Abstract:
Circuits, methods, and apparatus that provide sampling networks that avoid undesired transient voltages. One example provides a sampling network that includes a switch such that charge is transferred to an integrator in two separate steps instead of one. This switch connects the first side of a capacitor to an intermediate voltage after it is connected to an input voltage and before it is connected to a reference voltage, where the reference voltage is the output of a one-bit digital-to-analog converter. This intermediate switching allows charge to be transferred from a sampling capacitor to an integrating capacitor in two steps, thus avoiding undesirable transient voltages.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional patent application No. 60/944,789, titled INPUT SAMPLING NETWORK THAT AVOIDS UNDESIRED TRANSIENT VOLTAGES, by Rangan et al., filed Jun. 18, 2007, and is related to U.S. patent application number 12/141,099, titled HIGHLY LINEAR BOOTSTRAPPED SWITCH WITH IMPROVED RELIABILITY, by Sharma, filed Jun. 18, 2008, which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     Sampling networks are useful building blocks that find applications in many types of integrated circuits. Common uses for sampling networks include switched capacitor filters, sample and hold circuits, track and hold circuits, and others. When combined with an integrator, they can form an input stage for a sigma-delta modulator. 
     Sampling networks are efficiently implemented using MOSFET transistors. As a result, sampling networks have become much more popular with the increasing use of MOSFETs in analog and mixed signal applications. 
     In sigma-delta modulators, two signals are added together by sampling the signals and integrating the result. In one specific example, an analog signal being converted to a digital word is sampled and a reference voltage or signal that is the output of a one-bit digital-to-analog converter is subtracted from it. 
     The sampling of these signals can create voltages that exceed the supply voltage ranges of an integrated circuit on which these actives are performed. This in turn can lead to transient voltages that cause parasitic diodes to conduct and clamp voltages. This causes errors in the integrated output voltage. 
     Accordingly, what is needed are circuits, methods, and apparatus that provide sampling networks that avoid these undesirable transient voltages. 
     SUMMARY 
     Accordingly, embodiments of the present invention provide circuits, methods, and apparatus that provide sampling networks that avoid undesired transient voltages. An exemplary embodiment of the present invention provides a sampling network that includes a switch such that charge is transferred to an integrator in two separate steps instead of one. This two step approach reduces the magnitude of resulting transient voltages. 
     Another exemplary embodiment of the present invention provides a sampling network having an additional switch. The additional switch connects the first side of a capacitor to an intermediate voltage after it is connected to an input voltage and before it is connected to a reference voltage, where the reference voltage is the output of a one-bit digital-to-analog converter. This intermediate switching allows charge to be transferred from a sampling capacitor to an integrating capacitor in two steps, thus avoiding undesirable transient voltages. 
     Various embodiments of the present invention may incorporate one or more of these or the other features described herein. A better understanding of the nature and advantages of the present invention may be gained with reference to the following detailed description and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of a sampling network according to an embodiment of the present invention; 
         FIG. 2  is a timing diagram illustrating the operation of the circuitry in  FIG. 1 ; 
         FIG. 3  is another timing diagram illustrating the operation of the circuitry in  FIG. 1 ; 
         FIG. 4  is a flow chart illustrating the operation of a sampling network according to an embodiment of the present invention; and 
         FIG. 5  illustrates a sigma delta modulator (SDM) that may be improved by the incorporation of an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1  is a schematic of a sampling network according to an embodiment of the present invention. The sampling network includes a sampling capacitor C 1  and a number of switches, SW 1 , SW 2 , SW 3 , SW 4 , and SW 5 . These switches may be formed using a MOSFET or other type of transistor. Alternately, the switches may be implemented using circuits comprising multiple transistors, as well as other components, such as capacitors. For example, in a specific embodiment of the present invention, one or more of the switches are charge-boost or bootstrapped switches such as those shown in U.S. patent application Ser. No. 12/141,099, titled HIGHLY LINEAR BOOTSTRAPPED SWITCH WITH IMPROVED RELIABILITY, by Sharma, filed Jun. 18, 2008, which is hereby incorporated by reference. The sampling network is shown with an integrator that includes amplifier A 1  and capacitor C 2 . 
     During operation of conventional sampling networks, charge is transferred from one capacitor to another, where it is accumulated or integrated. When charge is transferred between capacitors, large transient voltages may appear under certain signal and state conditions. These excessive voltages may be clamped or limited by parasitic diodes or other circuit elements, thereby creating errors in the integrator output voltage. Accordingly, an embodiment of the present invention transfers charge from sampling capacitor C 1  to integrating capacitor C 2  in two steps such that these excessive voltages are avoided. 
     In this embodiment of the present invention, a switch SW 5  is included to connect node N 1  to a common mode voltage VCM for an intermediate period of time before N 1  is connected to the reference voltage VREF. In one specific embodiment of the present invention, the common mode voltage VCM is one-half the power supply voltage. For example, in an embodiment of where the power supplies are a positive power supply VCC at 5 V and ground, the common mode voltage is one-half VCC or 2.5 V. In this way, when switch SW 2  closes, the charge on capacitor C 1  is transferred to capacitor C 2  in two steps, thereby reducing the transient voltages at node N 2 . 
     In the specific example of  FIG. 1 , there are four clock or control signals, PH 1 , PH 2 , PHR, and PHF. When PH 1  is high, PH 2  is low. Conversely, when PH 2  is high, PH 1  is low. These signals may be complements of each other, they may be non-overlapping clocks, or they may have other relationships to each other. When PH 1  is high, both PHR and PHF are low, as is PH 2 . When PH 1  returns low, PH 2  is asserted high, as is PHR. After some period of time, PHR returns low, and signal PHF is asserted high. When PH 2  returns low, PHF returns low as well. In this way, during the time PH 2  is high closing switch SW 2 , either switch SW 5  or SW 3  is closed. 
     Specifically, when PH 1  is high, switches SW 1  and SW 4  are closed. Thus, as shown in the first line of the table of this figure, node N 1  is connected to VIN, while node N 2  is connected to VCM. At this time, the voltage across the sampling capacitor C 1  is equal to VCM minus VIN. It should be noted that in this table, (I) indicates an instantaneous voltage reached when a switch is closed, while (S) indicates a steady state reached some time after the switch is closed. 
     When PH 2  is asserted high, switch SW 2  is closed, and switches SW 1  and SW 4  are open. At this time, PHR is also high so switch SW 5  is closed. Node N 1  is thus equal to VCM, and the instantaneous voltage at node N 2  is equal to the common mode voltage VCM added to the voltage across the capacitor, which is equal to VCM minus VIN. Accordingly, the instantaneous voltage at node N 2  is equal to twice the common mode voltage VCM minus the input voltage. Again, in a specific embodiment of the present invention, VCM is equal to 2.5 V, while VIN ranges from 1 V to 4 V. Accordingly, the instantaneous voltage at node N 2  can be as high as 4 V and as low as 1 V. Since these voltages are within the supply voltage range of 5V, excessive voltages that can cause clamping at node N 2  do not result. 
     If a steady state is reached, the amplifier A 1  drives its inverting input voltage to the common mode voltage VCM. At this time, each node of the capacitor has a voltage equal to VCM. In other embodiments of the present invention, PHR is on long enough to transfer some of the charge from the sampling capacitor C 1  to the integrating capacitor C 2 , though the steady-state condition is not reached. 
     Later, switch SW 5  opens and switch SW 3  closes when PHF goes high. At this time, node N 1  is connected to VREF. Since the voltage across the sampling capacitor C 1  is near zero and cannot change instantaneously, the instantaneous voltage at node N 2  is equal to VREF. Again, the VREF signal is provided by the output of a one-bit digital-to-analog converter having a high voltage near VCC, the supply voltage, and a low voltage near ground. Thus, the instantaneous voltage at node N 2  after switch SW 3  closes does not exceed the supply range and unwanted transient voltages are avoided. Once steady state is reached, node N 2  returns to a voltage of VCM due to the operation of the integrator. The voltage across the capacitor is thus VCM minus VREF. 
       FIG. 2  is a timing diagram illustrating the operation of the circuitry in  FIG. 1 . In this example, the reference voltage, which may be the output from a one-bit digital-to-analog converter or comparator, is low, while the input voltage is high. When the clock signal PH 1  is high, node N 1  is connected to the input voltage VIN, while node N 2  is connected to the common mode voltage VCM. Accordingly, the voltage across the capacitor is equal to VCM minus VIN. 
     When PH 1  returns low, PH 2  and PHR are asserted high. Accordingly, node N 1  is connected to VCM, while node N 2  is connected to an output terminal, which may in turn be connected to an integrator as shown in  FIG. 1 , or other circuitry. Again, since the voltage across the capacitor cannot change instantaneously, node N 2  changes the same amount that node N 1  is changed, specifically an amount equal to VCM minus VIN. Accordingly, the resulting voltage at node N 2  is equal to twice VCM minus VIN. Given enough time, node N 2  is driven to the voltage VCM by the integrator. At this time, the net voltage across the capacitor is zero. 
     When in the clock signal PHR returns low, clock signal PHF is asserted high. At this time, node N 1  is connected to VREF, which in this example is zero. Node N 2  remains connected to the output terminal. Again, the voltage across the capacitor cannot change instantaneously and therefore node N 2  changes by the same amount as N 1 , specifically VREF-VCM. Once again, under steady-state conditions, node N 2  reaches VCM and the difference across the capacitor is equal to VCM minus VREF. 
     As can be seen in this example, the voltage at node N 2  does not exceed twice VCM minus VIN. Again, if VCM is 2.5V, when VIN is IV, node N 2  is equal to 4.0V and when VIN is 4.0, N 2  is equal to 1.0V. This range is well within the supply range, which is 5V in this example. This prevents excessive transient voltages on node N 2 , which may cause voltage clamping and a resulting loss of signal. 
       FIG. 3  is another timing diagram illustrating the operation of the circuitry in  FIG. 1 . In this example, VIN remains high, while the reference voltage VREF, which can be the output of a one-bit digital-to-analog converter or comparator, is high. Again, when PH 1  is high, node N 1  is equal to VIN while node N 2  is equal to VCM. 
     When PH 1  returns low, PHR and PH 2  are asserted high. At this time, node N 1  is connected to VCM, while node N 2  is connected to an output terminal. Again, the voltage across the capacitor cannot change instantaneously, and thus node N 2  is driven to twice VCM minus VIN. After enough time, the integrator drives node N 2  to VCM and the net voltage across the capacitor is zero. 
     When PHR returns low, PHF is asserted and node N 1  is coupled to VREF. Again, the voltage across the capacitor does not change instantaneously, therefore, since the voltage across the capacitor is zero, node N 2  is driven to VREF. Again, this voltage settles to VCM, at which time the voltage across the capacitor is equal to VCM minus VREF. Again, in this case, the lowest voltage that node N 2  reaches is twice VCM minus VIN. Where VCM is 2.5V and VIN is 4.0, this voltage is equal to 1.0V. The highest the voltage reaches is VREF, which in this case is equal to the positive supply voltage. In this way, excessive transient voltages on node N 2 , which may cause clamping and loss of signal, are avoided. 
       FIG. 4  is a flow chart illustrating the operation of an embodiment of the present invention. In acts  410 A and  410 B, the first terminal of the capacitor is connected to an input voltage, while a second terminal of the capacitor is connected to a common mode voltage. In acts  420 A and  420 B, the first terminal of the capacitor is connected to the common mode voltage, while a second terminal the capacitor is connected to an output, which in this example is connected to an input of an integrating amplifier. In act  430 A and  430 B, the first terminal of the capacitor is connected to a reference voltage, while the second terminal of the capacitor maintains the connection to the output terminal. 
     Sampling networks provided by embodiments of the present invention may be used in many applications, such as switched capacitor filters, sample and hold circuits, and sigma delta modulators. One example is shown in the following figure. 
       FIG. 5  illustrates a sigma delta modulator (SDM) that may be improved by the incorporation of an embodiment of the present invention. This particular example is a 4th order cascaded integrator feed forward (CIFF) sigma delta modulator. In this example, the summing node and input integrator may be implemented using the circuitry of  FIG. 1 . In this figure, integrators (ints) feed gain stages a 1 -a 5 , whose outputs are summed and fed in turn to a quantizer Q. 
     The above description of exemplary embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form described, and many modifications and variations are possible in light of the teaching above. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated.