Switched capacitor circuit compensation apparatus and method

[0001] The present invention generally relates to switched capacitor circuits, and particularly relates to compensating such circuits with respect to certain process, temperature, and supply variations.

[0002] Switched capacitor circuits form the fundamental building blocks for a variety of circuits, such as Delta-Sigma Analog-to-Digital Converters (ΔΣADCs) and discrete time analog filters. However, while their usage varies widely, all such circuits generally include a core switched capacitor architecture based on Operational Transconductance Amplifiers (OTA) that are configured as integrating amplifiers. This circuit's sampling input is coupled to a sampling capacitor through a first switch, and the sampling capacitor is coupled to an amplifier input through a second switch. That same input typically is coupled to the integration amplifier, which forms the feedback loop of the amplifier.

[0003] In a sampling phase, the first switch is closed to sample the input voltage, and the second switch is open to isolate the sampling capacitor from the amplifier. Then, in an integration phase, the first switch is opened to isolate the sampling capacitor from the sampling input, and the second switch is closed to connect the sampling capacitor to the amplifier input. That closure causes the amplifier to generate output current in reaction to the step change in its input voltage caused by closure of the second switch. Essentially, this stage involves the transfer of charge from the sampling capacitor to the integration capacitor.

[0004] Settling errors arise if the sampling phase ends before the sampling capacitor has fully charged (or discharged) to the level of the signal being sampled, or if the integration phase ends before completion of charge transfer to the integration capacitor. Such settling errors represent a potentially significant source of non-linear distortion in switched capacitor circuits. In practice, settling errors arise almost inevitably because switched capacitor circuits vary from their nominal or designed-for parameters.

[0005] Such variations arise for a variety of reasons, including, but not limited to, changes in the circuit fabrication process or changes in the environmental conditions in which the circuits are used. With circuit fabrication process variations, the switched capacitors can be larger or smaller than intended and/or the OTA's peak output current may be more or less than specified by the design. Environmental changes may include temperature and power-supply induced changes in circuit behavior, and changed behavior resulting from the use of switched capacitor clock frequencies different from the designed-for frequencies or the use of sampling-to-integration phase duty cycles that are not at or around 50%.

[0006] The present invention comprises a method and apparatus to compensate switched capacitor circuits, such as those used in filtering and analog-to-digital conversion functions. In an exemplary embodiment, the output current of a switched capacitor amplifier automatically is adjusted upward or downward as needed to ensure that circuit's settling time remains within the limits set by the clock frequency of the switched capacitor circuit. Thus, an exemplary method of controlling a settling time of a switched capacitor circuit comprises controlling a reference current to increase and decrease as needed to charge a reference capacitor at a targeted slew rate that is set by a clock frequency of the switched capacitor circuit, and generating a charging current that controls the settling slew rate of the switched capacitor circuit to be proportional to the reference current, such that a magnitude of the charging current depends on a capacitance of the reference capacitor and on the clock frequency. The method may further comprise configuring the reference capacitor such that process-related variations in its capacitance track process-related variations in an effective capacitance of one or more switched capacitors driven by the charging current. Similarly, the reference capacitor may be configured such that its environmentally related changes in capacitance track corresponding changes in the effective capacitance of the switched capacitor circuit.

[0007] An exemplary circuit according to the present invention comprises a switched capacitor circuit comprising one or more switched capacitors and an amplifier configured to generate an output current proportional to a reference current, wherein a settling time of the switched capacitor circuit depends on an effective capacitance of the one or more switched capacitors as seen by the output current; and further comprises a compensation circuit configured to increase and decrease the reference current as needed to charge a reference capacitor at a targeted slew rate determined by a clock frequency of the switched capacitor circuit. An exemplary compensation circuit comprises a delay locked loop circuit configured to increase the reference current if a detected slew rate of the compensation circuit is less than the targeted slew rate, and to decrease the reference current if the detected slew rate is greater than the targeted slew rate. The exemplary circuit may be implemented as an Integrated Circuit (IC) device, and may be advantageously used wireless communication devices, such as in filters and analog-to-digital converters used in radio base stations and mobile terminals, for example.

[0008] FIG. 1 is a diagram of a switched capacitor circuit and an associated compensation circuit according to an exemplary embodiment of the present invention.

[0009] FIGS. 2A and 2B are diagrams of an exemplary delay locked loop implementation of the compensation circuit of FIG. 1.

[0010] FIGS. 3 and 4 are graphs illustrating control of the compensation circuit's reference current as functions of the switched capacitor clock frequency and the capacitance of a reference capacitor.

[0011] FIG. 5 is a diagram of exemplary variable delay cell for use in the delay locked loop of FIGS. 2A and 2B.

[0012] FIG. 6 is a diagram of an exemplary delay locked loop that includes clock duty cycle regeneration.

[0013] FIG. 7 is a diagram of an exemplary mobile terminal according to one or more embodiments of the present invention.

[0014] FIG. 8 is a diagram of an exemplary radio base station according to one or more embodiments of the present invention.

[0015] FIG. 1 illustrates a circuit 10 comprising a switched capacitor circuit 12 and an associated compensation circuit 14. Switched capacitor circuit 12 may be configured as part of an Integrated Circuit (IC) for use in signal filtering or analog-to-digital conversion, for example. As is understood by those skilled in the art, switched capacitor circuit 12 operates in a sampling phase, and in an integration phase. In the sampling phase, switch S1 is closed and switch S2 is opened, thus sampling capacitor C_S samples the input signal, i.e., it is charged to the voltage of the input signal. Switch S1 then opens and switch S2 closes, thus placing the switched capacitor circuit 12 in its integration phase, wherein the output current, I_OUT, from amplifier 16 transfers charge from the sampling capacitor C_S to the integration capacitor C_I. Amplifier 16 thus operates as a transconductance amplifier that generates an output current responsive to a differential voltage applied to its (+) and (−) input terminals.

[0016] In fact, amplifier 16 drives a maximum I_OUT if its input voltage differential is more than a small value, V_CLIP, that typically is on the order of a few tens of milli-volts. Thus, when circuit 12 switches to its integration phase of operation, the inverting (−) input of amplifier 16 is driven to the sampling voltage on C_S, which causes amplifier 16 to drive a maximum I_OUT into the effective capacitance formed by the switched capacitors C_S and C_I (C_EFF=(C_S×C_I)/(C_S+C_I). The amplifier's output current thus functions as a charging current that remains at its maximum value after S2 closes until the differential voltage on the inverting (−) and non-inverting (+) inputs is driven back to V_CLIP, at which point the output current asymptotically falls off as the differential voltage is driven toward zero. The amplifier's output current thus functions like a constant current source that linearly charges the effective capacitance at a slew rate defined by the magnitude of I_OUT and the circuit's effective capacitance, C_EFF.

[0017] In conventional switched capacitor circuits, the magnitude of I_OUT is fixed. That approach is undesirable because the effective capacitance can vary widely between integrated circuit devices, and particularly between fabrication lots of such devices. If the effective capacitance is smaller than intended, then it takes I_OUT a shorter amount of time to charge C_EFF, thus allowing the circuit to meet its settling time requirements, i.e., charge transfer is completed before the circuit switches back to the next sampling phase. However, if C_EFF is larger than intended, which commonly happens where capacitors are fabricated using integrated circuit fabrication processes, then the magnitude of I_OUT may not be great enough to complete charge transfer within the required settling time and non-linear settling errors result that cause signal distortion. Similar errors can arise if the clock frequency of the conventional switched capacitor circuit is changed. For example, if the clock frequency is increased, the sampling and integration windows are shortened, thus leaving less time for the circuit to complete charge transfer in the integration phase. Even without a frequency increase, the allowable settling time may be shortened because of a duty cycle imbalance of the switched capacitor circuit's input clock, i.e., long sampling interval followed by a short integration interval or vice versa.

[0018] In any case, over-designing the amplifier current represents a common approach to accounting for “worst case” operating conditions of too much capacitance, higher than nominal clocking frequencies, and/or improper duty cycles. The downside to such an approach is relatively high current consumption due to the “oversized” amplifier output current. The present invention addresses that and other shortcomings of conventional switched capacitor circuits by providing in-circuit adjustment of I_OUT to account for process, temperature, and supply (PTS) variations. Thus, compensation circuit 14 provides a bias signal to amplifier 16 of switched capacitor circuit 12 that increases or decreases the magnitude of the maximum or peak amplifier output current such that the slew rate of switched capacitor circuit 12 changes as a function of input clock frequency and, for a given clock frequency, is maintained over a wide range of effective capacitance.

[0019] As illustrated in FIGS. 2A and 2B, compensation circuit 14 comprises variable delay cells 20 and a phase detector 22 that includes an output circuit 24 that generates the bias signal as a function of the phase difference between corresponding edges in an input clock signal and a delayed version of the clock signal as provided by variable delay cell 20. In operation, an input clock signal, i.e., the clock signal to be used for the switched capacitor circuit 12, is input to the variable delay cell 20 and to the phase detector 22. Variable delay cell 20 outputs a delayed version of the clock signal that serves as the other input to phase detector 22. The bias signal output from phase detector 22 thus varies as a function of the phase error between corresponding edges (rising or falling) in the input clock signal and the delayed clock signal.

[0020] The bias signal is used to set the magnitude of a reference current I_REF that charges a reference capacitor, C_REF, and the compensation circuit 14 is configured such that the bias signal increases or decreases under closed loop control such that the magnitude of I_REF is driven to the value needed to charge C_REF to a target voltage V_REF within one clock time of the input clock signal. That is, the delay of the variable delay cell 20 is driven to a one period delay of the input clock signal, and the corresponding magnitude of I_REF is the value needed to charge the reference capacitor C_REF to the target voltage V_REF within that delay period. As such, the value of I_REF is driven to that needed to achieve a targeted slew rate that depends on the capacitance of C_REF and on the input clock frequency. Thus, I_REF is generated as, $\begin{array}{cc}{I}_{\mathrm{REF}}=\frac{V_{\mathrm{REF}}{C}_{\mathrm{REF}}}{T},& \left(1\right)\end{array}$
where T is the input clock period or some portion thereof. With this relationship, then, the value of I_OUT is determined as,
I_OUT=f(I_REF)=k ·I_REF.   (2)

[0021] Of course, one may configure circuit 10 such that I_REF and I_OUT take on nominal values corresponding to a nominal operating frequency, nominal capacitance values, and nominal supply voltages and temperatures. Thus, one may set V_REF such that I_REF and I_OUT have desired nominal magnitudes for a given nominal frequency and nominal reference and effective capacitances according to, $\begin{array}{cc}{V}_{\mathrm{REF}}=\frac{I_{\mathrm{REF}}T}{C_{\mathrm{REF}}},& \left(3\right)\end{array}$
or set V_REF to a well controlled value, such as a bandgap voltage.

[0022] FIGS. 3 and 4 graphically depict the relationship of I_REF to the input clock frequency and to the capacitance of C_REF. For example, in FIG. 3, one sees that the magnitude of I_REF decreases with an increasing clock period, i.e., as the frequency of the switched capacitor circuit's input clock decreases, the required slew rate is reduced and the magnitude of I_REF correspondingly decreases. Conversely, in FIG. 4, one sees that the magnitude of I_REF increases with increasing reference capacitance. That is, for a given clock frequency, the magnitude of I_REF increases to maintain the targeted slew rate if the capacitance of C_REF increases and decreases if the capacitance of C_REF decreases.

[0023] Such variations in capacitance are commonplace in the semiconductor fabrication process. As is well understood by those skilled in the art, controlling the absolute value of capacitors fabricated within semiconductor devices is quite difficult. Thus, the absolute values of C_REF, C_I, and C_S all may change significantly from wafer to wafer, or even from device to device. However, the ratio of one capacitor to another within a given device can be well controlled. Thus, the designer can ensure that C_REF maintains a closely fixed relationship to the integration and sampling capacitors, C_I and C_S. In other words, if C_S and C_I are larger than their nominal values, e.g., 120 pF rather than 100 pF, then C_REF will be larger than its nominal value by the same factor (1.2). Likewise, if the capacitance of C_I and C_S is less than nominal, the capacitance of C_REF will be less than its nominal value by the same factor.

[0024] Such tracking between C_REF and C_I/C_S is accomplished by maintaining a known geometric scaling between the capacitors and by implementing C_REF in the same device layers (i.e., the same metal and oxide layers of the semiconductor device). By using the same chip layers, C_REF is subject to the same variations in oxide thickness, etc., as are the switched capacitors C_I and C_S. Such matching also results in good environmental tracking between C_REF and the switched capacitors C_I and C_S. That is, changes in the capacitance of C_REF track changes in the capacitance of C_I and C_S over changes in operating temperature and voltage.

[0025] FIG. 5 illustrates an exemplary configuration for the variable delay cell 20, and further illustrates an exemplary differential pair arrangement for amplifier 16. Delay cell 20 comprises a comparator 30, a D flip-flop (DFF) 32, an exclusive-or (XOR) gate 34, transistors Q1, Q2, and Q3a/Q3b, and a bandgap voltage reference D1. Note that other voltage reference types may be used, and that D1 may be omitted and an input port provided such that V_REF is received from an external source. An exemplary amplifier 16 comprises differential pairs of transistors Q5a/Q5b and Q6a/Q6b.

[0026] Regardless, in operation, the input clock signal is applied to one input of XOR gate 34, and its other input is tied to the “Q” output of DFF 32. The output from XOR gate 34 directly or indirectly drives the gates of transistors Q2 and Q3a. If the output of XOR gate 34 is high (“1”), Q2 is turned on and Q3a is turned off. In that state, C_REF is grounded and no current flows into it. Thus, the non-inverting input (+) of comparator 30 remains below its inverting input (−), which is at V_REF. Thus, when the output of XOR gate 34 switches low (“0”), Q2 turns off, Q3a turns on, and C_REF begins charging at a rate determined by the magnitude of I_REF flowing through Q1, which is set by the value of the bias signal provided by phase detector circuit 22. Note that Q3b may be used as an enabling or gating device for discharging C_REF and may or may not be included.

[0027] When C_REF charges up to about V_REF, the output of comparator 30 changes state, which clocks DFF 32, which in turn changes the output state of DFF 32. The output of DFF 32 serves as the delayed version of the clock signal and thus drives back into XOR gate 34 and into one of the phase inputs of phase detector 22. With this arrangement, if DFF 32 clocks early (less than one clock cycle between input and delayed clock signals), the bias signal increases, which lowers the gate-to-source voltage of Q1 and thus decreases the magnitude of I_REF and thereby reduces the slew rate of the voltage on C_REF. Conversely, if DFF 32 clocks late, the bias signal decreases, which increases the gate-to-source voltage on Q1 and thus increases the magnitude of I_REF and thereby increases the slew rate of the voltage on C_REF.

[0028] With this arrangement, I_REF increases if the value of C_REF increases because it takes a greater magnitude of I_REF to maintain the targeted slew rate with increasing C_REF. Similarly, if the input clock frequency increases, the clock period decreases and it takes a greater magnitude of I_REF to charge C_REF to V_REF within the shortened clock period. Of course, if the frequency decreases and/or C_REF decreases, I_REF is reduced a proportionate amount. Thus, compensation circuit 14 varies I_REF as needed to maintain a targeted slew rate that is set by the input clock frequency, wherein the targeted slew rate is defined as the rate needed to charge C_REF to V_REF within the allotted time, i.e., within one cycle delay time.

[0029] By tying the bias input of amplifier 16 to the same bias signal as controls I_REF, the output current I_OUT of amplifier 16 may be made to vary as a function of I_REF. Thus, by scaling the biasing transistor Q4 of amplifier 16 in known relationship to Q1, which itself may comprise a composite transistor circuit, the value of I_OUT increases or decreases in relation to I_REF by a factor that accounts for the relationship between C_REF and C_EFF. If C_REF equals C_EFF, then a one-to-one scaling of I_OUT to I_REF may be desired, but those skilled in the art will recognize that essentially any desired proportionality can be configured based on setting the relative device sizes.

[0030] This exemplary method establishes the slew rate of amplifier 16 as, $\begin{array}{cc}\mathrm{SlewRate}=\frac{I_{\mathrm{OUT}}}{C_{\mathrm{EFF}}}=\left(\frac{C_{\mathrm{REF}}}{C_{\mathrm{EFF}}}\right)N{V}_{\mathrm{REF}}{f}_s,& \left(4\right)\end{array}$
where N equals the number of unit delays cells in the variable delay cell 20, and where f_s equals the frequency of the input clock signal.

[0031] According to the above exemplary details, compensation circuit 14 adjusts I_REF as needed to maintain a targeted slew rate that is determined by the switched capacitor circuit's clock frequency. For a given clock frequency I_REF increases and decreases as needed to charge C_REF at the targeted slew rate and the slew rate is thus maintained over a range of C_REF capacitance. The output current I_OUT of amplifier 16 is made to change in proportion to I_REF and, thus, if the capacitance of C_REF tracks the effective capacitance driven by I_OUT, then the settling time of switched capacitor 12 will be maintained irrespective of whether C_I and C_S are bigger or smaller than their designed-for sizes and irrespective of whether their capacitance changes over time and temperature.

[0032] Of course, the delay cell 20 of FIG. 5 assumed that the input clock signal had a duty cycle close to fifty-percent. If the actual input clock signal deviates too far from a fifty-percent duty cycle, then the actual allowable settling time of the switched capacitor circuit 12 may be significantly shorter than used by the delay cell 20 to set the targeted slew rate. Therefore, an alternative exemplary embodiment of the present invention includes input clock reconditioning to ensure that the switched capacitor circuit 12 operates with a clock signal that has a desired duty cycle, e.g., a fifty-percent duty cycle.

[0033] FIG. 6 illustrates an alternate embodiment of compensation circuit 14, wherein a clock reconditioning circuit 50 drives the phase detector 22. Circuit 50 comprises the previously illustrated variable delay cell 20, and another delay cell 52, which may be implemented with a short fixed or variable delay. Circuit 50 further comprises DFFs 54 and 56. In operation, circuit 50 generates a reconditioned output clock signal that has the same frequency as the input clock signal but with essentially any desired duty cycle that is independent of the input clock signal's duty cycle.

[0034] The input clock signal, i.e., the clock signal with to-be-reconditioned duty cycle intended for switched capacitor circuit 12, clocks DFF 54. The output of DFF 54 serves as an input to delay cell 52, which provides a delayed output edge for every input edge it receives. The delayed edge serves as a reset input to DFF 54 and, thus, DFF 54 generates relatively narrow output pulses responsive to the incoming train of input clock pulses-the width of its output pulses is determined by the delay of delay cell 52 and various propagation delays. The output of DFF 54 also drives one phase input of phase detector 22, and drives the reset input of DFF 56. The output of DFF 56 in turn drives the clock input of variable delay cell 20 (see FIG. 2A or FIG. 5). The clock output of variable delay cell 20 drives the other phase input of phase detector 22, feeds back as the clock input of DFF 56, and serves as the reconditioned clock signal output. With this approach, the duty cycle of the clock output signal will be 50% if the variable delay cell 20 delays rising and falling edges of its input signal by an equal amount. Any other duty cycle of the clock output signal can be accomplished by delaying rising and falling edges by unequal amounts. The switches S₁ and S₂ in the switched capacitor circuit will be controlled by the clock out (CLOCK OUT) signal.

[0035] Whether or not clock reconditioning is used, the combination of switched capacitor circuit 12 with compensation circuit 14 is useful in a broad range of circuit devices, including the previously mentioned filtering and analog-to-digital conversion applications. FIG. 7 illustrates a mobile terminal 100 for use in a wireless communication network and depicts an exemplary embodiment of the present invention applied to the received signal processing chain of the mobile terminal 100. The illustrated mobile terminal 100 comprises an antenna assembly 102, a receiver circuit 104, a transmitter circuit 106, a baseband processor 108, a system controller 110, and a user interface 112 (e.g., keypad, display, speakers, etc.). As used herein, it should be understood that the term “mobile terminal” is given broad construction and means wireless communication handsets, e.g., cellular radiotelephones, Portable Digital Assistants (PDAs), laptop/palmtop computers, etc. Broadly, the term mobile terminal as used herein refers to any type of so-called “pervasive communication device.”

[0036] In operation, radio signals received via antenna assembly 102 are input to a low noise amplifier 120, and the resultant signal is split into in-phase (I) and quadrature (Q) signal streams via quadrature mixers 122-1 and 122-2, based on a local oscillator frequency provided by frequency synthesizer 126. The down-mixed I and Q signals are then passed through filters 128-1 and 128-2, respectively, for input to sampling ADCs 130-1 and 130-2. In an exemplary embodiment, ADCs 130 each include one or more compensated switched capacitor circuits 10 according to the present invention. The ADCs 130 generate digitized I and Q signal samples that are provided to baseband processor 108 for received signal processing, e.g., demodulation/decoding operations.

[0037] In this embodiment, the ADCs 130 may be fabricated as part of a radio receiver circuit that includes one or more ICs. As such, the compensated switched capacitor circuits 10 may be fabricated as part of a mixed signal ASIC or other IC that is used to implement receiver circuit 104. In that manner, the compensated switched capacitor circuits 10 reduce errors in the analog-to-digital conversion process by adjusting their charging currents as needed to ensure that the appropriate settling times are maintained over a range of operation conditions, irrespective of whether the effective circuit capacitances are more or less than their nominal values.

[0038] By adjusting their charging currents up or down to hit the targeted settling time as a function of effective capacitance and input clock frequency, the compensated switched capacitor circuits 10 obviate the need to maintain unnecessarily high charging currents that would otherwise be needed to ensure proper settling time performance under worst case conditions. As such, the use of the compensated switched capacitor circuits 10 in mobile terminal 100 reduce its overall power consumption and thereby improve battery life in addition to reducing received signal processing errors.

[0039] Of course, the present invention may be embodied elsewhere in mobile terminal 100, such as anywhere that switched capacitor filters or other switched capacitor circuits are used. Indeed, the present invention may be embodied in network radio base stations in their filtering and/or analog-to-digital conversion circuits. In FIG. 8, a radio base station 150 comprises transmit/receive antennas 152, receiver circuits 154, transmitter circuits 156, reverse link signal processing circuits 158, forward link signal processing circuits 160, and a base station controller interface circuit 162. Receiver circuits 154 includes pluralities of received signal processing changes that may be used to receive the individual signals from a plurality of mobile stations, wherein each received signal processing chain includes amplifiers 164, mixer circuits 166, filter circuits 168, and one or more ADCs 170 that include one or more compensated switched capacitor circuits 10 according to one or more embodiments of the present invention.

[0040] More broadly, the present invention can be embodied in essentially any type of device that uses some form of switched capacitor circuits. As such, the present invention is not limited by the foregoing discussion but rather is limited by the following claims and the reasonable equivalents thereof.