Abstract:
Methods and systems for producing a digital temperature reading are provided. In an embodiment, one or more current sources and one or more switches are used to selectively provide a first amount of current (I 1 ) and a second amount of current (I 2 ) to the emitter of a transistor (Q 1 ), during different time slots of a time period, to thereby produce a first base-emitter voltage (Vbe 1 ) and a second base-emitter voltage (Vbe 2 ), where I 1 =I 2 *M, and M is a known constant. An analog-to-digital converter (ADC) digitizes analog signals representative of the magnitudes Vbe 1  and Vbe 2 . A difference is determined between the magnitudes of Vbe 1  and Vbe 2 . A digital calculator produces a digital temperature reading (DTR) based on the difference between the magnitudes of Vbe 1  and Vbe 2.

Description:
FIELD OF THE INVENTION  
       [0001]    The present invention relates to temperature sensors, and more specifically to methods and systems that provide a digital output that is indicative of temperature. 
       BACKGROUND  
       [0002]    There has been a growing demand in thermal management systems for integrated temperature sensors with on-chip PN junction diodes/transistors, analog-to-digital converters (ADCs) and bus interfaces. For example, these so-called “smart” temperature sensors are widely used in PCs, laptops and LCD displays to monitor the system temperature. Such applications typically require low-cost temperature sensors with a desired accuracy within ±1.0 degree Celsius (deg C.). In other words, it is desired that the inaccuracy be below ±1.0 deg C. 
         [0003]    In order to convert temperature to a digital value, both a well-defined temperature-dependent signal and a temperature-independent reference signal are required. The temperature-dependent signal can be obtained by comparing the difference in the base-emitter voltages of two bipolar (BJT) transistors at different current densities. The temperature-independent reference signal can be obtained by using a bandgap reference voltage, which also depends on the base-emitter voltages. The base-emitter voltage Vbe of a single transistor in its forward-active region has a typical slope of about −2 mV/deg K and an extrapolated value at 0 deg K of roughly 1.2 V. The delta Vbe between the base-emitter voltages of two such transistors biased at different collector currents is proportional to absolute temperature (ptat) and has a value and a temperature coefficient which are typically one order of magnitude smaller than those of Vbe. Equations 1 and 2 below define such relationships. 
         [0000]        Vbe =( kT/q )ln( Ic/Is )   (Eq. 1) 
         [0000]      delta  Vbe=Vbe 1 −Vbe 2=( kT/q )ln[( Ic 1 *Is 2)/( Ic 2 *Is 1)]=( kT/q )ln( M )   (Eq. 2) 
         [0004]    Where,
       Is is the saturation current,   Ic is the collector current,   k is Boltzmann&#39;s constant,   q is the electron charge,   T is the temperature in degree Kelvin, and   M=(Ic 1 *Is 2 )/(Ic 2 *Is 1 ).       
 
         [0011]    Because of the small value and the small temperature coefficient of the delta Vbe, special care has to be taken to avoid the occurrence of errors due to mismatch of bipolar transistors and op-amp offset. For example, where M=8, a mere 0.53 mV of mismatch in Vbe (or offset voltage of the op-amp used to sense the delta Vbe; the offset voltage directly added to the delta Vbe) produces a 1% error in the absolute temperature, which is translates to a 3 deg C. error at ambient temperature. Where M=3, this mismatch or offset should be smaller than 10 uV to obtain a negligible temperature error (0.1 deg C.). Previous designs have employed nested-chopping/auto zeroing to reduce temperature errors due to the effects of mismatch in Vbe and offset voltage of amplifiers used in the readout circuit. 
         [0012]    Various temperature sensors, some of which are temperature to digital converts, are known in the prior art, including those listed below, each of which are incorporated herein by reference: K. Kujik, “A precision reference voltage source,” IEEE J. Solid State Circuits, vol. SC-8, pp. 222-226, June 1973; B. Song and P. Gray, “A precision curvature-compensated CMOS bandgap reference,” IEEE J. Solid State Circuits, vol. SC-18, pp. 634-643, December 1983; M. Tuthill, “A switched-current, switched-capacitor temperature sensor in 0.6 um CMOS,” IEEE J. Solid State Circuits, vol. SC-33, pp. 1117-1122, July, 1998; M. Pertijs, A Niederkorn, B. Mckillop, A. Baker, J. Huijsing, “A CMOS smart temperature sensor with a 3 sigma inaccuracy of ±0.5 deg C. from −50 to 120 deg C.,” IEEE J. Solid State Circuits, vol. SC-40, pp. 454-461, February 2005; M. Pertijs, K. Makinwa, J. Huij sing, “A CMOS smart temperature sensor with a 3 sigma inaccuracy of ±0.1 deg C. from −55 to 125 deg C.,” IEEE J. Solid State Circuits, vol. SC-40, pp. 2805-2815, December, 2005; J. Huijsing, R. J. van de Plassche and W. Sansen, “Analog Circuit Design”, Boston/Dordrecht/London: Kluwer Academic, 1996, pp. 350-351; and M. Pertijs, A. Bakker, J. Huijsing, “A High-Accuracy Temperature Sensor with Second-order Curvature Correction and Digital Bus interface”, in Proc. ISCAS, May 2001, pp. 368-371. 
         [0013]    Nevertheless, each of the above mentioned temperature sensor have certain aspects that need improving. In other words, there is still a desire to improve upon previous temperature sensors, including previous temperature to digital converters. 
       SUMMARY OF THE INVENTION  
       [0014]    Embodiments of the present invention are directed to methods and systems for producing a temperature reading, which can be a digital output indicative of temperature. In accordance with an embodiment of the present invention, a transistor (Q 1 ) is configured to have its base connected to its collector and its collector connected to ground. A first amount of current (I 1 ) and a second amount of current (I 2 ) are provided to the emitter of the transistor (Q 1 ), during different time slots of a time period, to thereby produce a first base-emitter voltage (Vbe 1 ) and a second base-emitter voltage (Vbe 2 ), where I 1 =I 2 *M, and M is a known constant. Analog signals indicative of magnitudes of Vbe 1  and Vbe 2  are digitized, and a difference between the magnitudes of Vbe 1  and Vbe 2  is determined digitally. A digital temperature reading (DTR) is determined based on the digitally determined difference between the magnitudes of Vbe 1  and Vbe 2 . 
         [0015]    In accordance with an embodiment, the DTR is produced using the following equation: 
         [0000]        DTR=K *Data/[ K *Data/(2 ̂N )+1], 
         [0016]    where K is a constant, Data is an N-bit digital value indicative of the difference between the magnitude of Vbe 1  and the magnitude of Vbe 2 , and N is the number of bits of the Data, which is an integer ≧2. 
         [0017]    In accordance with an embodiment, I 1  is produced during a first contiguous time slot, and I 2  is produced during a second contiguous time slot the follows the first time slot. In an alternative embodiment, I 1  is produced during one or more time slots that need not be contiguous, and I 2  is produced during one or more further time slots that need not be contiguous. 
         [0018]    In accordance with an embodiment of the present invention, I 1  is produced by simultaneously and continually turning on M current sources during a first time slot, and I 2  is produced by separately turning on each of the M current sources during different portions of a second time slot that equals the first time slot. In such embodiment, each of the different portions of the second time slot can be equal to 1/M th  of the first time slot. In an alternative embodiment, I 1  is produced by continually turning on a current source during a first time slot, and I 2  is produced by turning on the same current source for only 1/M th  of a second time slot that equals the first time slot. 
         [0019]    In accordance with an embodiment of the present invention, one or more current sources and one or more switches are used to selectively provide the first amount of current (I 1 ) and the second amount of current (I 2 ) to the emitter of the transistor (Q 1 ), during different time slots of a time period, to thereby produce the first base-emitter voltage (Vbe 1 ) and the second base-emitter voltage (Vbe 2 ). In accordance with an embodiment, the one or more switches are turned on and off at times that corresponds to transitions of a clock signal. Further, a voltage-to-current converter is used to convert Vbe 1  and Vbe 2  to currents. Additionally, the turning off a further switch, at the output of the voltage-to-current converter, for short time periods centered about transitions of the clock signal, is used reduce the effects of charge-injection due to the turning on and off of the one or more switches used to provide I 1  and I 2  to the emitter of the transistor (Q 1 ). 
         [0020]    In accordance with an embodiment of the present invention, an analog-to-digital converter (ADC) is used to digitize the analog signals representative of the magnitudes Vbe 1  and Vbe 2 . In accordance with an embodiment, a digital up/down counter is used to determine a different between the magnitudes of Vbe 1  and Vbe 2 . The up/down counter can be considered part of the ADC, but need not be. In accordance with an embodiment, a digital calculator is used to calculate the digital temperature readings (DTRs) based on the outputs of the ADC. 
         [0021]    In accordance with an embodiment, Vbe 1  and Vbe 2  are converted to currents using a voltage-to-current converter that includes a first resistor (R 1 ), to thereby produce the analog signals indicative of the magnitudes of Vbe 1  and Vbe 2 . Additionally, a further transistor (Q 3 ) is used to produce a third base-emitter voltage (Vbe 3 ), where the base and collector of the third transistor are connected together, the collector is connected to ground, and a current proportional to absolute temperature (Iptat) is provided to the emitter of the further transistor (Q 3 ). In such embodiment, Vbe 3  is converted to a reference current (Iref) using a further voltage-to-current converter that includes a second resistor (R 2 ), where Iref=Vbe 3 /R 2 . Additionally, Iref is used as a reference when digitizing the analog signals indicative of the magnitudes of Vbe 1  and Vbe 2 . In this embodiment, the DTR can be produced using the following equation: 
         [0000]        DTR=K *( R 2 /R 1)*( kT/q )ln( M )/[ K *( R 2 /R 1)*( kT/q )ln( M )+ Vbe 3]*(2 ̂N ), 
         [0022]    where K is a constant, k is Botzmann&#39;s constant, T is the temperature in degrees Kelvin, q is the electron charge, and N is an integer ≧2. 
         [0023]    In an alternative embodiment of the present invention, a current proportion to absolute temperature (Iptat) is provided to the emitter of the same transistor (Q 1 ) to thereby produce a third base-emitter voltage (Vbe 3 ). In such embodiment, the DTR can be produced using the following equation: 
         [0000]        DTR=K *( kT/q )ln( M )/[ K *( kT/q )ln( M )+ Vbe 3]*(2 ̂N ), 
         [0024]    where K is a constant, k is Botzmann&#39;s constant, T is the temperature in degrees Kelvin, q is the electron charge, and N is an integer ≧2. 
         [0025]    This summary is not intended to be a complete description of the embodiments of the present invention. Further and alternative embodiments, and the features, aspects, and advantages of the present invention will become more apparent from the detailed description set forth below, the drawings and the claims. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0026]      FIG. 1A  shows a temperature-to-digital converter circuit concept according to an embodiment of the present invention. 
           [0027]      FIG. 1B  shows an exemplary timing diagram that can be used with the circuit of  FIG. 1A . 
           [0028]      FIG. 2A  shows a circuit implementation of the concept shown in  FIG. 1A , according to an embodiment of the present invention. 
           [0029]      FIG. 2B  shows an exemplary timing diagram that can be used with the circuit of  FIG. 2B . 
           [0030]      FIG. 3  shows how currents I 1  and I 2  in  FIGS. 1A and 2A  can be generated, in accordance with an embodiment of the present invention. 
           [0031]      FIG. 4  shows a circuit that can be used to produce the reference current, I 3 , of  FIG. 2A , according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION  
       [0032]    Embodiments of the present invention relate to high accuracy and low cost temperature sensor circuits. In accordance with embodiments of the present invention, a sensor circuit employs an ADC to digitize different voltages of Vbe in different time slots, and then uses a digital correction scheme to obtain an accurate delta Vbe in the digital domain and thereafter a digital temperature reading (DTR). 
         [0033]      FIG. 1A  shows a temperature circuit concept according to an embodiment of the present invention. Referring to  FIG. 1A , the circuit is shown as using three currents I 1 , I 2  and I 3 , a bipolar transistor. Q 1 , an analog-to-digital converter (ADC)  102  and a digital-calculator  104 . Switches S 1 , S 2 , S 3  and S 4  are used to provide the three different currents I 1 , I 2  and I 3  or no current (when S 4  is connected to ground) to the input of the ADC  102 , in a time multiplexed manner, as can be appreciated from the timing diagram of  FIG. 1B . As shown in  FIG. 1A , I 1 =M*Ibias, I 2 =Ibias and I 3 =Iptat, where Ibias is a bias current, Iptat is a proportional-to-temperature current, and M is a constant. Also shown in  FIG. 1A  is that the ADC  102  receives a reference voltage (Vref), which is an arbitrary voltage. As can be appreciated from the following equations, Vref can vary with temperature and have a poor accuracy, since it is canceled out. 
         [0034]    The ADC  102  generates three digital values for 3 Vbe voltages corresponding to the three different currents I 1 , I 2  and I 3  in different time slots, and an additional digital value for a 0V voltage: 
         [0000]        D 1=(2 ̂N )*( Vbe 1 +Vos )/ V ref   (Eq. 3) 
         [0000]        D 2=(2 ̂N )*( Vbe 2 +Vos )/ V ref   (Eq. 4) 
         [0000]        D 3=(2 ̂N )*( Vbe 3 +Vos )/ V ref   (Eq. 5) 
         [0000]        D 4=(2 ̂N )* Vos/V ref   (Eq. 6) 
         [0000]    Here, Vos is the input offset voltage of the ADC  102 , and N is the number of bits of resolution of the ADC  102 , which is an integer ≧2. The digital-calculator  104  uses these outputs to calculate the digital temperature reading (DTR) using the following equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
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         [0000]    Here, K is a constant provided by digital logic of the digital calculator  104 , and gives a bandgap reference value equal to K*(kT/q)ln(M)+Vbe 3  that is be zero-temperature dependent at a reference temperature. Therefore, DTR is linearly changed with temperature and can be used as a temperature reading for the 1 st  order approximation. When Vbe 3  is biased with Iptat, K can be further increased slightly to compensate the bandgap reference voltage&#39;s curvature and to achieve the temperature sensor&#39;s accuracy better than ±0.3 deg C. 
         [0035]    In order to calibrate the process variation of Vbe 3 , the Iptat current can be used for trimming. Further, the effects of Vos and Vref variations are completely removed with the digital correction according to the above equation for DTR. In addition, the effect of the bipolar transistor&#39;s variation on delta Vbe is also removed because the same device (i.e., Q 1 ) is used to provide Vbe 1  and Vbe 2 . 
         [0036]      FIG. 2A  shows a practical implementation of the circuit concept shown in  FIG. 1A , using CMOS process, in accordance with an embodiment of the present invention. In  FIG. 2 , voltage signals, including Vbe, are converted into current signals using a voltage-to-current (V2C) converter  206 . In the embodiment shown, the V2C  206  is shown as including an operational amplifier U 1 , an NMOS transistor Q 2  and a resistor R 1 . However, other configurations are also possible, which are within the scope of the present invention. 
         [0037]    In  FIG. 2A , the two currents I 1  and I 2  are used to produce Vbe 1  and Vbe 2 , which are converted to currents that are provided to the ADC  102 , and the third current  13  (also referred to as Iref) proportional to a Vbe 3  is used as the reference current Iref for the ADC  102 . As can be appreciated from the discussion below, this arrangement reduces the conversion times for the ADC  102  from 4 time slots to 2 time slots for each temperature reading, and reduces analog and digital hardware. Exemplary circuits for generating currents I 1 , I 2  are discussed below with reference  FIG. 3 , and an exemplary circuit for generating Iref is discussed below with reference to  FIG. 4 . 
         [0038]    Further, in  FIG. 2A , the ADC  102  is implemented as a charge-balancing ADC. In accordance with a specific embodiment, the ADC  102  includes an integrator  208 , a comparator  210 , a D-flip-flop  212 , and an up/down counter  214 , as shown in  FIG. 2A . The integrator  208  is shown as including an op-amp U 2  and a capacitor C 1 . The op-amp U 2  has an inverting input (vin), a non-inverting input (vip) and an output. The capacitor C 1  is connected between the output and the inverting (vin) input of the op-amp U 2 . The non-inverting input (vip) receives a bias voltage (vbias). The comparator  210  includes an op-amp U 3  having an inverting input (vin), a non-inverting input (vip) and an output. The non-inverting input (vip) of the op-amp U 3  receives the output of the integrator  208 ; the inverting input (vin) of the op-amp U 3  receives the bias voltage (vbias); and the output of the op-amp U 3 , which is the output of the comparator  210 , is provided to the D input of the D-flip-flop  212 . The CLK input of the D-flip-flop  212  receives a clock signal (CLOCK). The Q output of the D-flip-flop  212  is used to turn on and off the switch S 3 , as well as being provided to the D input of the up/down counter  214 . The CLK input of the up/down counter  214  also receives the clock signal (CLOCK). An up/down bit (U/DB) input of the up/down counter  214  receives an up/down bit signal. In accordance with an embodiment, as shown in  FIG. 2B , the up/down bit signal should be high when the switch S 1  is turned on (i.e., closed) and the switch S 2  is turned off (i.e., open), which will cause the up/down counter  214  to count up; and the up/down bit signal should be low when the when the switch S 1  is turned off (i.e., open) and the switch S 2  is turned on (i.e., closed), which will cause the up/down counter  214  to count down. In this arrangement, the output of the up/down counter  214  is indicative of the difference between Vbe 1  and Vbe 2 . This output of the up/down counter  214 , which is the output of the ADC  102 , is provided to the digital calculator  104 . 
         [0039]    It is also possible that the up/down counter  214  be external to the ADC  102 . In other words, the up/down counter  214 , or a block that performs its function, can be between the ADC  102  and the digital calculator  104 . It is also possible that the up/down counter  214 , or at least its function, be implemented within the digital calculator  104 . 
         [0040]    The ADC  102  shown in  FIG. 2A , which is a charge-balancing ADC, acts as a first order delta-sigma modulator type quantizer. Other types of ADCs, or more generally, other types of quantizers, can be used, while still being within the scope of the present invention. 
         [0041]    In accordance with an embodiment, the digital calculator  104  can be implemented, e.g., using simple CMOS logic, or simple register transfer level (RTL) circuit. It is also possible that the digital calculator  104  be implemented using software coding with a digital signal processor (DSP) or a microprocessor, if available. These are just a few examples of how the digital calculator  104  can be implemented. Other implementations are also within the scope of the present invention. 
         [0042]    Still referring to  FIG. 2A , when the output of the D-flip-flop  212  is high (causing the up/down counter  214  to count up or down), the switch S 3  within the ADC  102  will be closed and Iref will be added to the input of the integrator  208 . This is used to achieve the so-called charge-balancing of the ADC. During the period of conversion, current provided to the input of the ADC  102  (also referred to as the input current to the ADC, or simply as the input current) is integrated all the time; at the same time, Iref is added to the input of the integrator  208  for certain clock periods to bring the integrator&#39;s output voltage back. The net result is that the charge integrated from the input current in the whole conversion period is equal to the charge integrated from Iref for certain clock periods. The up/down counter  214  is used to record how many clock periods are used, and to produce the digital output. In accordance with a specific embodiment, the whole conversion period is divided into 2 time slots, with each time slot including 2̂N clock periods, where N is number of ADC bits. Other variations are also possible, and within the scope of the present invention. 
         [0043]    In accordance with an embodiment, the device Q 1  is a vertical PNP bipolar transistor made from a parasitic structure of P+ to NWELL to P-SUB in a CMOS process. The device Q 1  can also be a NPN or PNP transistor, or a simple diode when a BJT or BiCMOS process is used. The resistor R 3  is used to compensate for the effect of the base resistance (rb). 
         [0044]    The overall operation of the circuit of  FIG. 2A  will now be described with reference to  FIG. 2A  and the timing diagram of  FIG. 2B . As can be seen from  FIG. 2B , a temperature-to-digital conversion time is divided into two equal time slots. Each time slot contains 2̂N clocks for N-bit resolution. In the 1 st  time slot, the switches S 1  are on (i.e., closed) and the switch S 2  is off (i.e., open), resulting in the bias current I 1 =M*Ibias generating Vbe 1  for the bipolar device Q 1 . This voltage (Vbe 1 ) is converted into a current by the V2C converter  206 , and is digitized by the ADC  102  with the up/down bit (U/DB) being high for counting up. Equations for Vbe 1 , I 1  and D 1  are shown below. 
         [0000]        Vbe 1=( kT/q )ln[ M*Ibias *beta/(beta+1)/ Is]   (Eq. 8) 
         [0000]        I 1 =[Vbe 1 +rb *( M*Ibias )/(beta+1)+ Vos]/R 1   (Eq. 9) 
         [0000]        D 1 =I 1 /Iref* 2̂ N=I 1/( Vbe 3 /R 2)*(2 ̂N )   (Eq. 10) 
       Here, beta is the common-emitter current gain, and Vos is the offset voltage of the op-amp  U 1 . D 1  is the number of times to count up in the 1 st  time slot, which is indicative of Vbe 1 . 
       [0045]    Still referring to  FIGS. 2A and 2B , in the 2 nd  time slot, the switches S 1  are off (i.e., open) and the switch S 2  is on (i.e., closed). This results in the bias current I 2 =Ibias generating Vbe 2  for the same device Q 1 . This voltage is converted into a current by the V2C  206 , and is digitized by the ADC  102  with U/DB being low for counting down. Equations for Vbe 2 , I 2  and D 2  are shown below. 
         [0000]        Vbe 2=( kT/q )ln[ Ibias *beta/(beta+1)/ Is]   (Eq. 11) 
         [0000]        I 2 =[Vbe 2+( rb +( M −1)* rb )* Ibias /(beta+1)+ Vos]/R    (Eq. 12) 
         [0000]        D 2 =I 2 /Iref* 2 ̂N=I 2/( Vbe 3 /R 2)*(2 ̂N )   (Eq. 13) 
       Here, D 2  is the number of times to count down in the 2 nd  time slot, which is indicative of Vbe 2 . 
       [0046]    At the end of the conversion time (i.e., after the 2 nd  time slot in  FIG. 2B ), the digital output from the ADC  206  is: 
         [0000]      Data= D 1 −D 2=[( kT/q )ln( M )]*( R 2 /R 1)/ Vbe 3*(2 ̂N )   (Eq. 14) 
         [0000]    In Equation 14, the Data, since it is equal to the difference between D 1  and D 2 , is indicative of the delta Vbe (i.e., indicative of Vbe 1 −Vbe 2 ). Here, resistors R 1  and R 2  can be same type of resistors, and therefore (R 2 /R 1 ) can be a constant ratio not dependent on temperature. As can be appreciated from Equation 14, the errors due to Is, beta and rb of device Q 1  in delta Vbe (represented by the Data), and Vos of the op-amp U 1 , are removed. 
         [0047]    In order to obtain a digital reading which is linearly changed with temperature, the digital-calculator block  104  uses the Data output of the ADC  102  to calculate the digital temperature reading (DTR) using the following equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
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                                
                               
                                   
                               
                                
                               3 
                             
                           
                           ] 
                         
                         * 
                         
                           ( 
                           
                             2 
                             ^ 
                             N 
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                      
                     15 
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    Here, K is a constant provided by digital logic of the digital calculator  104 , and provides a bandgap reference value equal to K*(R 2 /R 1 )*(kT/q)ln(M)+Vbe 3  that is zero-temperature dependent at a reference temperature, as K is in the Equation 7 above. K can be further increased slightly for curvature correction. Further, to calibrate for process variation of Vbe 3 , the Iptat current can be used for trimming. The resulting DTR is a digital reading linearly varied with temperature according to the Equation 15. The implementation of the digital-calculator block should be simple since Equation 15 only involves multiplication with a constant, addition with a constant, and division. 
         [0048]      FIG. 3A  shows how currents I 1  and I 2  of  FIG. 2A  can be generated using a technique which can be referred to as a dynamic element matching. Typically, currents for use in a temperature-to-digital converter circuit are generated simultaneously. In contrast, in the embodiment of  FIG. 3A , the currents I 1  and I 2  are generated in different time slots, enabling both currents to be provided to the emitter of the same device (Q 1 ) during different times. For illustration, here the ratio I 1 /I 2 =8. For any desired ratio of M/1, M current sources can be used, as will be appreciated from this discussion. In accordance with an embodiment, during the 1 st  time slot, all current sources are on; and during the 2 nd  time slot, only one current source is alternatively on. For an example, during the 1 st  time slot, all the switches SW 1  . . . SW 8  are on for 8 clock periods; and during the 2 nd  time slot, only SW 1  is on for the 1 st  clock period, and then only SW 2  is on for the 2 nd  clock period, etc., until only SW 8  is on for the 8 th  clock period. This can be appreciated from the timing diagram of  FIG. 3B . 
         [0049]    It is also possible that a single current source with a single switch be used to produce currents I 1  and I 2 . For example, where I 1 /I 2 =M=8, this can be accomplished by closing the switch for an entire 1 st  time period that is 8 clock periods in length (to generate I 1 ), and then closing the switch for only 1 clock period of a 2 nd  time period that is also 8 clock periods in length (to generate I 2 ). 
         [0050]    As can be appreciated from the above description, I 1  and I 2  are amounts of currents that are produced during different time slots of a time period. In the above described embodiments, I 1  is produced during a 1 st  contiguous time slot, and I 2  is produced during a 2 nd  contiguous time slot that follows the 1 st  time slot. But this need not be the case. For example, I 2  can be produced before I 1 . It is also possible that I 1  and I 2  are produced during time slots that are not contiguous, so long as the up/down counter ( 214  in  FIG. 2A ) is counting in one direction when I 1  (or a portion thereof) is being produced, and in the opposite direction when I 2  (or a portion thereof) is being produced, and so long as the ratio of I 1 /I 2  is a known constant (M). For example, referring to  FIG. 3A and 2A , all the switches in  FIG. 3A  can be closed during a 1 st  clock period while the up/down counter ( 214  in  FIG. 2A ) is counting up (as instructed by the U/DB); then only switch Sw 1  can be closed during a 2 nd  clock period while the up/down counter is counting down; then all the switches Sw 1  . . . Sw 8  in  FIG. 3A  can be closed during a 3 rd  clock period while the up/down counter is counting up; then only switch Sw 2  can be closed during a 4 th  clock period while the up/down counter is counting down; . . . until at the 16 th  clock period only switch Sw 8  is closed while the up/down counter is counting down. One of ordinary skill in the art, reading the above description, will also appreciate that other variations are also possible, which are within the scope of the present invention. 
         [0051]    In order to reduce the effect of charge-injection due to the turning on and off of the switches Sw 1 , Sw 2  . . . , an additional switch SwN can be added, as shown in  FIG. 3A . In accordance with specific embodiments of the present invention, the switch SwN is turned off for a short time period with its center corresponding to the clock&#39;s transition edges. 
         [0052]      FIG. 4  shows a simple implantation for a further voltage-to-current converter  406 , which can be used to produce the reference current Iref (i.e., I 3  in  FIG. 2A ), using a further transistor (Q 3 ) and a proportional to absolute temperature current (Iptat). Here, the resistor R 2  is preferably the same type resistor as the resistor R 1  in  FIG. 2A . As shown, a simple chopping technique, using choppers  408  and  410 , is used to reduce the effects of Vos of the op-amp U 4  and the mismatch of the current mirror  412 , because the Vbe is preferably a magnitude (e.g., 10×) larger than the delta Vbe. The Iptat current source is trimmed for the process variation of Vbe 3 . 
         [0053]    While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. 
         [0054]    The present invention has been described above with the aid of functional building blocks illustrating the performance of specified functions and relationships thereof. The boundaries of these functional building blocks have often been arbitrarily defined herein for the convenience of the description. Unless otherwise specified, alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. Any such alternate boundaries are thus within the scope and spirit of the claimed invention.