals TRUE and INVERTED are available at the control terminals 34 and 36 of the differential switch pair coincident with their transfer into the storage elements 40 and 42. Once these signals are stored and are providing control over the dif¬ ferential switch pair, the transfer switches TSWl and TSW2 are opened to provide isolation from the input MSB which may be subject to modification of its logic state due to the transfer of data into a DAC of which the novel switch 30 is a part . An implementation of the novel switch 30 is illus- trated in FIG.4, employing P-channel FETs as transfer switches TSWl and TSW2 within latch 32. Weak cross-coupled inverters INV4 and INV5 serve as storage elements 40 and 42. As noted in the background section above, cross-coupled inverters are known in the art and, briefly, have two sta- ble states which they may assume. Once forced into one of those states, they will maintain it. Because the two states desired for the differential switch control terminals 34 and 36 coincide with the two states available from the cross-coupled inverters, the inverters provide the func- tions of storage elements 40 and 42, although they are not independent storage elements in the sense of a pair of mem¬ ory cells, and cannot store binary patterns corresponding to the (undesirable) states which would simultaneously turn both switches swa and swb "on" or "off" . As described in greater detail in relation to FIG.6, the inverters INV4 and INV5 are "weak" in that they have limited drive current capability and therefore may be easily forced, by whatever device drives the MSB input and the inverter INV3, into a desired state. Alternatively, a pair of cross-coupled inverters INV6 and INV7, one of which (INV7) is a gated inverter, may be employed, as a storage element, as illustrated in FIG.5A, with one pair for each storage element 40 and 42. As will be described, this implementation eliminates the need for weak inverters. The inverter INV3, inputs MSB and ck and outputs TRUE and INVERTED are as described in relation to FIG.3. In this implementation, the storage elements 40 and 42 are identical; the detailed description given below for storage element 40 will also apply to storage element 42. A signal ck is asserted to transfer a single-bit sig- nal MSB through a transfer switch TSWl, when MSB is valid, to the input of inverter INV6. A signal ckb, the non-over¬ lapping inverse of ck, opens switch SWI1 when ck closes TSWl. The switch SWI1 is connected between the output of the inverter INV7 and the input of the inverter INV6. Con- sequently the output of the inverter INV7 is isolated from the device driving the input of the inverter INV6 and, be¬ cause there is no contention between the output of INV7 and the device driving the input of inverter INV6, inverters INV6 and INV7 needn't be "weak" inverters. When the ck input is "de-asserted", transfer switch TSWl opens and switch SWIl closes. During the short time that neither switch is closed, stray capacitance maintains the state of inverter INV6 until the switch SWIl closes. With switch SWIl closed, the inverter INV7 provides positive feedback to the inverter INV6, thereby "latching" the inverters and providing the TRUE and INVERTED control signals for the differential switch control terminals 34 and 36 (not shown) . In some applications, the differential switch pair requires more drive current than "weak" inverters may pro- vide. Employing standard inverters and the additional switch SWIl as illustrated in FIG.5A eliminates buffers which would otherwise be required to drive the differential switch pair 38. Similarly, FIG.5B illustrates an implementation of the storage element 40 (also applicable to storage element 42) which employs a gated inverter 46 to achieve the same iso¬ lation between the input to the inverter INV6 and the out¬ put from a feedback inverter, in this case, gated inverter 46. This isolation is achieved, as described below, by causing the output of the inverter 46 to "float" . The in¬ put to inverter INV6 is the switched data input, i.e. the input to the storage element 40, and its output TRUE drives the differential switch control inputs 34, as illustrated in FIGs 3 and 4 above. The gated inverter 46, illustrated in detail in FIG.5C, is cross-coupled with the inverter INV6. The same advantage accrues to the use of a gated inverter in this implementation as the use of the inverter switch SWIl in the implementation of FIG.5A, i.e., the cross-coupled inverters needn't be weak inverters and, therefore, buffers which otherwise may be required to drive the differential switch pair's control terminals 34 and 36 may be eliminated. The gated inverter 46 shown in FIG.5C is controlled by the control signals ck and ckb previously identified. The inverter 46 is composed of two n-channel FETs, nl and n2, and two p-channel FETs, pi and p2. FETs n2 and pi are con¬ nected as a conventional CMOS inverter, FETs nl and p2 are connected in series between the inverter formed by n2 and pi and positive and negative supplies V + and V , respec¬ tively. When the ck signal is asserted (driven HIGH) , transferring the MSB signal into storage element 40, it also forces p2 into a non-conducting state. At the same time ckb is driven LOW, forcing nl into a non-conducting state. The inverter is "floating" in this state and what¬ ever device drives the input to inverter INV6 can also drive the output of gated inverter 46 without contention from the gated inverter 46. When ck is de-asserted, after the binary value from MSB has been transferred to the input of the inverter INV6, FETs nl and p2 are turned on, thereby providing supply voltages to the inverter 46. At this point the input 50 to the gated inverter 46 will be driven to the updated level by the output of inverter INV6, and the output 48 of the gated inverter 46 will drive the input of the inverter INV6 to the same level transferred to it by the transfer switch TSWl. in this way an updated value of MSB is transferred to the input of the inverter INV6 without contention from the output of the gated inverter 46. This value is then latched, through positive feedback from the gated inverter 46, at the output of the storage element 40. In some applications, notably those employing a DAC in waveform reconstruction, the DACs differential switches' data inputs, LSB 14-MSB 18, are rapidly updated. As illus¬ trated in FIG.5D, a capacitor Cl of sufficient capacity to hold its value throughout an individual switch's longest update period, i.e. the longest period during which trans- fer switch TSWl is left open, could be used as a storage element 40. After transfer switch TSWl charges the capaci¬ tor Cl to the binary value present at MSB, it opens so that only leakage paths are available to charge or discharge the capacitor Cl (assuming that Cl drives a high impedance in- put) . So long as the voltage across the capacitor does not fall to the following device's low input threshold before being recharged, the capacitor can serve as a storage ele¬ ment. It may be necessary to buffer the capacitor Cl, if, for example, storage element 40 is to drive one of the dif- ferential switch pair 38 of FIG.4 and the pair 38 comprises bipolar transistors. This may be accomplished using an inverter INV8 which provides sufficient drive current for one of the differential switch control terminals 34 or 36. In a preferred embodiment, the novel switch 30 employs a master/slave architecture, as illustrated by the sche¬ matic of FIG.6. All the inverters employed within this pre¬ ferred embodiment are CMOS inverters, the relative strengths of which will be discussed below. A master latch 50 latches control signals during a first phase of the en- able input ck. Then, during the second phase of ck, these signals are transferred to a slave latch 52. As described in relation to FIG.2A above, the master/slave architecture provides greater flexibility in re-loading the latches, in that the data value of the input MSB need be valid for only a short time around the transition of the enable signal ck. A buffer section 54 which follows the slave latch 52 in- creases the drive capability Over that of the slave latch 52. The outputs of the buffer section drive the control inputs 34 and 36 of the differential switch pair swa and swb which, in the preferred embodiment, are p-channel FETs. The switch 30 accepts an enable signal at an input ck and data at an input MSB. Within a DAC, the data appearing at the input MSB would represent one bit of a digital code which is to be converted into an analog output. Data ap¬ pearing at the MSB input is inverted by a CMOS inverter INV9 and the true and complement signals thus formed are passed through switches P5 and P4, respectively, when the signal ck goes low. In this embodiment, switches P5 and P4 are p-channel FETs. Inverters INV10 and INVll form a cross-coupled weak inverter "master" latch that accepts signals passed through the switches P5 and P4. A pair of inverters INV12 and INV13 buffer the outputs from the latch. When the enable signal ck goes high, n-channel MOSFET switches N4 and N5 pass the outputs from the buffers INVl2 and INV13 to a "slave" latch formed by cross-coupled weak inverters INV14 and INVl5. At the same time switches P4 and P5 turn off, thereby isolating the master latch from the MSB signal and its complement. The INVERTED and TRUE outputs from the slave latch are buffered by inverters INVl6 and INVl8 connected in series and INVl7 and INVl9 connected in series, respectively. The buffer inverters INVl8 and INVl9 drive the control terminals 34 and 36 of the differential switch pair 38. In the preferred embodi¬ ment, switches swa and swb are p-channel MOSFETs. Since the signal at the MSB input is complemented before being latched, the skew created by the delay of inverter INV9 is eliminated during the latching process. Consequently, a single binary signal is converted into de-skewed differen¬ tial switch pair control signals available to drive inputs 34 and 36. To accommodate the difference in mobility between holes and electrons, the size (i.e., channel width to length ratio) of the p-channel FET within each inverter INV9-INV17 is preferably about 3.2 times that of the corre¬ sponding n-channel FET, and the p-channel FETs of inverters INVl8 and INVl9 are about twice the size of their corre¬ sponding n-channel FETs. Additionally, the channel widths of the weak inverters, (INV10, INVll, INVl and INVl5) are approximately .4 that of the other inverters. This allows the devices which drive the weak inverters to do so with little contention from the weak inverters. The forgoing description of specific embodiments of the invention has been presented for the purposes of illus¬ tration and description. It is not intended to be exhaus¬ tive or to limit the invention to the precise forms dis- closed, and many modifications and variations are possible in light of the above teachings. For example, the novel switches may be employed within a DAC which is itself a part of an analog to digital converter, such as a succes¬ sive approximation converter. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the inven¬ tion. It is intended that the scope of the invention be limited only by the claims appended hereto.

Claims: WE CLAIM : 1. A ske less differential switch, comprising: control and complement inputs for receiving a control signal and a complement of said control signal which may be skewed with respect to said control signal, an enable input (CK) , storage elements (40,42), switches (TSWl, TSW2) connected to provide transmission paths under control of said enable input from each of said control and complement inputs to said storage elements, said storage elements also connected to store said signals and to provide de-skewed output signals having the same binary value as the respective signal stored within said storage elements, and a differential switch pair (38) having complementary control inputs, one of said control inputs (34) being connected to receive said control signal, the other said control input (36) being connected to receive said complement signal from said storage elements, so that said differential switch pair is controlled by outputs from said storage elements. 2. The switch of claim 2 further comprising a intermediate switch pair (N4,N5) and intermediate storage element pair (INV14, INV15), said intermediate switch and storage element pairs interposed between said control (34) and complement (36) inputs and said first switch pair such that, said intermediate switch pair provides a transmission path from each of said inputs to one of said intermediate storage elements under control of said enable input (CK) , and said intermediate storage elements are connected to receive said control and complement signals from said inputs through said intermediate switches, store said signals and provide output signals to said first switch pair having the same values as the respective signals stored within said in¬ termediate storage elements. 3. The switch of claim 2, wherein said storage elements comprise a pair of cross-coupled weak inverters. 4. The switch of claim 2, wherein each said storage element (INV10, INVll) comprises a pair of cross-coupled inverters, one weak and one strong, the output of said strong inverter is connected to one of said differential control inputs and the input of said strong inverter comprises said storage el¬ ement input. 5. A digital to analog converter, comprising: an analog output section(15), a digital section (13) having a binary output and an enable output connected to provide control over said analog output section, said analog output section comprising a differential switch (30) which connects a first or second conducting ter¬ minal to a third conducting terminal thereby producing an analog output, said differential switch further having a control input (CK) connected to said binary output and an enable input connected to said enable output (MSB) , said differential switch further comprising, an inverter (INV3) connected to receive a signal from said control input and to produce a complement of said control signal, control and complement inputs for receiving a con¬ trol signal and a complement of said control signal which may be skewed with respect to said control signal, an enable input, storage elements (40,42), switches (TSWl, TSW2) connected to provide trans¬ mission paths under control of said enable input from each of said control and complement inputs to said storage elements, said storage elements also connected to store said signals and to provide de-skewed output signals having the same binary value as the respective signal stored within said storage elements, and a differentially connected switches (38) having complementary control inputs, one of said differentially connected switch control inputs being connected to receive said control signal, and the other differentially connected switch control input being connected to receive said comple¬ ment signal from another of said storage elements . 6. The digital to analog converter of claim 12, wherein said differential switch further comprises a intermediate switch pair (N4,N5) and intermediate storage element pair (INV14, INV15) , said intermediate switch and storage element pairs interposed between said control and complement inputs and said first switch pair such that: said intermediate switch pair provides a transmission path from each of said inputs to one of said intermediate storage elements under control of said enable input, and said intermediate storage elements receive said control and complement signals from said inputs through said intermediate switches, store said signals and provide output signals to said first switch pair having the same values as the respective signals stored within said intermediate stor- age elements. 7. The switch of claim 6, wherein said storage elements comprise a pair cross-coupled weak inverter pair. 8. A differential switch, comprising: control and complementary inputs for receiving a control signal and its complement, a latch (50) which latches said control and complementary signals simultaneously, said latch having two outputs, a pair of differentially connected switches (38) with two control terminals, said switches connecting one of two conducting terminals to a third conducting terminal, depend- ing upon the state of the control terminals which are connected to said latch outputs. 9. A digital to analog converter comprising: an analog output section (15) , a digital section (11) having a binary output and an enable output connected to provide control of said analog output section, said analog output section comprising a differential switch (30) which connects a first or second conducting ter¬ minal to a third conducting terminal thereby producing an analog output, said differential switch further comprising, control and complementary inputs for receiving a control signal and its complement, a latch (50) having control and complementary in¬ puts which latches said control and complementary signals simultaneously, said latch having two outputs, and a pair of differentially connected switches (38) with two control terminals, said switches connecting one of two conducting terminals to a third conducting terminal, de¬ pending upon the state of the control terminals said control terminals being connected to said latch outputs. 10. The digital to analog converter of claim 9, wherein each of said differential switches further comprises an intermediate latch (52) interposed between said control and complement inputs and said latch, said intermediate latch connected to simultaneously latch said control and complement inputs under control of an enable input, and to provide latched control and complementary signals to said control and complementary inputs of said latch.

Inventors: MERCER DOUGLAS A, REYNOLDS DAVID, ROBERTSON DAVID H, STROUD ERNEST T, MERCER, DOUGLAS, A., REYNOLDS, DAVID, ROBERTSON, DAVID, H., STROUD, ERNEST, T.
Applicants: ANALOG DEVICES INC, ANALOG DEVICES, INC. (ADI)
Application year: 1996
Main classification: H03K3
IPC codes: H03K17