BACKGROUND

[0001] Analog comparators are relatively common electronic circuits that are used in a wi de variety of applications to compare the amplitude of two signals. Latched comparators are a type of comparator that use positive feedback to maintain an output state of the comparator. For example, output state change of a latched comparator may be triggered by a transition or state of a clock signal. After the output state change, the output state of the comparator is maintained by positive feedback and the remainder of the comparator circuitry may operate at a reduced power level. Latched comparators are often used in event driven or power sensitive applications, such as analog-to-digital converters (ADCs).

SUMMARY

[0002] In accordance with at least one aspect of this description, a comparator includes a pair of back-to-back negative- AND (NAND) gates and a delay circuit coupled to the pair of back-to-back NAND gates. The delay circuit is configured to modulate a triggering clock signal by an input voltage to generate a delayed clock signal with a delay that is based on the input voltage. Each of the pair of back-to-back NAND gates is configured to receive the delayed clock signal and generate a comparator output signal based on the delayed clock signal.

[0003] Another aspect of this description is a delay circuit that includes a first inverter, a second inverter, and a pair of cross-coupled capacitors. The first inverter is coupled to a first transistor and a second transistor. The first inverter is configured to receive a first intermediate delayed clock signal and generate a positive delayed clock signal to be received by a first NAND gate of a comparator. The second inverter is coupled to a third transistor and a fourth transistor. The second inverter is configured to receive a second intermediate delayed clock signal and generate a negative delayed clock signal to be received by a second NAND gate of the comparator. The pair of cross- coupled capacitors are coupled to the first and second inverters.

[0004] Yet another aspect of this description is a comparator that includes a differential input pair of transistors and a delay circuit. The differential input pair of transistors includes a first input transistor and a second input transistor. The delay circuit includes a first inverter, a second inverter, and a pair of cross-coupled capacitors. The first inverter is coupled to a first transistor and a second translator. The first inverter is configured to receive a first intermediate delayed clock signal and generate a positive delayed clock signal to be received by the first input transistor. The second inverter is coupled to a third transistor and a fourth transistor. The second inverter is configured to receive a second intermediate delayed clock signal and generate a negative delayed clock signal to be received by the second input transistor. The pair of cross-coupled capacitors are coupled to the first and second inverters.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] FIG. 1 shows an illustrative block diagram of an ADC in accordance with various examples.

[0006] FIG. 2 shows an illustrative circuit diagram of a comparator in accordance with various examples.

[0007] FIG. 3 shows an illustrative circuit diagram for a delay circuit of a comparator in accordance with various examples.

[0008] FIG. 4A shows an illustrative timing diagram of various signals in a delay circuit of a comparator in accordance with various examples.

[0009] FIG. 4B shows an illustrative timing diagram of various signals in a delay circuit of a comparator in accordance whh various examples.

[0010] FIG. 5 shows an illustrative block diagram of a flash ADC comparison circuit in accordance with various examples.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0011] In mis description, the term "couple" or "couples" means either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection, or through an indirect connection via other devices and connections. Also, in this description, the recitation "based on" means "based at least in part on." Therefore, if X is based on Y, then X may be based on Pand any number of other factors.

[0012] ADCs are used to convert analog signals into a digital representation of the same signal. ADCs are used in a wide variety of applications, ranging from medical and entertainment to communications (both voice and data). One key design block of any ADC is the comparator. The comparator generates digital output signals based on the comparison of an analog input signal whh a reference voltage. [0013] For example, in a SAR ADC, the comparator compares a sampled analog input signal to the analog output of an internal DAC and outputs a digital output signal to a SAR circuit that generates a digital code of the analog input signal voltage which is provided to the DAC. The SAR circuit is initialized so that the most significant bit (MSB) is equal to digital one. This digital code is output into the DAC which converts the digital code into an analog equivalent based on a reference signal. The DAC's analog output is then received by the comparator. If the analog input signal has a voltage that is greater than the voltage of the DAC's analog output, the comparator will output a HIGH signal (thus causing the SAR to keep the MSB as one); however, if the analog signal has a voltage that is less than the voltage of the DAC's analog output, the comparator will output a LOW signal (thus causing the SAR to reset the MSB as zero). Each bit in the SAR is tested in a similar manner against the analog input signal by the comparator until every bit has been set The resulting code is output as the digital output signal that represents the analog input signal. Comparators are similarly important in sigma-delta modulated ADCs, flash ADCs, high speed radio frequency (RF) sampling converters, etc

[0014] Because the cornparator is essential to ADCs, the speed of the comparator affects the speed of the entire ADC. In other words, by increasing the speed of the cofnparator, the speed of the ADC may also be increased. Conventional high speed ADCs include comparators that require input information (e-g., the analog input signal and reference signal) until the comparator gives a valid decision (e.g., generates the output signal). In 65nm technology, the time for a comparator to make a decision is typically greater than 100ps. Therefore, ADCs using conventional comparators typically require an amplifier that drives the comparators to hold the input for at least 100ps. In other words, conventional systems require the amplifier that drives the comparator to hold the input signal for a relatively long period of time. Therefore, the operation of the ADC is slowed. Furthermore, because the input signal is held for a relatively long period of time, timing complexities are introduced into these conventional systems.

|0015] In accordance with various examples, a relatively high speed latch comparator is provided. Such a comparator includes, in various examples, a voltage controlled delay circuit that converts the input voltage information into time information. The input clock of the comparator (LATP) is delayed by the delay circuit in proportion to the input voltage. Therefore, the delayed input signals (which can include a pair of differential delayed input signals (INPD and INMD) include the input mfbnnation (eg., input voltage) in the tune domain. Depending on which of the signals INPD or UNMD arrives at the latch first, the latch triggers, allowing the comparator to generate an output signal. For such a comparator, the delay generated by the delay circuit is relatively low (e.g., approximately 25ps). Furthermore, input voltage movements (eg., change in the input voltage) after the signals INPD and INMD are generated will not affect the decision of the comparator (e.g., will not affect the outputted signal). Tlwrefbre, input information is not needed when the comparator is making a decision. In this way. the speed of the comparator is increased, and timing complexities are reduced. Additionally, the input information is not corrupted during processing.

[0016] FIG. 1 shows an illustrative block diagram of an ADC 100 in accordance with various examples. The ADC 100 includes, in an embodiment, a comparison circuit 102, DAC 110, and a sumniation circuit 106. The summation circuit 106 is configured to receive an analog input signal 122 (eg., a time-varying analog voltage) and an analog feedback signal 134 which is an analog version of the comparison circuit output signal 130. The summation circuit 106 is further configured to sum the analog input signal 122 and the analog feedback signal 134 to generate the summed analog signal 126.

[0017] The comparison circuit 102 includes at least one comparator 104. For example, the comparator 104 can be configured to compare the summed analog input signal 126 with a reference signal 124 and generate a digital comparison circuit output signal 130. Thus, for example, if the summed analog input signal 126, which corresponds with the analog input signal 122, has a voltage mat is greater man the voltage of the reference signal 124, the comparator 104 will output a HIGH signal as the digital comparison circuit output signal 130; however, if the summed analog input signal 126 has a voltage that is less than the voltage of the reference signal 124, the comparator 104 will output a LOW signal as the digital comparison circuit output signal 130. The DAC 110 receives the digital comparison circuit output signal 130 and generates an analog version of the digital comparison circuit output signal 130 as the analog feedback signal 134.

[0018] As discussed above, the comparator 104 can be one of numerous comparators in the comparison circuit 102. Thus, for example, the comparison circuit 102 can include a number of comparators that generate separate bits of the digital comparison circuit output signal 130.

[0019] FIG. 2 shows an illustrative circuit diagram of comparator 104 in accordance with various examples. The comparator 104 includes, in an example, a pair of back-to-back negative- AND (NAND) gates that are made up of the differential input pair of transistors 260-262 and the regeneration transistors 254-256 and 264-266. For example, one NAND gate can include the transistors 254, 260, and 264 while the second NAND gate can include the transistors 256, 262, and 266. The comparator 104 also includes timing transistors 252 and 258 which are configured to receive the triggering clock signal LATP and delay elements 202-204. The delay elements 202- 204 are coupled to the back-to-back NAND gates and are configured to modulate the triggering clock signal LATP by an input voltage to generate a delayed clock signal with a delay that is based on the input voltage. For example, the input voltage received by the comparator 104 can be a differential signal composed of the positive input signal (INP) 222a and the negative input signal (INM) 222b. More particularly, the delay element 202 is configured to receive INP 222a while delay element 204 is configured to receive INM 222b. Additionally, the delay elements 202-204 are configured to receive a reference signal, which, in an example, is a differential signal composed of the positive reference signal (REFP) 224a and the negative reference signal (REFM) 224b. More particularly, the delay element 202 is configured to receive REFM 224b while delay element 204 is configured to receive REFP 224a. In some examples, the reference signals REFP 224a and REFM 224b are set so that the threshold of the comparator 104 is equal to the voltage of REFP 224a minus the voltage of REFM 224b.

[0020] The delay element 202 is configured to generate a positive delayed clock signal (INPD) 226a whose delay is proportional to the voltage level of the INP 222a. In other words, the input transistor 260 receives a delayed version of the triggering clock signal LATP, the delay amount being proportional to INP 222a. The delay element 204 is configured to generate a negative delayed clock signal (INMD) 226b whose delay is proportional to the voltage level of INM 222b. In other words, the input transistor 262 receives a delayed version of the triggering clock signal LATP, the delay amount being proportional to INM 222b. In this way, the delay in the delayed clock signal is proportional to the input voltage. In other words, the input voltage signal is converted into time information before being processed by the back-to-back NAND gates.

[0021] FIG. 3 shows an illustrative circuit diagram for a delay circuit 300 of comparator 104 in accordance with various examples. The delay circuit 300 can include the delay elements 202-204 shown in FIG. 2. The delay circuit 300 includes, in an example, the transistors 302-318, inverters 322-324, and a pair of cross-coupled capacitors that include capacitors 326-328. In some examples, the transistors 302-312 are n-channel metal-oxide-semiconductor field-effect (NMOS) transistors. However, in other examples, the transistors 302-312 can be p-channel metal-oxide- semiconductor field-effect (PMOS) transistors, bipolar junction transistors (BJTs) or any combination thereof (e.g., a combination of NMOS transistors and PMOS transistors). The transistors 314-318 are, in an example, PMOS transistors; however, the transistors 314-318 can be NMOS transistors, BJTs or any combination thereof (e.g., a combination of NMOS transistors and PMOS transistors).

[0022] The transistors 302-304 are configured to receive the triggering clock signal LATP which then is provided to the transistors 306-312. More particularly, transistors 306-308 are in parallel with one another, and transistors 310-312 are in parallel with one another. The transistor 306 is configured to receive at its gate INP 222a. Therefore, the resistance of the transistor 306 is proportional to the voltage level of INP 222a. The transistor 308 is configured to receive at its gate REFM 224b. Therefore, the resistance of the transistor 308 is proportional to the voltage level of REFM 224b. Transistor 310 is configured to receive at its gate INM 222b. Therefore, the resistance of the transistor 310 is proportional to the voltage level of INM 222b. Transistor 312 is configured to receive at its gate REFP 224a. Therefore, the resistance of the transistor 312 is proportional to the voltage level of REFP 224a.

[0023] For example, the parallel transistors 306-308 receive the triggering clock signal LATP and generate an intermediate delayed clock signal 352a (a delayed version of LATP) that is based on the resistance of the parallel transistors 306-308 (and thus based on the voltage level of INP 222a and REFM 224b). Thus, if the resistance of the parallel transistors 306-308 is relatively large, then the delay in LATP will be relatively large in the resulting intermediate delayed clock signal 352a. However, if the resistance of the parallel transistors 306-308 is relatively small, then the delay in LATP will be relatively small in the resulting intermediate delayed clock signal 352a. In this way, the resistance (caused by the voltage levels of INP 222a and REFM 224b) of the parallel transistors 306-308 determines the delay in the intermediate delayed clock signal 352a and thus, INPD 226a.

[0024] Similarly, the parallel transistors 310-312 receive the triggering clock signal LATP and generate an intermediate delayed clock signal 352b (a delayed version of LATP) that is based on the resistance of the parallel transistors 310-312 (and thus based on the voltage level of INM 222b and REFP 224a). Thus, if the resistance of the parallel transistors 310-312 is relatively large, then the delay in LATP will be relatively large in the resulting intermediate delayed clock signal 352b. However, if the resistance of the parallel transistors 310-312 is relatively small, then the delay in LATP will be relatively small in the resulting intermediate delayed clock signal 352b. In this way, the resistance (caused by the voltage levels of INM 222b and REFP 224a) of the parallel transistors 310-312 determines the delay in the intermediate delayed clock signal 352b and thus, INMD 226b.

[0025] The delay difference between the intermediate delayed clock signal 352a and the intermediate delayed clock signal 352b can be relatively small; therefore, the pair of cross-coupled capacitors that includes the capacitors 326-328 are included in some examples to provide more resolution in the delay difference between INPD 226a and INMD 226b. For example, the inverter 322 is configured, in an example, to receive the intermediate delayed clock signal 352a from the parallel transistors 306-308 and invert the intermediate delayed clock signal 352a to generate INPD 226a. Similarly, the inverter 324 is configured, in an example, to receive the intermediate delayed clock signal 352b from the parallel transistors 310-312 and invert the intermediate delayed clock signal 352b to generate INMD 226b. Due to the presence of the capacitors 326-328 in a cross-coupled configuration, if INPD 226a transitions HIGH before INMD 226b transitions HIGH, then the transition of INMD 226b to HIGH is delayed. Similarly, if INMD 226b transitions HIGH before INPD 226a transitions HIGH, then the transition of INPD 226a to HIGH is delayed. The higher resolution in the delay difference between INPD 226a and INMD 226b enables the comparator 104 to provide an accurate decision as its output signal.

[0026] In one specific example to generate a high resolution delay difference between INPD 226a and INMD 226b, the drains of transistors 306-308 are connected to the input terminal of inverter 322 and a first terminal of capacitor 326. Thus, the first terminal of capacitor 326 is connected to the input terminal of inverter 322. The drains of transistors 310-312 are connected to the input terminal of inverter 324 and a first terminal of capacitor 328. Thus, the first terminal of capacitor 328 is connected to the input terminal of inverter 324. The second terminal of capacitor 326 is connected to the output terminal of inverter 324 and thus, to INMD 226b. The second terminal of capacitor 328 is connected to the output terminal of inverter 322 and thus, to INPD 226a.

[0027] FIG. 4A shows an illustrative timing diagram 400 of LATP, INPD 226a and INMD 226b in delay circuit 300 of comparator 104 in accordance with various examples. More particularly, the timing diagram 400 shows the timing of the signals LATP, INPD 226a, and INMD 226b if the voltage of INP 222a minus the voltage of INM 222b is greater than the voltage of REFP 224a minus the voltage of REFM 224b (i.e., ( INP— INM ) > ( REFP— REFM )). Thus, as shown in FIG. 4A, when ( INP— INM ) > ( REFP— REFM), INPD 226a triggers before INMD 226b triggers.

[0028] FIG. 4B shows an illustrative timing diagram 450 of LATP, INPD 226a, and INMD 226b in delay circuit 300 of comparator 104 in accordance with various examples. More particularly, the timing diagram 450 shows the timing of the signals LATP, INPD 226a, and INMD 226b if the voltage of INP 222a minus the voltage of INM 222b is less than the voltage of REFP 224a minus the voltage of REFM 224b (i.e., ( INP— INM) < ( REFP— REFM)). Thus, as shown in FIG. 4B, when ( INP— INM) < ( REFP— REFM), INMD 226b triggers before INPD 226a triggers.

[0029] FIG. 5 shows an illustrative block diagram of flash ADC comparison circuit 102 in accordance with various examples. The example comparison circuit 102 includes, in an example, delay circuits 300, 500, and 550, first level comparators 502-504 and 554, interpolation comparators 506-518, and dummy comparators 520-528 and 556-558. The comparators 502-528 and 554-556 are, in an example, similar to the comparator 104 from FIG. 2 without the delay elements 202-204. The delay circuits 500 and 550 are similar, in an example, to the delay circuit 300 except with one or more different input signals. For example, while the delay circuit 300 receives INP 222a, INM 222b, REFP 224a, and REFM 224b, the delay circuit 500 receives different reference signals (e.g., REFP 574a and REFM 574b) in addition to INP 222a and INM 222b and the delay circuit 550 receives the triggering clock signal LATP-IN as the only input.

[0030] The comparators 554-558 are configured to generate triggering clock signals for successive levels of comparators, each level delayed from the previous level. For example, the clock signal LATP IN is used as the triggering clock signal for the first level comparators 502- 504, LATP1 is used as the triggering clock signal for the second level of comparators that includes interpolation comparator 506 and the dummy comparators 520-522, the triggering clock signal LATP2 is used as the triggering clock signal for the third level of comparators that includes interpolation comparators 508-510 and the dummy comparators 524-528, and the triggering clock signal LATP3 is used as the triggering clock signal for the fourth level of comparators that includes interpolation comparators 512-518.

[0031] The first level comparator 502 compares the positive delayed clock signal generated by the delay circuit 500 with the negative delayed clock signal generated by the delay circuit 500 and generates a differential comparator output signal (e.g., a positive comparator output signal and a negative comparator output signal). Similarly, the first level comparator 504 compares the positive delayed clock signal generated by the delay circuit 300 with the negative delayed clock signal generated by the delay circuit 300 and generates a differential comparator output signal (e.g., a positive comparator output signal and a negative comparator output signal). The interpolation comparator 506 receives the negative comparator output signal from the comparator 504 and the positive comparator output signal from the comparator 502 and generates a second level differential comparator output signal (e.g., a positive comparator output signal and a negative comparator output signal). The dummy comparators 520-522 are utilized to match the timing difference generated by the interpolation comparator 506 for the third level of comparators.

[0032] The third level of comparators works in a similar manner as the second level of comparators except that there are now two interpolation comparators (e.g., interpolation comparators 508-510). For example, the interpolation comparator 508 receives the negative comparator output signal from the interpolation comparator 506 and the positive comparator output signal from the dummy comparator 522 and generates a third level differential comparator output signal (e.g., a positive comparator output signal and a negative comparator output signal). The interpolation comparator 510 receives the positive comparator output signal from the interpolation comparator 506 and the negative comparator output signal from the dummy comparator 520 and generates a third level differential comparator output signal (e.g., a positive comparator output signal and a negative comparator output signal). The dummy comparators 524-528 are utilized to match the timing difference generated by the interpolation comparators 508-510 for the fourth level of comparators.

[0033] The fourth level of comparators works in a similar manner as the second level of comparators and the third level of comparators except that there are now four interpolation comparators (e.g., interpolation comparators 512-518). For example, the interpolation comparator 512 receives the negative comparator output signal from the dummy comparator 524 and the positive comparator output signal from the interpolation comparator 510 and generates a single bit differential comparator output signal (e.g., a positive comparator output signal and a negative comparator output signal) that makes up a single bit of the comparison circuit 102 output signal 130. The interpolation comparator 514 receives the negative comparator output signal from the interpolation comparator 510 and the positive comparator output signal from the dummy comparator 526 and generates a single bit differential comparator output signal (e.g., a positive comparator output signal and a negative comparator output signal) that makes up a single bit of the comparison circuit 102 output signal 130. The interpolation comparator 516 receives the negative comparator output signal from the dummy comparator 526 and the positive comparator output signal from the interpolation comparator 508 and generates a single bit differential comparator output signal (e.g., a positive comparator output signal and a negative comparator output signal) that makes up a single bit of the comparison circuit 102 output signal 130. The interpolation comparator 518 receives the negative comparator output signal from the interpolation comparator 508 and the positive comparator output signal from the dummy comparator 528 and generates a single bit differential comparator output signal (e.g., a positive comparator output signal and a negative comparator output signal) that makes up a single bit of the comparison circuit 102 output signal 130.

[0034] Thus, the input information (INP 222a, INM 222b, REFP 224a, REFP 224b, REFP 574a, and REFM 574b) is needed only for the duration of the delay element of the delay circuits 300 and 500 to modulate the input triggering clock LATP IN. The entire conversion can follow asynchronous operation and thus, only one input triggering clock is needed. Furthermore, as the last level of comparators (e.g., the fourth level of comparators) is in a decision phase, the first level of comparators can begin processing the next input sample. Moreover, each level of interpolation gives a gain for the next level of comparators, thus, all the comparators need not be designed for noise.

[0035] Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.