Patent ID: 12237843

REFERENCE NUMERALS

N1—first NMOS transistor;N2—second NMOS transistor;N3—third NMOS transistor;N4—fourth NMOS transistor;N5—fifth NMOS transistor;N6—sixth NMOS transistor;N7—seventh NMOS transistor;N8—eighth NMOS transistor;M1—first transistor;M2—second transistor;M3—third transistor;M4—fourth transistor;K1—first switch;K2—second switch;K3—third switch;K4—fourth switch;K5—fifth switch;VDD—operating voltage;Vip0—first input signal;Vin0—second input signal;Vip1—first common mode input signal;Vin1—second common mode input signal;Vip2—first differential mode input signal;Vin2—second differential mode input signal;Dop—first output signal of the pre-amplifier;Don—second output signal of the pre-amplifier;Voutp—first output signal of the comparator;Voutn—second output signal of the comparator;a—first node;b—second node;c—third node;d—fourth node;11—input unit;12—adjustment unit;13—load unit;131—a pair of gain adjustment transistors;132—a pair of first offset transistors;133—a pair of second offset transistors;14—constant current source;15—latch.

DETAILED DESCRIPTION

As described in the background, in a case with high precision requirements, a cascaded multi-stage pre-amplifier (Preamp) connected to the latch is typically used as a comparator, to overcome the shortcomings such as high noise and large offset of the single-stage latch structure. Since the pre-amplifier has an offset voltage, when the comparator is applied in the case with high precision requirements, the input differential mode voltage of the comparator may be less than the offset voltage of the comparator, where the input signal of the comparator will be “submerged” by the offset voltage, causing the comparator to output an incorrect comparison result. Therefore, this offset voltage needs to be eliminated in practical applications. The traditional elimination technology of the offset voltage of the comparator is to add a coupling capacitor between adjacent pre-amplifiers. The schematic diagram of an elimination technology of an offset voltage of a comparator with a cascaded multi-stage pre-amplifier structure is shown inFIG.1. When the comparator is in a reset state, an input end of a first-stage pre-amplifier (Preamp1) is connected to a common mode voltage, and the offset voltage of each stage of pre-amplifier (Preamp1, Preamp2, . . . , Preamp (n)) is stored in the coupling capacitor between each adjacent stages, thus, the offset voltage can be eliminated when the comparator is in an amplify state. However, the problems of the conventional structure inFIG.1are as follows: 1. the gain of each pre-amplifier stage cannot be large to abate the influence of the offset voltage, therefore, under conditions with high-precision requirements, many pre-amplifier stages are required to be connected in series, which increases the power consumption of the comparator. 2. The added coupling capacitor between each adjacent pre-amplifier stages significantly reduces the bandwidth of each pre-amplifier, thereby lowering the speed of the comparator. 3. The multi-stage coupling capacitors cause an obvious increase in the layout area of the circuit and the cost of the circuit.

The embodiments of the present disclosure will be described below through specific examples. Those skilled in the art can easily understand other advantages and effects of the present disclosure according to the contents disclosed by the specification. The present disclosure may also be implemented or applied through other different specific implementation modes. Various modifications or changes may be made to all details in the specification based on different points of view and applications without departing from the spirit of the present disclosure.

It needs to be stated that the drawings provided in the following embodiments are just used for schematically describing the basic concept of the present disclosure, thus only illustrating components related to the present disclosure and are not drawn according to the numbers, shapes, and sizes of components during actual implementation, the configuration, number, and scale of each component during the actual implementation thereof may be freely changed, and the component layout configuration thereof may be more complex. It should be understood that the structures, proportions, sizes, and the like, which are illustrated in the drawings of the present specification, are only used to clarify the contents disclosed in the specification for understanding and reading by those skilled, and are not intended to limit the implementation of the present disclosure, thus are not technically meaningful. Any modification of the structure, change of the scale, or adjustment of the size should still fall within the scope of the technical contents disclosed by the present disclosure without affecting the effects and achievable objectives of the present disclosure.

Embodiment 1

The present disclosure provides a comparator, referring toFIG.2, which includes an input unit11, a load unit13, a control switch, and an adjustment unit12.

An input end of the input unit11is connected to a first input signal Vip0and a second input signal Vin0.

The load unit13is connected to the input unit11, and the load unit13includes a pair of gain adjustment transistors131, whose gate voltages are adjusted to change a gain of the comparator.

The adjustment unit12is connected to the input unit11, and the gate voltages of the pair of gain adjustment transistors131are adjusted according to an enable state of the control switch.

In an embodiment, referring toFIG.2, the adjusting unit12includes a first transistor M1, a second transistor M2, a seventh NMOS transistor N7, and an eighth NMOS transistor N8. A first electrode of the first transistor M1is connected to a power supply voltage VDD, a second electrode of the first transistor M1is connected to a first node a, and a control electrode of the first transistor M1is connected to a second node b. A first electrode of the second transistor M2is connected to the power supply voltage VDD, a second electrode of the second transistor M2is connected to a third node c, and a control electrode of the second transistor is connected to a fourth node d. A source of the seventh NMOS transistor is grounded, a gate of the seventh NMOS transistor is connected to the first node a, and a drain of the seventh NMOS transistor is connected to the first node a. A source of the eighth NMOS transistor is grounded, a gate of the eighth NMOS transistor is connected to the third node c, and a drain of the eighth NMOS transistor is connected to the third node c.

In an embodiment, referring toFIG.2, the pair of gain adjustment transistors131include a fifth NMOS transistor N5and a sixth NMOS transistor N6. A source of the fifth NMOS transistor N5is grounded, a gate of the fifth NMOS transistor N5is connected to the first node a or the second node b according to the enable state of the control switch, and a drain of the fifth NMOS transistor N5is connected to the second node b. A source of the sixth NMOS transistor N6is grounded, a gate of the sixth NMOS transistor N6is connected to the third node c or the fourth node d according to the enable state of the control switch, and a drain of the sixth NMOS transistor N6is connected to the fourth node d.

In an embodiment, referring toFIG.2, the load unit13further includes a pair of first offset transistors132and a pair of second offset transistors133, where the pair of first offset transistors132and the pair of second offset transistors133are connected to the input unit11in parallel, the pair of first offset transistors132include a first transconductance and a first output impedance, and the pair of second offset transistors133include a second transconductance and a second output impedance. The first transconductance and the second transconductance are offset according to an enable state of the control switch, or the first output impedance and the second output impedance are offset according to an enable state of the control switch.

In an embodiment, referring toFIG.2, the load unit13further includes a pair of first offset transistors132and a pair of second offset transistors133, where the pair of first offset transistors include a first NMOS transistor N1and a second NMOS transistor N2, and the pair of second offset transistors include a third NMOS transistor N3and a fourth NMOS transistor N4. A source of the first NMOS transistor N1is grounded, a gate of the first NMOS transistor N1is connected to the fourth node d, and a drain of the first NMOS transistor N1is connected to the second node b; a source of the second NMOS transistor N2is grounded, a gate of the second NMOS transistor N2is connected to the second node b, and a drain of the second NMOS transistor N2is connected to the fourth node d; a source of the third NMOS transistor N3is grounded, a gate of the third NMOS transistor N3is connected to the second node b, and a drain of the third NMOS transistor N3is connected to the second node b; and a source of the fourth NMOS transistor N4is grounded, a gate of the fourth NMOS transistor N4is connected to the fourth node d, and a drain of the fourth NMOS transistor N4is connected to the fourth node d.

In an embodiment, referring toFIG.2, the comparator further includes a constant current source14which provides a current to the comparator. The input unit11includes a third transistor M3and a fourth transistor M4, where a first electrode of the third transistor M3is connected to the constant current source14, a second electrode of the third transistor M3is connected to the second node b, and a control electrode of the third transistor M3is connected to the first input signal Vip0; and a first electrode of the fourth transistor M4is connected to the constant current source14, a second electrode of the fourth transistor M4is connected to the fourth node d, and a control electrode of the fourth transistor M4is connected to the second input signal Vin0.

In some embodiments, as shown inFIG.3, the input unit11, the load unit13, the control switch, and the adjustment unit12constitute a pre-amplifier stage (Preamp)14of the comparator, and the comparator further includes a latch stage15, where a first output signal Vop of the pre-amplifier stage and a second output signal Von of the pre-amplifier stage are input signals for the latch stage, respectively, and a first output signal Voutp and a second output signal Voutn of the latch stage are output signals for the comparator.

In some embodiments, referring toFIG.2, the control switch includes a first switch K1, a second switch K2, a third switch K3, a fourth switch K4, and a fifth switch K5.

The first switch K1in an on state enables the gate of the fifth NMOS transistor N5to connect to the second node b.

The second switch K2in an on state enables the gate of the fifth NMOS transistor N5to connect to the first node a.

The third switch K3in an on state enables the gate of the sixth NMOS transistor N6to connect to the fourth node d.

The fourth switch K4in an on state enables the gate of the sixth NMOS transistor N6to connect to the third node c.

The fifth switch K5in an on state enables the two output ends of the comparator to be connected to each other.

The second switch K2is turned on after the first switch K1is turned on, the first switch K1is turned off before the second switch K2is turned on; the fourth switch K4is turned on after the third switch K3is turned on, the third switch K3is turned off before the fourth switch K4is turned on; and the fifth switch K5is turned on after the second switch K2and the fourth switch K4are turned on.

In some embodiments, the operating sequence of the comparator is shown inFIG.4. The comparator has three working states, which are respectively an offset store state, an amplify state, and a reset state.

In some embodiments, the first input signal includes a first common mode voltage, the second input signal includes a second common mode voltage, the control switch cuts off the connection between the adjustment unit and the pair of gain adjustment transistors, so that the pair of gain adjustment transistors, the pair of first offset transistors, and the pair of second offset transistors are connected in parallel to the input unit, the first transconductance and the second transconductance offset each other, and the pair of gain adjustment transistors and the pair of second offset transistors are in a diode connection state, resulting in the comparator in the offset store state.

In an embodiment, when the comparator is in the offset store state, the schematic diagram of the comparator is shown inFIG.5, the switch K2, the switch K4, and the switch K5are turned off, the switch K1and the switch K3are turned on, and the gates of the third transistor M3and the fourth transistor M4of the input unit in the comparator are input common mode signals (Vip1and Vin1). It should be noted that the input common mode signals of the third and fourth transistors should be the same in theory, however, the equivalent input signals of the two ends of the comparator are different in practice due to the offset of the comparator. Therefore, the common mode signals are represented as Vip1and Vin1, respectively. In this case, the following equations can be obtained according to the KCL current law of the output ends (Von and Vop) of the pre-amplifier stage:

gmS⁢Vip⁢1+V⁢o⁢nro⁢3+gm⁢5⁢Vop+V⁢o⁢nro⁢5+(gm⁢7+gm⁢9)⁢Von=0(1)gm⁢4⁢Vin⁢1+V⁢o⁢pro⁢4+gm6⁢Von+Vopro⁢6+(gm⁢8+gm⁢10)⁢Vop=0(2)

In the equations, gm3is a transconductance of the third transistor M3, gm4is a transconductance of the fourth transistor M4, gm5is a transconductance of the first NMOS transistor N1, gm6is a transconductance of the second NMOS transistor N2, gm7is a transconductance of the third NMOS transistor N3, gm8is a transconductance of the fourth NMOS transistor N4, gm9is a transconductance of the fifth NMOS transistor N5, gm10is a transconductance of the sixth NMOS transistor N6, ro3is an impedance of the third transistor M3, ro4is an impedance of the fourth transistor M4, ro5is an impedance of the first NMOS transistor N1, and ro6is an impedance of the second NMOS transistor N2.

Due to the symmetry of the circuit, a voltage gain of a small signal of the pre-amplifier stage can be expressed as:

A1=gm⁢31ro⁢3+1ro⁢5-gm⁢5+gm⁢9+gm⁢7(3)

If N1and N3, as well as N2and N4inFIG.5, are designed as MOS transistors with the same size, then equation (3) can be simplified as:

A1=gm⁢3gm⁢9(4)

From equation (4), it can be seen that the gain of the small signal of the pre-amplifier stage is very small.

Moreover, the N3/N5and N4/N6transistors of the comparator are in the diode connection state, and the equivalent output impedance is small, therefore, the gain of the comparator is small, and the amplifying multiple of the offset voltage is small.

In some embodiments, the first input signal includes a first differential mode voltage, the second input signal includes a second differential mode voltage, the control switch enables the connection between the adjustment unit and the pair of gain adjustment transistors, so that the first output impedance and the second output impedance offset each other, resulting in the comparator in the amplify state.

In an embodiment, when the offset store state is finished, the comparator comes into the amplify state schematically shown inFIG.6, in which the gates of the third transistor M3and the fourth transistor M4of the input unit in the comparator are input differential mode signals (Vip2and Vin2), the switches K2and K4are turned on, and the switches K1, K3, and K5are turned off.

In this case, the following equations can be obtained according to the KCL current law of the output ends (Von and Vop) of the pre-amplifier:

gm⁢3⁢Vip⁢2+Vonro⁢3+gm⁢5⁢Vop+Vonro⁢5+Vonro⁢9+gm⁢7⁢Von+Vonro⁢7+gm⁢9⁢Vnx=0(5)gm⁢4⁢Vin⁢2+Vopro⁢4+gm⁢6⁢Von+Vopro⁢6+Vopro⁢10+gm⁢8⁢Vop+Vopro⁢8+gm⁢10⁢Vpx=0(6)Vnx=-gm⁢1(ro⁢1⁢1gm⁢11)(7)Vpx=-gm⁢2(ro⁢2⁢1gm⁢12)(8)

The following can be deduced from equation (5) to equation (8):

A2=gm⁢31ro⁢3+1ro⁢5+1ro⁢9+1ro⁢7-gm⁢9(9)

In the equations, gm1is a transconductance of the first transistor M1, gm2is a transconductance of the second transistor M2, m3is a transconductance of the third transistor M3, gm4is a transconductance of the fourth transistor M4, gm5is a transconductance of the first NMOS transistor N1, gm6is a transconductance of the second NMOS transistor N2, gm7is a transconductance of the third NMOS transistor N3, gm8is a transconductance of the fourth NMOS transistor N4, gm9is a transconductance of the fifth NMOS transistor N5, gm10is a transconductance of the sixth NMOS transistor N6, gm11is a transconductance of the seventh NMOS transistor N7, gm12is a transconductance of the eighth NMOS transistor N8, ro1is an impedance of the first transistor M1, ro2is an impedance of the second transistor M2, ro3is an impedance of the third transistor M3, ro4is an impedance of the fourth transistor M4, ro5is an impedance of the first NMOS transistor N1, ro6is an impedance of the second NMOS transistor N2, ro7is an impedance of the third NMOS transistor N3, ro8is an impedance of the fourth NMOS transistor N4, ro9is an impedance of the fifth NMOS transistor N5, and ro10is an impedance of the sixth NMOS transistor N6.

It can be seen from equation (9) that if the denominator of equation (9) tends to be 0, then the gain of the small signal of the pre-amplifier stage can tend to be infinite, so that the comparator can provide a large gain of small signal in the amplify state. The equivalent output impedances of N2and N1are negative and are offset by the equivalent output impedances of N3and N4which are in a diode connection mode, respectively, so that the equivalent output impedances of the pre-amplifier stage of the comparator become the equivalent output impedances of N5and M3(N6and M4) which are in a parallel connection. The output impedances of the MOS transistors M3, M4, N5, and N6are large, because they all operate in a saturation region, therefore, a large output impedance of the pre-amplifier stage can be achieved and then a large voltage gain of the pre-amplifier stage can be realized. Furthermore, the input signals Vip2and Vin2of the comparator are significantly amplified.

Afterward, the comparator comes into the reset state, where the switches K5, K4, and K2are turned on, and the switches K1and K3are turned off.

From the above description of the operating states of the comparator, it can be seen that when the comparator is in the offset store state, the amplifying multiple (A1) of the offset voltage (Voff) is small, and when the comparator is in the amplify state, the amplifying multiple (A2) of the input differential mode signal (Vip2−Vin2) is large. Therefore, under the condition that there exists an offset voltage of the comparator, the output differential mode voltage of the comparator can be expressed as: A2*(Vip2−Vin2)−A1*Voff. Since A2is much larger than A1, the offset voltage Voffof the comparator can be ignored without affecting the comparison result of the comparator.

It should be noted that, in some embodiments, the first transistor, the second transistor, the third transistor, and the fourth transistor may be PMOS transistors or NMOS transistors, and a person skilled in the art may adjust a specific wiring pattern according to whether a PMOS transistor or an NMOS transistor is used.

In order to further verify the above advantages of the present disclosure, the above structures are carefully designed under a 180 nm CMOS process, where a load capacitor is 10 fF, a clock frequency is 50 MHz, a power supply voltage is 1.8V, and a common mode voltage is 0.9V. When |Voutp−Vouton|=0.9V, it is considered that the comparator completes a comparison. After simulation, a comparison of gains in the offset store state and the amplify state of the comparator of the present disclosure is shown inFIG.7. It can be seen fromFIG.7that the gain of the pre-amplifier stage of the comparator is about 1.2 dB in the offset store state and the gain of the pre-amplifier stage of the comparator is about 51 dB in the amplify state. A comparison of equivalent input noises in the offset store state and the amplify state of the comparator of the present disclosure is shown inFIG.8. When the temperature changes from −40° C. to 120° C., the equivalent input noise of the comparator is in a range of 41 uV rms to 59 uV rms in the offset store state, and the equivalent input noise of the comparator is in a range of 8 uV rms to 15 uV rms in the amplify state. From the simulation results, it can be seen that the offset voltage of the comparator can be significantly suppressed, and the accuracy of the comparator can be significantly improved based on the elimination technology of the offset voltage of the comparator proposed by the present disclosure.

The gain of the comparator with traditional structures is unchanged, the amplifying multiples of the offset voltage of the comparator and the input differential mode voltage of the comparator are the same, and the offset of the comparator is stored by inserting a capacitor between adjacent pre-amplifier (Pre-amp) stages, which seriously reduces the bandwidth of the comparator, affects the speed of the comparator, and significantly increases the layout area of the comparator and the cost of the circuit. The comparator described in an embodiment of the present disclosure provides a small gain when the comparator is in the offset store state, so that the offset voltage of the comparator is amplified by a small multiple, and the comparator provides a large gain when the comparator is in the amplify state, so that the input differential mode signal of the comparator is amplified by a large multiple. Thus, the influence of the offset voltage of the comparator on the comparison result of the comparator is eliminated. The elimination method of the offset of the comparator proposed by the present disclosure improves the speed of the comparator, reduces the layout area, and obviously eliminates the offset voltage of the comparator.

The present disclosure further provides an analog-to-digital converter which includes the above-mentioned comparator. The operation efficiency of the analog-to-digital converter is improved by the fast and accurate comparison of the comparator. In an embodiment, the analog-to-digital converter includes the comparator according to any one of the above embodiments. Reference about specific structure of the analog-to-digital converter may be made to the prior art, therefore, details will not be described herein.

In summary, the comparator provided by the present disclosure includes an input unit, a load unit, a control switch, and an adjustment unit, where the input ends of the input unit are connected to a first input signal and a second input signal; the load unit is connected to the input unit, and the gain of the comparator is adjusted by changing the gate voltage of the gain adjustment pair transistors of the load unit; the adjustment unit is connected to the input unit, and the enable state of the control switch adjusts the gate voltage of the gain adjustment pair transistors. The present disclosure also provides the analog-to-digital converter. The gain of the comparator is small in the offset store state, so that the offset voltage of the comparator is amplified by a small multiple, and the gain of the comparator is large in the signal amplify state, so that the input differential mode signal of the comparator is amplified by a large multiple, which eliminates the influence of the offset voltage the comparator on the comparison result of the comparator, improves the speed of the comparator, and reduces the layout area.

The above-mentioned embodiments are just used to exemplarily describe the principle and effects of the present disclosure instead of limiting the present disclosure. Those skilled in the art can make modifications or changes to the above-mentioned embodiments without going against the spirit and the range of the present disclosure. Therefore, all equivalent modifications or changes made by those who have common knowledge in the art without departing from the spirit and technical concept disclosed by the present disclosure shall be still covered by the claims of the present disclosure.