Abstract:
An input control circuit that can be used to drive analog switches of analog modules such as an analog-to-digital converter (ADC) enables a sampling switch to receive a higher input voltage than the voltage rating of the devices comprising the sampling switch without risk of damage and without the need for a resistor divider network. The input control circuit and switch both receive an input voltage to be processed and the input control circuit generates a control signal for the switch that is derived from a pre-charged capacitor. The control circuit permits the design and manufacture of high voltage analog modules using low voltage devices, which can save on mask costs without any performance trade-offs.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to integrated circuits and, more particularly, to an integrated circuit for enabling lower voltage-rated devices to handle higher input voltages. 
     Analog functional modules (for example, an analog-to-digital converter (ADC) of a system on chip (SOC) device sometimes need to handle input voltages that exceed the voltage rating of the devices constituting the analog functional module. One known way of handling high analog input voltage ranges employs a resistor divider network connected at the front end of the analog module. However, the resistor divider network constitutes a continuous load on the input. Disconnecting the resistor divider network when the module is idle reduces the load but still leaves the component devices at risk of damage if a high voltage should appear on the input. A further disadvantage associated with use of a resistor divider network, particularly in the case of an ADC, is that the input voltage swing is reduced (typically by 50%) along with a potential signal to noise reduction of 6 db. Also, disadvantageously, a trade-off between speed and input loading (and gain error) has to be made by the designer. 
     Thus, it would be advantageous to provide a circuit for an analog device that enabled the analog device to handle higher voltages than the voltage ratings of its constituent devices without the need for a resistor divider network. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention, together with objects and advantages thereof, may best be understood by reference to the following description of preferred embodiments together with the accompanying drawings in which: 
         FIG. 1  is a schematic circuit diagram of a front-end circuit suitable for an analog module or device and which includes an input circuit in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The detailed description set forth below in connection with the appended drawings is intended as a description of presently preferred embodiments of the invention, and is not intended to represent the only forms in which the present invention may be practised. It is to be understood that the same or equivalent functions may be accomplished by different embodiments that are intended to be encompassed within the spirit and scope of the invention. In the drawings, like numerals are used to indicate like elements throughout. Furthermore, terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that module, circuit, device components, structures and method steps that comprises a list of elements or steps does not include only those elements but may include other elements or steps not expressly listed or inherent to such module, circuit, device components or steps. An element or step proceeded by “comprises . . . a” does not, without more constraints, preclude the existence of additional identical elements or steps that comprises the element or step. 
     In one embodiment, the present invention provides a control circuit for an analog device that includes a sampling switch arrangement having a control input and arranged to receive an analog voltage to be sampled. The control circuit comprises an input terminal for receiving the analog voltage to be sampled, an output terminal for connection to the control input of the sampling switch arrangement, a first switch arrangement, a charging capacitor operably coupled between the input terminal and the first switch arrangement and to the output terminal, and a charging circuit operably coupled to the capacitor for charging the capacitor to a preset voltage. The first switch arrangement connects the output terminal to ground in an ‘off’ mode of operation and transfers a voltage from a terminal of the charging capacitor to the output terminal in an ‘on’ mode of operation. 
     In another embodiment, the invention provides an analog device comprising a control circuit and a sampling switch. The control circuit comprises an input terminal for receiving an analog voltage to be sampled, an output terminal, a first switch arrangement, a charging capacitor operably coupled between the input terminal and the first switch arrangement, and a charging circuit operably coupled to the capacitor for charging the capacitor to a preset voltage. The first switch arrangement connects the output terminal to ground in an ‘off’ mode of operation and transfers a voltage from a terminal of the charging capacitor to the output terminal in an ‘on’ mode of operation. The sampling switch comprises a first NMOSFET (metal oxide semiconductor field effect transistor) connected in series between second and third NMOSFETs where a drain terminal of the second NMOSFET receives the analog voltage to be sampled, a drain terminal of the third NMOSFET comprises an output of the sampling circuit, gate terminals of the second and third NMOSFETs are connected to the capacitor, and a gate terminal of the first NMOSFET is connected to the output terminal of the control circuit. 
     In one example, the input circuit can be used with an analog module (such as an ADC) to process 3.3 volt input signals using devices rated at 1.8 volts with reliability and precision. 
     Referring now to  FIG. 1 , an electrical circuit  100  suitable for use as a front-end circuit for an ADC is shown. A control circuit  101  has an input terminal  102  and a control output  103 . A sampling switching circuit  104  receives, on line  105 , an input analog voltage to be sampled. In an ‘on’ mode of operation the sampling switching circuit  104  transfers the received input voltage to an output terminal  106  that is connected to a sampling capacitor  107 . In an ‘off’ mode of operation no input voltage is transferred as the sampling switching circuit  104  is turned off in this mode. The output  106  of the sampling switching circuit  104  is connected to ADC circuitry (not shown) and can go higher than device ratings in situations such as ADC sampling of other inputs. The input terminal  102  of the control circuit  101  also receives the input voltage on line  105 . In one embodiment, the circuit  100  is implemented in a SOC device, which typically includes a plurality of other functional modules. 
     The control circuit  101  includes a charging capacitor  108 . The control circuit  101  also includes a first switch arrangement  109  that comprises first, second, and third N channel metal oxide semiconductor field effect transistors (NMOSFETs)  110 ,  111 ,  112  respectively and a P-channel MOSFET (PMOSFET)  113 . The first switch arrangement  109  connects the control output  103  to ground in an ‘off’ mode of operation and transfers a voltage from a terminal of the charging capacitor  108  to the control output  103  in an ‘on’ mode of operation. 
     Referring now to the sampling switching circuit  104 , in this embodiment, the sampling switching circuit  104  comprises three NMOSFETs connected in series. A fourth NMOSFET  114  and a fifth NMOSFET  115  have gate terminals connected to a first terminal of the charging capacitor  108  of the control circuit  101 . A sixth NMOSFET  116  connected in series between the fourth and fifth NMOSFETs  114 ,  115 , has a gate terminal connected to the control output  103  of the control circuit  101 . In the ‘off’ mode the control output  103  and therefore the gate terminal of the sixth NMOSFET is grounded and in the ‘on’ mode this gate terminal is connected to a first terminal of the charging capacitor  108 . 
     The control circuit  101  also includes seventh, eighth, ninth, and tenth NMOSFETs  117 ,  118 ,  119  and  120 , respectively, which facilitate charging of the charging capacitor  108  in a manner to be described below. The control circuit  101  also includes a second switch arrangement  121  comprising a tri-state inverter  122  and an eleventh NMOSFET  123 . The second switch arrangement  121  ensures over-voltage protection of the PMOSFET  113 . The control circuit  101  also includes twelfth and thirteenth NMOSFETs  124 ,  125  that are connected in series between the input terminal  102  and a second terminal of the charging capacitor  108 . 
     The electrical circuit  100  will now be described in greater detail. The sampling switching circuit  104  accurately passes a voltage appearing on the input line  105  to the output  106  and as mentioned above comprises three NMOSFETs  114 ,  115 ,  116  connected in series. The fourth NMOSFET  114  has a drain terminal connected to a first terminal of the sampling capacitor  107  whose second terminal is connected to ground, and a source terminal connected to a drain terminal of the sixth NMOSFET  116 . A source terminal of the sixth NMOSFET  116  is connected to a source terminal of the fifth NMOSFET  115 . A drain terminal of the fifth NMOSFET is connected to the input line  105 . A gate-to-source voltage for each of the NMOSFETs  114 ,  115 ,  116  that comprise the sampling switching circuit  104  is provided by the charging capacitor  108 , once charged. 
     A drain terminal of the twelfth NMOSFET  124  is connected to the input line  105 . A gate terminal of the twelfth NMOSFET  124  is connected to the first terminal of the charging capacitor  108 . A source terminal of the twelfth NMOSFET  124  is connected to a drain terminal of the thirteenth NMOSFET  125  whose gate terminal is connected to the control output  103 . A source terminal of the thirteenth NMOSFET  125  is connected to the second terminal of the charging capacitor. The twelfth and thirteenth NMOSFETs  124 ,  125  are used to connect the input voltage to the second terminal of the charging capacitor  108  in order to create a sufficient gate-to-source voltage at each of the NMOSFETs  114 ,  115 ,  116  comprising the sampling switching circuit  104  in the ‘on’ mode. 
     Circuitry for controlling the charging of the charging capacitor  108  will now be described. A drain terminal of the seventh NMOSFET  117  is connected to the second terminal of the charging capacitor  108 . A gate terminal of the seventh NMOSFET  117  is connected to a supply voltage supply VDD and a source terminal of the seventh NMOSFET  117  is connected to a drain terminal of the eighth NMOSFET  118 . A source terminal of the eighth NMOSFET  118  is connected to ground and its gate terminal receives a first mode select signal that is generated elsewhere on the SOC. The first mode select signal is either zero volts or VDD. When the eighth NMOSFET  118  is turned on, the second terminal of the charging capacitor  108  is grounded. A drain terminal of the ninth N MOSFET  119  is connected to the first terminal of the charging capacitor  108 . A gate terminal of the ninth NMOSFET  119  is connected to a voltage supply, which in this example is twice VDD. A source terminal of the ninth NMOSFET  119  is connected to a drain terminal of the tenth NMOSFET  120  whose source terminal is connected to the voltage supply VDD. A gate terminal of the tenth NMOSFET  120  is connected to a second mode select signal, which is generated elsewhere on the SOC. The second mode select signal is either VDD or twice VDD. When the tenth NMOSFET  120  is turned on, the first terminal of the charging capacitor  108  is connected to the voltage supply VDD. Thus, the seventh, eighth, ninth and tenth NMOSFETs  117 ,  118 ,  119 ,  120  are used to pre-charge the charging capacitor  108  to a voltage level of VDD. 
     In the first switch arrangement  109 , a source terminal of the first NMOSFET  110  is connected to ground and a drain terminal of the first NMOSFET  110  is connected to a source terminal of the second NMOSFET  111 . A gate terminal of the first NMOSFET  110  is connected to a third mode select signal, which is generated elsewhere on the SOC. The third mode select signal is either zero volts or VDD. A gate terminal of the second NMOSFET  111  is connected to VDD and a drain terminal of the second NMOSFET  111  is connected to a source terminal of the third NMOSFET  112 . A gate terminal of the third NMOSFET  112  is connected to a fourth mode select signal, which is generated elsewhere on the SOC. The fourth mode select signal is either VDD or twice VDD. A drain terminal of the third NMOSFET  112  is connected to the control output  103  and to a drain terminal of the PMOSFET  113 . A source terminal of the PMOSFET  113  is connected to the first terminal of the charging capacitor  108 . A gate terminal of the PMOSFET  113  is connected to the second switch arrangement  121 . When the first and third NMOSFETs  110 ,  112  are turned on, the control output  103  is grounded and consequently the sampling switching circuit  104  is off. When the PMOSFET  113  is on, it permits the voltage stored on the first terminal of the charging capacitor  108  to be transferred to the control output  103  (and therefore turns on the sampling switching circuit  104 ). 
     The 11th NMOSFET  123  of the second switch arrangement  121  has a drain terminal connected to the gate terminal of the PMOSFET  113  and to a first output of the inverter  122 . A gate terminal of the eleventh NMOSFET  123  is connected to the control output  103 . A second output of the inverter  122  is connected to the second terminal of the charging capacitor  108  and an input of the inverter  122  is connected to a fifth mode select signal, which is generated elsewhere on the SOC. The fifth mode select signal is either zero volts or VDD. The eleventh NMOSFET  123  is used to protect the PMOSFET  113  against an overvoltage appearing on the first terminal of the charging capacitor  108 . 
     In operation, the control circuit  101  permits the sampling switching circuit  104  to transfer an input voltage on line  105  to its output  106  and sampling capacitor  107 , which can be up to a level of twice the rating of the NMOSFETs  114 ,  115 ,  116  comprising the sampling switching circuit without risk of damage to these NMOSFETs. Further, the MOSFETs comprising the control circuit  101  are also protected from any over-voltage condition. In this example, the input voltage to be sampled can have any level between 0 volts and twice VDD while voltages across terminals of the MOSFETs are limited to VDD. 
     In order to protect the sixth NMOSFET  116  (which is the main switching element used to transfer the input voltage to the sampling capacitor  107 ), it has to be ensured that voltage differences across its terminals never rise above the rated level while at the same time there should be a sufficient gate-to-source voltage present in order to reduce ON resistance to a level that permits transfer of the input voltage with the required precision. In a particular application of analog multiplexing, a sampling switching circuit typically comprises several transistors connected in parallel and in such an application, both input and output voltages can vary from 0 to twice VDD. To ensure that the sixth NMOSFET  116  is held off in the ‘off’ mode, its gate voltage has to be reliably held at zero volts. However, this can pose a reliability risk for the sixth NMOSFET  116  because its source and/or drain terminals could rise to a level as high as twice VDD. However, the presence of the fourth and fifth NMOSFETs connected in series either side of the sixth NMOSFET  116  prevent this situation from occurring. 
     The control circuit  101  ensures that when operating in the ‘on’ mode, all three NMOSFETs  114 ,  115 ,  116  comprising the sampling switching circuit  104  have a gate voltage that is higher than the input voltage (this is ensured of the action of the charging capacitor  108 ). Further, when operating in the ‘off’ mode, the gate terminal of the sixth NMOSFET  116  is grounded and the gate terminals of the fourth and fifth NMOSFETs  114 ,  115  are held at VDD (this is ensured by the action of the components of the first switch arrangement  109 ). Thus, the components of the sampling switching circuit  104  devices are protected while still reliably keeping the sampling switching circuit  104  off. 
     To generate gate-to-source voltages for the NMOSFETs of the sampling switching circuit  104  in the ‘on’ mode, the charging capacitor  108  is used. During the ‘off’ mode, charging is enabled by setting the first and second mode select signals so that the second terminal of the charging capacitor  108  is grounded via the seventh and eighth NMOSFETs  117 ,  118  and the first terminal of the charging capacitor  108  is connected to the voltage supply VDD via the ninth and tenth NMOSFETs  119 ,  120 . Once the charging capacitor  108  has been charged to a desired voltage (Vc), the first and second mode select signals are reset so that the charging capacitor is isolated from ground and VDD. At this stage, the second terminal of the charging capacitor  108  is now connected to the input  105  through the twelfth and thirteenth NMOSFETs  124 ,  125 . It will be noted that the second terminal of the charging capacitor can be as high as twice VDD. Now the voltage level on the first terminal of the charging capacitor is Vc plus the level of the input voltage on line  105 . It will be noted that this voltage could be as high as 3VDD if the charging capacitor has been charged to VDD and the input level is at twice VDD. The ninth and tenth NMOSFETs  119 ,  120  are protected from a voltage as high as 3VDD because their gates are set at 2VDD in this charging phase. 
     Now that the charging capacitor  108  has been charged, an ‘on’ mode of operation can follow. The third, fourth and fifth mode select signals are set so that the resulting action of the first switch arrangement  108  permits the voltage on the first terminal of the charging capacitor  108  to be transferred to the gate terminals of the NMOSFETs of the sampling switching circuit  104 , thereby turning it on, and the eleventh NMOSFET  123  also to turn on. 
     The gate terminals of the fourth and fifth NMOSFETs  114 ,  115  need to be at a level equal to the input voltage level plus VDD in the ‘on’ mode while they must be connected to VDD in the ‘off’ mode. This is achieved by directly connecting these gate terminals to the first terminal of the charging capacitor  108 . The gate terminal of the sixth N MOSFET  116 , however, needs to vary from zero volts in the ‘off’ mode to a voltage level equal to the input voltage level plus VDD in the ‘on’ mode (to ensure that its gate-to-source voltage will never rise beyond the voltage rating for this NMOSFET of VDD). To achieve this, the gate terminal of the sixth NMOSFET  116  is coupled to the first terminal of the charging capacitor  108  through the PMOSFET  113 . In the ‘on’ mode, the PMOSFET  113  shorts the gate terminal of the sixth transistor  116  to the first terminal of the charging capacitor  108 , whereas in the ‘off’ mode this gate terminal is connected to ground through the first, second and third NMOSFETs  110 ,  111 ,  112 . The gate terminal voltages of the first, second and third NMOSFETs  110 ,  111 ,  112  are chosen so that they can tolerate voltages as high as three times VDD (which could occur during an ‘off’ condition). 
     The gate terminal of the PMOSFET  113  needs to be shorted to the second terminal of the charging capacitor  108  in the ‘on’ mode while it should be at VDD in the ‘off’ mode. These conditions are fulfilled by the action of the inverter  122  and the eleventh NMOSFET  123 . Connecting the gate terminal of the PMOSFET  113  to the second terminal of the charging capacitor  108  ensures that the PMOSFET  113  does not turn off before the charging capacitor has discharged and the voltage on the gate terminal of the sixth NMOSFET  116  has risen to the required level. 
     Advantageously, the present invention provides an input control circuit that can be used to drive analog switch arrangements of analog devices or modules such as an ADC or multiplexer, for example and that can enable the switch arrangement to receive a higher input voltage than the voltage rating of the switch arrangement without risk of damage and without the need for a resistor divider network. Obviating the requirement for a resistor divider network eliminates its associated short-comings of excessive loading, speed reduction and the need for customer-end trimming. Furthermore, the invention also ensures that the components of the input circuit are also protected from any over-voltage conditions. As the invention permits the design and manufacture of high voltage analog modules using low voltage devices, this has the advantage of saving on mask costs. 
     It will be appreciated that while the example of  FIG. 1  has been described in the context of an ADC processing an input voltage, the control circuit  101  may be used with other analog functional modules and devices. 
     The connections as discussed herein may be any type of connection suitable to transfer signals from or to the respective nodes, units or devices, for example via intermediate devices. Accordingly, unless implied or stated otherwise, the connections may for example be direct connections or indirect connections. The connections may be illustrated or described in reference to being a single connection, a plurality of connections, unidirectional connections, or bidirectional connections. However, different embodiments may vary the implementation of the connections. For example, separate unidirectional connections may be used rather than bidirectional connections and vice-versa. Also, plurality of connections may be replaced with a single connection that transfers multiple signals serially or in a time multiplexed manner. Likewise, single connections carrying multiple signals may be separated out into various different connections carrying subsets of these signals. Therefore, many options exist for transferring signals. 
     Although specific conductivity types or polarity of potentials have been described in the examples, it will be appreciated that conductivity types and polarities of potentials may be reversed. 
     The description of the preferred embodiments of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or to limit the invention to the forms disclosed. It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiment disclosed, but covers modifications within the spirit and scope of the present invention as defined by the appended claims.