Patent Publication Number: US-8988146-B1

Title: Voltage amplifier for capacitive sensing devices using very high impedance

Description:
BACKGROUND 
     This invention relates to a switch used in electronic systems in which the voltage across a capacitor is amplified by a trans-capacitance operational amplifier. 
     Electronic systems which process the voltage provided by a sensing device approximately modeled by a capacitor (such as a MEMS gyroscope or a MEMS microphone) use a voltage amplifier implemented as a continuous&#39; time trans-capacitance operational amplifier. The operational amplifier feedback path comprises a capacitor in series with a switch. It is desirable that the switch is connected to the input of the operational amplifier (and hence to the sensing device), while the capacitor is connected to the output of the operational amplifier. The sensing device provides a very low voltage, so it is critical that the current leakage due to the imperfections of the switch in the “off” state is minimized. Therefore, the switch used in the sampling circuitry requires a very high impedance (&gt;1 TOhm) in the “off” state, and a low impedance (&lt;1 kOhm) in the “on” state. 
     Conventional techniques use one or two MOS transistors to implement the switch. These transistors must have a ratio W/L high enough to allow a low impedance in the “on” state. However, the larger this ratio is, the larger the leakage of the switch becomes in the “off” state. Such conventional techniques can only achieve an equivalent “off” impedance on the order of a few GOhms. To avoid the unacceptable leakage from the sensing device, conventional switches are connected at the output of the operational amplifier. This solution has significant disadvantages, such as the increase of the noise of the amplifier due to the switching of the capacitor in parallel to the sensing device. 
     SUMMARY 
     Aspects of the invention include a switch comprising one input, one output, a digital control signal, three series MOS transistors (all three being PMOS or all three being NMOS), and additional circuitry which properly biases the drains and sources of the said three MOS transistors not connected to either the said input or to the said output; wherein the digital control signal is in the “on” state then the said drains and sources are left floating; and when the digital control signal is in the “off” state then the said drains and sources are connected to electrical nodes which voltages minimize the leakage current flowing through the said input. 
     Aspects of the invention include a method to amplify a voltage across a capacitor comprising a switch connected in series with a capacitor and an operational amplifier, such that the switch is connected to the input of the operational amplifier and the capacitor to the output of the operational amplifier; wherein the said switch comprises one input, one output, a digital control signal, three series MOS transistors (all three being PMOS or all three being NMOS), and additional circuitry which properly biases the drains and sources of the said three MOS transistors not connected to either the said input or to the said output; wherein the digital control signal is in the “on” state then the said drains and sources are left floating; and when the digital control signal is in the “off” state then the said drains and sources are connected to electrical nodes which voltages minimize the leakage current flowing through the said input. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and advantages of the present invention will become better understood upon reading the following detailed description and upon reference to the drawings where: 
         FIG. 1  shows the schematic of a voltage amplifier which amplifies the voltage across a sensing device modeled as a capacitor, and which comprises a switch, according to some embodiments of the present invention. 
         FIG. 2  shows the schematic of a switch using PMOS transistors exhibiting a very high impedance in the “off” state, according to some embodiments of the present invention. 
         FIG. 3  shows the schematic of a switch using NMOS transistors exhibiting a very high impedance in the “off” state, according to some embodiments of the present invention. 
         FIG. 4  shows the schematic of a voltage amplifier which amplifies the voltage across a sensing device modeled as a capacitor, and which uses both a switch using PMOS transistors and a switch using NMOS transistors, according to some embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The following description illustrates the present invention by way of example and not necessarily by way of limitation. Any reference to an element is understood to refer to at least one element. A set of elements is understood to include one or more elements. Any recited connection is understood to encompass a direct operative connection or an in direct operative connection through intermediary structure(s). 
       FIG. 1  shows the schematic of a voltage amplifier which amplifies the voltage across a sensing device  110  modeled as a capacitor, according to some embodiments of the present invention. The control signal  160  sets the amplifier either in the sampling mode (in which mode the switch is “off”), or in the hold mode (in which mode the switch is “on”). The amplifier comprises an operational amplifier  130 , a switch  100 , and a feedback capacitor  120 . The DC bias block  180  sets the DC bias of the said operational amplifier inputs and ensures a stable functioning of the voltage amplifier. At the terminal Ref of the switch  100  we apply a signal which voltage substantially replicates the voltage of the input  140 , such as the signal of the input  140  itself. In the sampling mode the impedance of the switch needs to be low (&lt;1 kOhm). In the hold mode the amplifier must provide a very low input leakage current, otherwise the voltage across the sensing device is altered. Sensing devices  110  that can be approximately modeled as a capacitor include, but are not limited to, MEMS gyroscopes or MEMS microphones. In another embodiment, the signal applied at the terminal Ref of the switch is the non-inverting input  131  of the operational amplifier  130 . 
       FIG. 2  shows the schematic of a switch having very high impedance in the “off” state, according to some embodiments of the present invention. The switch has an input  271 , an output  272 , a digital control  273 , and another “Ref” input  274  which is connected externally to a signal which voltage is substantially equal to the voltage of the input  271 . The switch uses three series PMOS transistors  200 ,  210 , and  220 . The drain of  200  is connected to the input  271 . The source and the bulk of  200  are connected together to the drain of  210 , thus creating the node  270 . The source and bulk of  210  are connected together to the source and the bulk of  220 , thus creating the node  280 . The drain of  220  is connected to the output  272 . The control signal sets through the inverter  262  the voltages of the gates of  200 ,  210 , and  220 , to either VDD or VSS, thus turning them on or off. In addition, the unity-gain buffer  261  substantially replicates at its output the voltage of the input “Ref” node  274 . In addition, in the “on” mode, the control voltage leaves floating, through the devices  230  and  240 , the electrical nodes  270  and  280 . In addition, in the “off” mode, the control voltage forces, through the device  240 , the node  270  to the voltage of the input  274 , which is substantially equal to the voltage of the input  271 , hence increasing the threshold voltage of the transistor  200  due to its body effect and considerably reducing the leakage current drawn from the input  271 . In addition, in the “off” mode, the control voltage forces, through the device  230 , the node  280  to VDD. 
       FIG. 3  shows the schematic of a switch having very high impedance in the “off” state, according to some embodiments of the present invention. The switch has an input  371 , an output  372 , a digital control  373 , and another “Ref” input  374  which is connected externally to a signal which voltage is substantially equal to the voltage of the input  371 . The switch uses three series NMOS transistors  300 ,  310 , and  320 . The drain of  300  is connected to the input  371 . The source and the bulk of  300  are connected together to the drain of  310 , thus creating the node  370 . The source and bulk of  310  are connected together to the source and the bulk of  320 , thus creating the node  380 . The drain of  320  is connected to the output  372 . The control signal sets the voltages of the gates of  300 ,  310 , and  320 , to either VDD or VSS, thus turning them on or off. In addition, the unity-gain buffer  361  substantially replicates at its output the voltage of the input “Ref” node  374 . In addition, in the “on” mode, the control voltage leaves floating, through the devices  330  and  340 , the electrical nodes  370  and  380 . In addition, in the “off” mode, the control voltage forces, through the devices  340  and  360 , the node  370  to the voltage of the input  374 , which is substantially equal to the voltage of the input  371 , hence increasing the threshold voltage of the transistor  300  due to its body effect and considerably reducing the leakage current drawn from the input  371 . In addition, in the “off” mode, the control voltage forces, through the device  330 , the node  380  to VSS. In the embodiments using NMOS transistors, such as shown in  FIG. 3 , the said NMOS transistors must be implemented in an integrated circuits process which features NMOS transistors with the bulk not connected to the substrate, such as, but not limited to, a “triple-well” process. 
     The choice between the embodiment using PMOS transistors (as in  FIG. 2 ) and the embodiment using NMOS transistors (as in  FIG. 3 ) will be made based, among other factors, upon the range of voltages which the sensing devices can generate in relationship to VDD and VSS. This will have a strong impact upon the impedance of the switch in the “on” state. In another embodiment (shown in  FIG. 4 ), when neither an embodiment using PMOS transistors, nor an embodiment using NMOS transistors, provide a low enough impedance in the “on” state for the entire range of voltages across the sensing device, the two switches  400  and  401  can be used in parallel. 
     It will be clear to one skilled in the art that the above embodiments may be altered in many ways without departing from the scope of the invention. Accordingly, the scope of the invention should be determined by the following claims and their legal equivalents.