Abstract:
An input stage for a switched capacitor analog-to-digital converter has a differential voltage input receiving an input voltage, a differential reference voltage input receiving a chopped reference voltage, a common voltage connection, and a differential output. A pair of input capacitors is coupled between the differential voltage input and the differential output and a pair of reference capacitors is coupled between the differential reference voltage input. A switching unit is controlled by a first and second phase operable during the first phase to connect a first terminal of the input capacitors with the common voltage connection and couple the first terminal of the reference capacitors with the inverted differential voltage reference; and during a second phase to connect the first terminal of the input capacitors with the differential input voltage and couple the first terminal of the reference capacitors with the non-inverted differential voltage reference.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 14/181,904 filed on Feb. 17, 2014, which claims the benefit of U.S. Provisional Application No. 61/769,928 filed on Feb. 27, 2013, which are incorporated herein in their entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a 2-phase switched capacitor Flash analog-to-digital-converter (ADC), in particular a Flash ADC using a chopper voltage reference. 
     BACKGROUND 
     A block diagram of a conventional switched capacitor analog-to-digital-converter  100  (ADC) is shown in  FIG. 1 . An input signal to be converted is fed through a loop filter  110  to a quantizer  120  in which the signal is generally converted into an output bit stream having 1-bit or multiple bit resolution. The output signal is fed back through a digital-to-analog converter  130  (DAC) to the loop filter  110  where it is subtracted from the input signal. 
     This type of analog-to-digital converter introduces quantization noise and offset errors. In particular a reference voltage is used within the quantizer which introduces an offset voltage and therefore distorts the measured signal. 
     SUMMARY 
     There exists a need for an improved design of a switched capacitor DAC. According to an embodiment, an input stage for a switched capacitor analog-to-digital converter may comprise a differential voltage input receiving an input voltage, a differential reference voltage input receiving a chopped reference voltage, a common voltage connection, a differential output, a pair of input capacitors coupled between the differential voltage input and the differential output, a pair of reference capacitors coupled between the differential reference voltage input, and a switching unit controlled by a first and second phase operable during the first phase to connect a first terminal of the input capacitors with the common voltage connection and couple the first terminal of the reference capacitors with the inverted differential voltage reference; and during a second phase to connect the first terminal of the input capacitors with the differential input voltage and couple the first terminal of the reference capacitors with the non-inverted differential voltage reference. 
     According to a further embodiment, the first and second phase are defined by non-overlapping clock signals. According to a further embodiment, the reference voltage is chopped using the non-overlapping clock signals. According to a further embodiment, the reference voltage comprises a positive offset voltage during one phase and a negative offset voltage during the other phase. According to a further embodiment, during the first phase the second terminals of the input capacitors are coupled with a common voltage connection. According to a further embodiment, during the first phase the second terminals of the input capacitors are coupled with a negative input voltage. According to a further embodiment, the first terminal of a positive input capacitor and the first terminal of a positive reference capacitor are coupled with a positive output terminal and the first terminal of a negative input capacitor and the first terminal of a negative reference capacitor are coupled with a negative output terminal. According to a further embodiment, the common voltage connection is ground. According to a further embodiment, the common voltage connection is a virtual ground. 
     According to yet another embodiment, a method of providing an input signal to a quantizer of a sigma delta modulator having an input stage with a pair of input capacitors and a pair of reference capacitors, wherein a first terminal of each of the input capacitors and reference capacitors is connected with a positive and negative output terminal, respectively, may comprise during a first phase connecting the positive and negative output terminals with a common voltage and charging the reference capacitors with an inverted reference voltage; and during a subsequent second phase disconnecting the positive and negative output terminals from the common voltage and connecting the first terminals of the input capacitors with a non-inverted differential input voltage, respectively and the first terminals of the reference capacitors with the non-inverted reference voltage. 
     According to a further embodiment of the method, the first and second phase are defined by a non-overlapping clock signals. According to a further embodiment of the method, According to a further embodiment of the method, the reference voltage can be chopped using the non-overlapping clock signals. According to a further embodiment of the method, the reference voltage comprises a positive offset voltage during one phase and a negative offset voltage during the other phase. According to a further embodiment of the method, during the first phase the second terminals of the input capacitors are coupled with the common voltage. According to a further embodiment of the method, during the first phase the second terminals of the input capacitors are coupled with an inverted input voltage. According to a further embodiment of the method, the common voltage connection is ground. According to a further embodiment of the method, the common voltage connection is a virtual ground. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a block diagram of a sigma delta modulator. 
         FIG. 2  shows a conventional Flash ADC stage. 
         FIG. 3  shows a chopped reference voltage. 
         FIG. 4  shows an input stage for a sigma delta modulator according to a first embodiment. 
         FIG. 5  shows an input stage for a sigma delta modulator according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     According to various embodiments, a 2-phase switched capacitor Flash ADC using a chopper voltage reference can be provided wherein the voltage reference is averaged within the Flash ADC. The present disclosure solves the problem how to be able to use a chopper voltage reference in a 2-phase switched capacitor based Flash ADC without being impacted by the offset of the voltage reference and without additional circuitry to average the chopper voltage reference. 
     In sigma delta converters as for example shown in  FIG. 1 , Flash ADCs are often used to make the quantizer for multi level modulators. These Flash ADCs are often capacitive based where there is a switching network that samples the signal and the reference followed by a comparator that compares if the resulting sampled signal is higher than the threshold given by the reference voltage. 
     The Flash ADC often uses two phases (one reset phase and one compare phase) to be able to reset the capacitor charges between two independent comparisons. The voltage reference is often coming from a chopper voltage reference source to avoid 1/f noise and offset errors in the reference signal. 
     According to various embodiments, a 2 phase switched capacitor Flash ADC using a chopper voltage reference can be provided in which the voltage reference is averaged within the Flash ADC, so that no low pass filter is required to average the chopper signals. This also permits to have reference capacitors twice smaller in the Flash ADC. This will remove the need for having an un-buffered and not chopped voltage reference signal as the reference source for the Flash ADCs and therefore simplifies the design of multi-level modulators too. 
     A conventional Flash ADC is composed of multiple comparator stages (n comparators for n+1 levels for the ADC resolution). In each stage, each of the comparator stage is typically driven by a switched-capacitor circuit, as shown for example shown in  FIG. 2 . 
     This circuit is a differential input stage that has Vin+/− for the input signal and Vref+/− for the reference signal and VCM for a common-mode virtual ground or ground signal. Switch  210   a  connects Vin+ with a first terminal of capacitor  230   a  whose second terminal is coupled with the output terminal OP. Switch  215   a  connects Vref+ with a first terminal of capacitor  240   a  whose second terminal is coupled with the output terminal OP. Switch  215   b  connects Vref− with a first terminal of capacitor  240   b  whose second terminal is coupled with the output terminal OM. Switch  210   b  connects Vin− with a first terminal of capacitor  230   b  whose second terminal is coupled with the output terminal OM. 
     Switches  220   a - d  are provided to couple the first terminal of capacitors  230   a ,  240   a ,  240   b , and  230   b  with VCM. Switches  250   a  and  250   b  connect the output terminals OP and OM with VCM and switch  260  connects OP and OM. Switches  220   a - d ,  250   a, b  and  260  are controlled by clock signal P 1  and switches  210   a, b  and  215   a, b  are controlled by clock signal P 2 . 
     The outputs OP/OM are connected to a comparator (respectively positive and negative side). Typically here the circuit works in two phases P 1  and P 2  (non overlapping phases/clock signals) to transfer the charges to the comparator. In the P 1  phase, the switches connected to OP/OM are on and the top plate and the bottom plate of all the caps are reset to VCM. No charge is transferred to the comparator. The input switches connected to V in +/− and V ref +/− are opened. OP=OM=VCM in this phase. This is the RESET phase. 
     In phase P 2 , the transfer happens, hence this is the TRANSFER phase. The input switches  210   a, b  and  215   a, b  are on, so each input is sampled on their respective capacitors  230   a, b  and  240   a, b . The switches  250   a, b  and  260  connected to OP/OM are opened so the OP/OM signals are no more reset to VCM. Their value depends on the V in +/− and Vref+/− and the capacitor values. The OP-OM differential voltage gets positive if Cin(Vin+−Vin−)−Cref(Vref+−Vref−)&gt;0 which means Vin+−Vin−&gt;Cref/Cin*(Vref+−Vref−). So the effective comparator threshold for the differential voltage input Vin+−Vin− is Cref/Cin (Vref+−Vin−). Each of the comparator stage typically has a different comparator threshold, set by the Cref capacitor value and equally distributed to have a uniform quantization error. A negative threshold is simply realized in swapping Vref+ and Vref− at the inputs. 
     This conventional stage as shown in  FIG. 2  has a problem when the Vref+/− include a chopper modulation even if the modulation is happening between P 1  and P 2  which is a natural approach. If Vref+−Vref−=Vref+Voff is written in phase P 1  and Vref−Voff in phase P 2 , a model is obtained for the voltage reference that is chopped between P 1  and P 2  and that has an offset of Voff and an average of Vref as shown in  FIG. 3 . 
     In the conventional stage, the Vref transfer (at the Vref+/− inputs) only happens in P 2 . So it always transfers Vref-Voff in the case of a chopper modulated voltage reference. So in this case, the proper averaging is never realized and the Vref offset is always integrated in the output charge which is not the purpose of the chopper modulation. 
       FIG. 4  shows an improved input stage for a switched capacitor ADC according to various embodiments. Switch  410   a  connects Vin+ with the first terminal of capacitor  450   a  whose second terminal is coupled with output OP. Switch  420   a  connects Vref+ with the first terminal of capacitor  460   a  whose second terminal is coupled with output OP. Switch  420   b  connects Vref− with the first terminal of capacitor  460   b  whose second terminal is coupled with output OM. Switch  410   b  connects Vin− with the first terminal of capacitor  450   b  whose second terminal is coupled with output OM. Switches  440   a  and  440   b  are provided to couple the first terminals of capacitors  450   a  and  450   b  with VCM. Switches  470   a  and  470   b  are provided to couple the outputs OP and OM with VCM and switch  480  shorts outputs OP and OM. Switch  430   a  is provided to connect Vref- to the first terminal of capacitor  460   a  and switch  430   b  is provided to connect Vref+ with the first terminal of capacitor  460   b . Switches  440   a, b ,  430   a, b ,  470   a, b , and  480  are controlled by clock signal P 1 . Switches  410   a, b  and  420   a, b  are controlled by clock signal P 2 . 
     In the proposed structure according to various embodiments, as shown in the  FIG. 4 , there are no more switched connections to VCM on the Vref input switches. The switches  420   a,b  and  430   a, b  are now in a cross configuration and are connected to either Vref+ or Vref−. A set of switches  430   a, b  connects the capacitors  460   a, b  to Vref+/− in phase P 1  and a complement set of switches  420   a, b  connects to the opposite Vref−/+ in phase P 2 . So the total amount of charge transferred on the reference capacitors Cref is then: 
     −Cref(Vref+−Vref−) in phase P 1 , and 
     Cref(Vref−−Vref+) in phase P 2 . 
     In case of a chopped reference input that is toggled between P 1  and P 2 , we can write: Vref+−Vref−=Vref+Voff in phase P 1  and Vref−Voff in phase P 2 , the total charge transferred is: Charge (P 2 )−Charge (P 1 )=Cref{Vref+Voff)−(−Cref(Vref−Voff))=2*Cref*Vref. In this total sum, the Voff component is cancelled, due to the demodulation of the chopper done at the Vref+/− inputs with the switches  420   a, b  and  430   a, b  connected in a cross configuration on P 1  and P 2 . This means that the reference has been averaged between the two phases through the capacitor charge integration and the comparison result is thus independent of the voltage reference offset which is solving the conventional ADC problem. The input signal capacitors Cin are not changed in this configuration but can also be set in a cross configuration to gain a factor of 2× in the sampled signal (as long as the input signal is available during the two phases which may not always be true). With this new technique, a gain of 2× is realized in the Vref path, so the Cref can be divided by two to keep the same effective threshold, which is beneficial for settling time issues and die size. The amount of switches is still the same than in a conventional configuration so there is no real apparent drawback. The chopper modulation just needs to be synchronous with the phases P 1  and P 2  and the toggling needs to happen between P 1  and P 2  to realize the proper offset cancellation. 
       FIG. 5  shows an exemplary embodiment of a circuit where the gain factor of 2× is realized in the Vin path with the cross configuration. Switch  510   a  connects Vin+ with the first terminal of capacitor  550   a  whose second terminal is coupled with output OP. Switch  520   a  connects Vref+ with the first terminal of capacitor  560   a  whose second terminal is coupled with output OP. Switch  520   b  connects Vref− with the first terminal of capacitor  560   b  whose second terminal is coupled with output OM. Switch  510   b  connects Vin− with the first terminal of capacitor  550   b  whose second terminal is coupled with output OM. Switch  540   a  connects Vin− with the first terminal of capacitor  550   a  and switch  540   b  connects Vin+ with the first terminal of capacitor  550   b . Switches  570   a  and  570   b  are provided to couple the outputs OP and OM with VCM and switch  580  shorts outputs OP and OM. Switch  530   a  is provided to connect Vref− to the first terminal of capacitor  560   a  and switch  530   b  is provided to connect Vref+ with the first terminal of capacitor  560   b . Switches  540   a, b ,  530   a, b ,  570   a, b , and  580  are controlled by clock signal P 1 . Switches  510   a, b  and  520   a, b  are controlled by clock signal P 2 .