Abstract:
A transconductance difference amplifier ( 300 ) is described, for providing an output current dependent upon a difference between a first input voltage ( 302 ) and a second input voltage ( 304 ). The difference amplifier comprises an input sampling capacitor ( 306 ) having two conductors; a transconductance amplifier ( 312 ) having an input ( 318 ) coupled to a first conductor of said input sampling capacitor and a current output ( 314 ) for generating said output current; and an input switch ( 308, 310 ) for selectively coupling a second conductor of said input sampling capacitor to a first input of said difference amplifier for receiving said first input voltage and to a second input of said difference amplifier for receiving said second input voltage. The difference amplifier couples said second conductor to one of the inputs and applies a voltage to said first conductor and then couples said second conductor to the other input to provide a voltage change to said transconductance amplifier input dependent upon said input voltage difference.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention generally relates to transconductance amplifiers, that is amplifiers which produce an output current in response to an input voltage, and to analogue-to-digital converters (ADCs) employing such amplifiers. More particularly the invention relates to transconductance amplifiers in which an output current depends upon the difference between two input voltages, and to their applications.  
         BACKGROUND TO THE INVENTION  
         [0002]    In many ADC designs an analogue input voltage is compared to a reference voltage (or a plurality of reference voltages) to produce a voltage output which can be used to generate a digital output code. Exemplary voltage comparators are described in U.S. Pat. No.  6 , 150 , 851 , U.S. Pat. No. 6,356,148, U.S. Pat. No. 6,249,181 and D. R. Beck and D. J. Allstot, “An 8-bit, 1.8V, High Speed Analogue-to-Digital Converter” (http://students.washington.edu/beckdo/papers/techcon2000.doc) and P. Setty, J. Bamer, J. Plany, H. Burger and J. Sonntag, “A 5.75b 350MSamples/S or 6.75b 150MSamples/S reconfigurable/ADC for a PRML Read Channel”, Session  9  IEEE International Solid-State Circuit Conference 5-7 February 1998 (ISSCC98). Sample and Hold (S/H) circuits for ADCs are also known, such as those described in U.S. Pat. No. 6,169,427 and U.S. Pat. No. 5,963,156 and N. Waltari and K. Halonen, “1.0-Volt, 9-bit Pipeline CMOS ADC”, 26 th  European Solid-State Circuit Conference Stockholm, Sweden 19-21 September 2002 which all use a conventional (voltage output) operational amplifier with switched capacitor feedback. Also known are Successive-Approximation-Register (SAR) analogue-to-digital converters (see, for example, J. L. McCreary and P. Gray, IEEE JSSC SC-10 pp371-9, Dec  1975 ) which compare an analogue input with the output of a digital-to-analogue converter (DAC), which DAC may employ a binary-weighted capacitor array to generate an analogue output voltage using charge redistribution between the capacitors.  
           [0003]    The above-mentioned analogue-to-digital converters use voltage comparisons to generate a digital output. It will be appreciated that when comparing an input voltage to a reference voltage only gain, and not linearity is important since it is merely necessary to know whether the input is above or below the reference. However it is also known to simplify ADC circuits by summing currents generated by interpolating between reference voltages, and for this interpolation-type summing linearity is an important requirement. Interpolating ADCs are often used for low-resolution high-or medium-speed applications. FIG. 1 shows a generalised circuit diagram of an exemplary stage of a current-mode interpolating ADC. Examples of ADCs with current-mode interpolation are described in M. P. Flynn and D. J. Alstot, “CMOS Folding A/D Converters with Current-Mode Interpolation”, IEEE JSSC, vol.  31 , September 1996 pages 1248-1257; M. P. Flynn and B. Sheahan, “A 400MSample/S, 6-b CMOS Folding and Interpolating ADC” IEEE JSSC, vol.  33 , December 1998, pages 1932-1938; B-S Song, P. L. Rakers and S. F. Gillig, “A 1V, 6-b 50MSamples/S Current Interpolating CMOS ADC”, IEEE J. Solid-State Circuits, vol. 35, April 2000, pages 647-651.  
           [0004]    Referring to FIG. 1 a , the general principle is to generate a plurality of comparison levels (five in FIG. 1) from a smaller set of reference voltages (two, VrefA and VrefB, in FIG. 1) by interpolating between the outputs from a set of amplifiers (two in FIG. 1), each of which outputs represents the difference between an input signal and one of the references.  
           [0005]    In the interpolating stage  100  of FIG. 1 a  an input voltage on line  102  is provided to one input of first  104  and second  106  differential transconductance amplifiers. A second input to the first transconductance amplifier  104  is provided by a first reference voltage VrefA on line  108  and a second input to differential transconductance amplifier  106  is provided by a second reference voltage VrefB on line  110 . Each differential transconductance amplifier generates an output current in proportion to the difference between the voltages on its two inputs, the ratio of output current to input voltage difference being termed the transconductance. Amplifiers  104  and  106  are drawn as current sinks but preferably their output currents can be of either positive or negative polarity according to the polarity of Vin-VrefA and Vin-VrefB respectively.  
           [0006]    [0006]FIG. 1 b  illustrates one possible implementation of a transconductance amplifier  130  suitable for use for transconductance amplifiers  104  and  106 . The transconductance amplifier comprises a pair of input transistors  132 ,  134  with inputs from, respectively, Vin  102  and one of VrefA  108  and VrefB  110 , and a common source connection connected to a constant current sink  136 . The drain connections of transistors  132  and  134  are connected to respective input and output connections of a current mirror enclosed by dashed line  138 , and a current output connection  140  is taken from the junction of the drain of transistor  134  with the output of current mirror  138 . Each of transistors  132 ,  134  passes an incremental current given by its transconductance gm, multiplied by the incremental gate input voltage of the transistor so that the output current is given by Iout=Gm.(Vin−Vref) where the transconductance, Gm, of the amplifier is equal to gm.  
           [0007]    The output current from transconductance amplifier  104  is input to or drives a first current mirror  112  and the output current from transconductance amplifier  106  drives a second current mirror  114 . Current mirror  112  comprises a plurality of constant current generators  112   a - e . In a conventional manner, a voltage on line  116  sets the current through element  112   a  to be the same as the output current flowing into differential transconductance amplifier  104 . This same drive voltage is also provided to elements  112   b - e  to provide constant current outputs on lines  118   a - d  determined by the output current of transconductance amplifier  104 .  
           [0008]    [0008]FIG. 1 c  shows an example of a controllable current generator  150  suitable for use in the interpolating ADC stage  100  of FIG. 1a. An input transistor  151  and a constant current sink  152  are connected in series between power supply lines  154 ,  156 , a connection between transistor  151  and current sink  152  providing a current output Iout  158 . Input transistor  151  has a control voltage Vc applied to its gate connection to provide a controlled, unipolar current equal to the sum of the output current Iout and the current through the constant current sink  152 . Thus the output current may be of either polarity depending upon the magnitude of the controlled unipolar current through input transistor  151 . For a given input control voltage Vc the output current (or the outputs of a plurality of matched controllable current generators) can be scaled by scaling the dimensions of input (MOS) transistor  151 . The current sink  152  is preferably scaled in the same ratio to maintain a constant “zero current” point. In this way a plurality of differently-scaled, matched controllable current generators may be arranged to have zero output current for substantially the same input control voltage, and thus to provide zero output current substantially simultaneously. Where transistors comprising elements  112   a  and  112   b  have the same physical size the current on line  118   a  is substantially the same as the current through element  112   a . The sizes of transistors comprising elements  112   c, d, e  are reduced to 0.75×, 0.5× and 0.25×that of element  112   b  so that respective currents of 0.75×, 0.5× and 0.25× the output current from transconductance amplifier  104  are provided on lines  118   b ,  118   c  and  118   d . Generally current mirror  112  is fabricated on an integrated circuit so that the transistors comprising the current mirror are matched. Current mirror  114  likewise comprises elements  114   a - e , for example bipolar or FET transistors, and operates in a corresponding manner to provide, respectively, currents of 1.0×, 0.75×, 0.5×, and 0.25× the output current of transconductance amplifier  106  on lines  120   a - d . The current  11  on line  118   a  is provided to a comparator  122   a  to generate a digital output D1. Digital output D2 from comparator  122   b  is determined by a sum of the currents on lines  118   b  and  120   d ; output D3 from comparator  122   c  is determined by a sum of currents on lines  118   c  and  120   c ; output D4 from comparator  122   d  is determined by a sum of currents on lines  118   d  and  120   b  and digital output D5 from comparator  122   e  is determined by the current in line  120   a . Denoting the output current of transconductance amplifier  104  I A  and the output current of transconductance amplifier  106  I B , I A  is proportional to the difference between Vin on line  102  and VrefA on line  108  and I B  is proportional to the difference between Vin and VrefB on line  110 . The summed intermediate currents are proportional to a fraction of the difference between Vin and VrefA plus a fraction of the difference between Vin and VrefB. In mathematical terms:  
             I   1   =I   A   =G ( Vin−VrefA )  
             I   5   =I   B   =G ( Vin−VrefA )  
             I   2 =0.75  G ( Vin−VrefA )+0.25 G ( Vin−VrefB )  
           therefore  I   2   ∝Vin− (0.75  VrefA+ 0.25 VrefB )  
           and  I   2   ∝Vin− (3 VrefA+VrefB )/4.  
           [0009]    In other words these scaled currents generate output currents proportional to the residues [Vin−Vth(i)] i=1 . . . 5 where:  
             Vth (1)= VrefA,    
             Vth (2)=(3. VrefA+VrefB )/4,  
             Vth (3)=( VrefA+VrefB )/2,  
             Vth (4)=( VrefA+ 3 .VrefB )/4,  
             Vth (5)= VrefB.    
           [0010]    It can be seen that where Vin=Vth the output current is zero and that for Vin&gt;Vth the output current is positive and for Vin&lt;Vth the output current is negative. Thus the output currents  11  to  15  will cross zero at input voltages equal to thresholds Vth(1), . . . , N where N=(2+no. of intermediate values), in this case  5  (i.e. 2+3). In the interpolating stage circuit  100  of FIG. 1 the output currents  11  to  15  are applied to high input impedance comparators  122   a  to  122   e  respectively which make logic decisions at these thresholds. Thus input voltage Vin signal levels between VrefA and VrefB are converted to a digital format output on lines D1 to D5 as a so-called thermometer code. This code is then converted to a binary code by conventional means (not shown in FIG. 1). This may comprise, for example, a priority encoder or alternatively the functionality of the hardware may be specified in a hardware description language such as Verilog (Trade Mark) or VHDL, allowing a hardware synthesis tool to blend in the code conversion functionality with other back-end logic such as error correction logic. Although in FIG. 1 only two reference voltages, VrefA and VrefB, are illustrated more reference voltages may be employed for multiple (binary) bit conversion. The reference voltages are typically derived from a resistor string and the skilled person will appreciate that compared to a conventional/analogue-to-digital converter fewer taps on such a resistor string are necessary since fewer reference voltages are required. Furthermore the amplification of the residue signal also eases the requirements on the comparators, and in particular the requirements on their omitted input offset voltage and overdrive. Typically this saves the need for a preamplification stage before each comparator, so that the overall circuit is simpler despite the added current mirrors and linear transconductance amplifiers. One application of an interpolating ADC is as the second stage of a two-stage analogue-to-digital converter such as, the analogue-to-digital converter  200  shown in FIG. 2. In this ADC an input voltage Vin on line  202  is provided to an M-bit ADC  204  which outputs the M most significant bits (MSBs) of the digitised signal on line  206 . This coarse approximation to the input voltage Vin is converted back to an analogue voltage by an M-bit digital-to-analogue converter  208  and subtracted from the original input signal by subtractor  210  to leave a residual signal on line  212 . This residual signal is presented to an N-bit analogue-to-digital converter  214  which generates the N least significant bits (LSBs) on line  216  which are combined with the MSBs by suitable logic in combiner  218  to provide a digital output  220  with the required total number of binary bits. An example of such a two-stage ADC is described in “A 3.3-V, 10-b,  25 -MSample/s Two-Step ADC in 0.35-μm CMOS”, Hendrik van der Ploeg and Robert Remmers, IEEE Journal of Solid State Circuits, Vol 34, No 12, December 1999. The coarse and fine stage conversions ranges may be overlapped to ease constraints on the coarse stage ADC.  
           [0011]    It can be seen by inspection of FIG. 2 that the first stage ADC  204  must first make its decision and then the output of DAC  208  and of the difference amplifier  210  must settle before second stage comparators associated with ADC  214  make their decision. Thus the first stage must sample its input signal well before the second stage decision time. To avoid conversion errors it has previously been necessary to precede such a two-stage ADC with a sample-and-hold circuit to hold the input constant between the two sampling instants. This increases the complexity of the ADC, thus increasing its cost, and also increases the power consumption. It is therefore desirable to be able to dispense with such a sample-and-hold circuit, particularly whilst retaining the linearity desirable for interpolation-type summing and other linear signal processing.  
         SUMMARY OF THE INVENTION  
         [0012]    According to a first aspect of the present invention there is therefore provided a difference amplifier configured for providing an output current dependent upon a difference between a first input voltage and a second input voltage, the difference amplifier comprising an input sampling capacitor having two conductors; a transconductance amplifier having an input coupled to a first conductor of said input sampling capacitor and a current output suitable for generating said output current; and an input switch for selectively coupling a second conductor of said input sampling capacitor to a first input of said difference amplifier for receiving said first input voltage and to a second input of said difference amplifier for receiving said second input voltage; the difference amplifier being configured to, in a first state, couple said second conductor to one of said first and second inputs and apply a voltage to said first conductor and to, in a second state, couple said second conductor to the other of said first and second inputs to provide a voltage change to said transconductance amplifier input dependent upon said difference between said first and second input voltages.  
           [0013]    Preferably, the output current is substantially linearly dependent upon the difference between the first and second input voltages. The output current may be provided directly from the transconductance amplifier or the current generated by the transconductance amplifier may be mirrored or used in some other way to provide an output current for said difference amplifier.  
           [0014]    The input sampling capacitor allows one input voltage, say the first, to be sampled in the difference amplifier&#39;s first state, to set a charge on the input sampling capacitor. Then, when the second conductor of the input sampling capacitor is connected to the other input voltage, say the second, the voltage difference between the inputs is transferred to the input of the transconductance amplifier which provides a current output for generating, either directly or indirectly, the output current from the difference amplifier. A controller may be employed to control this sampling process. Provided that the transconductance amplifier is itself linear the difference amplifier is also substantially linear.  
           [0015]    In operation the first voltage is sampled onto the input capacitor at the end of the first state, but its value is held and subtracted from the second voltage during the second state. The output current resulting from the difference voltage may be sampled (by a following comparator) at the end of the second state, giving the second voltage additional time to settle. This sample and hold action is particularly useful for two-stage ADCs, where the first voltage can be the original input signal, and the second voltage is a reference voltage chosen by a “coarse” ADC also sampling the input signal. Since the second, reference voltage is not needed until the start of the second state the “coarse” ADC has time to settle, and the need for a separate sample-hold circuit can be avoided.  
           [0016]    In a refinement of the difference amplifier one or more additional input sampling capacitors may be provided. Such an additional input sampling capacitor may be switched between two reference voltages and where a plurality of difference amplifiers is employed, for example in an two-stage ADC, one of these reference voltages may be common to all the difference amplifiers and the other reference voltage applied to each difference amplifier may be tied to respective points in a ladder of reference voltages.  
           [0017]    In this way the effective operating points of the difference amplifiers may be spaced apart, for example substantially equidistantly, to provide two or more output currents which may be summed for use by an interpolating ADC. Each difference amplifier may be arranged to generate multiple scaled output currents, for example for use in current-mode interpolation.  
           [0018]    It will be recognized that the first conductor of the input sampling capacitor need not be connected to the transconductance amplifier input in the difference amplifier&#39;s first state and it could, for example, be coupled to the input of the transconductance amplifier via a switch. However in a preferred embodiment the voltage applied to the first conductor in the first state of the difference amplifier is a virtual earth voltage, that is for a differential transconductance amplifier a voltage on one of the inputs is maintained by a closed feedback path to be substantially the same as a (preferably fixed) voltage on the other differential input. This can be provided by means of a switched DC feedback path, either from the output of the transconductance amplifier or from a later stage in the difference amplifier such as after an output drive device, for example a device providing a mirrored or split output current. In a preferred embodiment a positive differential input of the transconductance amplifier is connected to a fixed reference voltage such as ground and a negative differential input of the transconductance amplifier is connected to the input sampling capacitor. Preferably a low resistance switch such as an FET switch is then provided to couple the negative differential input to a current output of the difference amplifier. This virtual earth connection allows the input offset voltage of the transconductance amplifier to be cancelled.  
           [0019]    The voltage change provided to the transconductance amplifier input in changing from the first to the second state of the difference amplifier may be substantially equal to the difference between the first and second input voltages, for example where there is only a single input sampling capacitor or the voltage change may be scaled, for example where there is some charge sharing between two or more input sampling capacitors.  
           [0020]    A differential difference amplifier may be constructed along similar lines, using a pair of the above-described difference amplifiers, but using a shared, differential transconductance amplifier so that, in effect, one input switch and one set of first and second input voltages is associated with each differential input (positive and negative) of the differential transconductance amplifier. In this way, the differential difference amplifier is responsive to a differential signal comprising two sets of voltage differences between two sets of said first and second input voltages. Generally speaking, one set of first input voltages will comprise a positive and negative first input voltage and one set of second input voltages will comprise a positive and negative second input voltage. The circuit may similarly be extended, as described above, by providing one or more additional sets of (differential) input sampling capacitors and/or by adding further (differential) transconductance amplifiers and/or by mirroring or otherwise providing a plurality of single-ended or differential outputs.  
           [0021]    Thus in a related aspect the invention also provides a differential difference amplifier for providing an output current dependent upon a differential signal at a differential input comprising two pairs of signal inputs, said differential signal comprising two voltage differences, a first being dependent upon a difference between first and second input voltages on a first pair of said pairs of signal inputs, a second being dependent upon a difference between third and fourth input voltages on a second pair of said pairs of signal inputs, said differential difference amplifier comprising: first and second input sampling capacitors, each having two conductors, for said first and second pair of signal inputs respectively; a differential transconductance amplifier having a differential input coupled to said first and second input sampling capacitors and an output for generating said output current; a pair of input switches, one for each of said pair of signal inputs, for selectively coupling said first and second input sampling capacitors respectively to one of said first and second input voltages and to one of said third and fourth input voltages; a pair of initialisation switches to bring plates of said first and second input sampling capacitors coupled to said differential transconductance amplifier to initial voltages; and a controller to control said input switches and said initialisation switches to apply said differential signal to said differential transconductance amplifier.  
           [0022]    The plates of the sampling capacitors may be brought to the same initial voltage or to different initial voltages, the output current may either comprise a single-ended or a differential output current and, again, the circuit may be extended by providing additional pairs of sampling capacitors for additional differential inputs, each additional pair of sampling capacitors having an associated pair of input switches for determining a pair of input voltage differences.  
           [0023]    According to another aspect of the present invention there is provided an analogue-to-digital converter comprising at least one transconductance amplifier configured to provide a plurality of output currents at a plurality of outputs; and a plurality of comparators coupled to said plurality of transconductance amplifier outputs for providing a digital output; at least one switched input sampling capacitor coupled to an input of said transconductance amplifier; and at least one switch configured to couple said input sampling capacitor alternately to a first reference voltage and to an analogue voltage for conversion.  
           [0024]    The analogue-to-digital converter may comprise, for example, a current-mode interpolating or folding converter, preferably comprising a plurality of stages. As previously described, by providing an input sampling capacitor and a switch to alternately couple the capacitor to an analogue voltage for conversion and to a reference voltage a sampling difference amplifier is provided which, in embodiments enables a prior sample-hold to be dispensed with. In embodiments a pair of input sampling capacitors may be provided for each of a plurality of transconductance amplifiers, to allow for combinations of output currents with zero crossings at regularly spaced input voltage thresholds, thereby creating a ladder of zero crossing threshold voltages for use in generating the digital output. In embodiments the digital output comprises a thermometer code which is converted to a binary representation.  
           [0025]    The analogue-to-digital converter may be used as the second stage in a two-stage analogue-to-digital converter in which a first analogue-to-digital converter provides a digital output to a first number of (most significant) bits accuracy to provide a coarse approximation to an analogue input signal. This coarse approximation may then be used as the reference voltage for the input sampling capacitor for generating one or more least significant output bits of the two-stage analogue-to-digital converter.  
           [0026]    According to a related aspect of the invention there is provided a method of generating a current in substantially linear dependence upon a voltage difference between first and second voltages using a circuit comprising a switch, a switched input capacitor, and a substantially linear transconductance amplifier, a first plate of the input capacitor being coupled to an input of the transconductance amplifier, a second plate of the input capacitor being switchably couplable to the first and second voltages, the method comprising coupling the second plate of the input capacitor to the first voltage whilst maintaining the first plate at a reference voltage to charge the input capacitor; and then coupling the second plate of the input capacitor to the second voltage and allowing the potential of the first plate to change by an amount dependent upon said voltage difference to cause said transconductance amplifier to generate an output current substantially linearly dependent upon said voltage difference.  
           [0027]    When the input capacitor is connected to the first voltage and to the reference voltage it is charged, that is it is brought to a defined state of charge by charge flowing onto or off the capacitor. Then by connecting the second plate of the input capacitor to the second voltage the difference between the first and second voltages is substantially transferred to the input of the transconductance amplifier.  
           [0028]    In another aspect the invention provides a method of generating a current substantially linearly dependent upon two voltage differences, a first voltage difference between first and second voltages and a second voltage difference between third and fourth voltages, the method employing a circuit comprising first and second switches, first and second switched input capacitors, and a substantially linear transconductance amplifier, a first plate of the first input capacitor and a first plate of the second input capacitor being coupled together and coupled to an input of the transconductance amplifier, a second plate of the first input capacitor being coupled to the first switch for switchable coupling to the first and second voltages and a second plate of the second input capacitor being coupled to the second switch for switchable coupling to the third and fourth voltages, the method comprising coupling the second plates of the first and second input capacitors to the first and third voltages respectively whilst maintaining the first plates of the capacitors at a reference voltage, to charge the input capacitors; and then coupling the second plates of the first and second input capacitors to the second and fourth voltages respectively and allowing the charge on the first plates of the capacitors to be shared such that the potential of the first plates changes by an amount dependent upon both said first and second voltage differences to cause said transconductance amplifier to generate output current substantially linearly dependent upon both said voltage differences.  
           [0029]    The input capacitors may be of different sizes or values, thus providing proportionate scaling of the respective input voltages.  
           [0030]    There is also provided a difference amplifier configured to operate in accordance with these methods.  
           [0031]    In a further aspect the invention provides a method of operating a two-stage analogue-to-digital converter, the two-stage analogue-to-digital converter comprising a first analogue to digital converter to provide a coarse approximation to an analogue input signal and a second analogue-to-digital converter comprising at least one difference amplifier configured for providing an output current dependent upon a difference between a first input voltage and a second input voltage, the difference amplifier comprising a transconductance amplifier for providing said output current for a comparator for providing a digital output, at least one switched input sampling capacitor coupled to an input of said transconductance amplifier, and at least one switch configured to couple said input sampling capacitor alternately to said first and second input voltages, the method comprising controlling said switch to couple said input sampling capacitor first to an analogue voltage for conversion and then to said coarse input signal approximation to provide a reference voltage.  
           [0032]    Preferably the difference amplifier output current is substantially linearly dependent upon the difference between the first and second input voltages. More preferably the method comprises using a plurality of difference amplifiers for comparing the coarse input signal approximation with a plurality of reference levels, preferably by offsetting the reference levels of the difference amplifiers using a second switched input sampling capacitor for each amplifier.  
           [0033]    The invention also provides an analogue-to-digital converter configured to operate in accordance with this method. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]    These and other aspects of the present invention will now be further described, by way of example only, with reference to the accompanying figures in which:  
         [0035]    [0035]FIG. 1 a  to  1   c , show respectively, a generalised circuit diagram of an exemplary stage of a current-mode interpolating analogue-to-digital converter, an example of a differential transconductance amplifier, and an example of a controllable current generator;  
         [0036]    [0036]FIG. 2 shows a typical two-stage analogue-to-digital converter; FIGS. 3 a  to  3   d  show, respectively, a difference amplifier according to a first embodiment of the present invention, waveforms for the circuit of FIG. 3 a , a differential version of the amplifier of FIG. 3 a , and timing waveforms for the circuit of FIG. 3 c;    
         [0037]    [0037]FIG. 4 shows a difference amplifier with multiple input capacitors, according to a second embodiment of the present invention;  
         [0038]    [0038]FIG. 5 shows a difference amplifier including a plurality of transconductance amplifiers according to a third embodiment of the present invention;  
         [0039]    [0039]FIG. 6 shows a difference amplifier with a mirrored output, according to a fourth embodiment of the present invention;  
         [0040]    [0040]FIG. 7 shows a difference amplifier with a split current output, according to a fifth embodiment of the present invention;  
         [0041]    [0041]FIG. 8 shows a current-mode interpolating ADC comprising a plurality of sampling difference amplifiers, according to an embodiment of an aspect of the present invention;  
         [0042]    [0042]FIGS. 9 a  and  9   b  show, respectively, a portion of a differential version of the interpolating ADC of FIG. 8, and a differential input current comparator; and  
         [0043]    [0043]FIG. 10 shows an embodiment of a differential difference amplifier according to an aspect of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0044]    Referring first to FIG. 3 a , this shows a schematic circuit diagram of a difference amplifier  300 . An input voltage Vin is provided on line  302  and a reference voltage Vref is provided on line  304 . An input sampling capacitor C1  306  has one plate coupled to a pair of switches S1  308 , S2  310 , respectively connecting lines  302  and  304  to capacitor  306 . Switches  308  and  310  are controllable to allow either line  302  or line  304  to be connected to input sampling capacitor  306  and may comprise MOSFET or FET switches. The skilled person will appreciate that the illustration of switches  308  and  310  in FIG. 3 a  is diagrammatic and that other functionally equivalent switching configurations may be employed.  
         [0045]    The other plate of the input sampling capacitor  306  is coupled to an inverting input of a differential transconductance amplifier  312  which provides a current output Iout on line  314 . A third switch  316  is connected between output line  314  and node X  318  between input sampling capacitor  306  and the inverting input of transconductance amplifier  312 . Again switch S3  316  is controllable and may comprise a MOSFET or a FET. A non-inverting differential input of transconductance amplifier  312  is connected to a fixed potential, as illustrated a ground potential Vgnd. Transconductance amplifier G1  312  has a transconductance of Gm. The three switches  308 ,  310  and  316  are controlled by a clock generator  322 , as described in more detail below with reference to FIG. 3 b.    
         [0046]    In many applications difference amplifier  300  is constructed on an integrated circuit, typically comprising part of an analogue-to-digital converter. The skilled person will understand that standard building blocks may be employed for the various elements and the transconductance amplifier may be implemented, for example, using a long-tailed bipolar or MOS pair.  
         [0047]    Referring now to FIG. 3 b , the circuit has two clock phases, Phi1 and Phi2. FIG. 3 b  shows the respective statuses  350 ,  352 ,  354  of switches S1  308 , S2  310  and S3  316  during these two clock phases as well as respective voltages  356 ,  358  at node Y  317  and node X  318 . During clock phase Phil switch S3  316  is closed to connect together the output and the (inverting) input of transconductance amplifier  312 , so that the voltage at node X  318  settles to the same voltage as that on the inverting input  320  of transconductance amplifier  312 , that is Vgnd. This is because node X  318  is a virtual earth, although the skilled person will understand that in this context the voltage at node X is substantially the same as the voltage on non-inverting input  320  but, in some embodiments, this voltage may not be zero volts.  
         [0048]    Also during clock phase Phil switch S1  308  is closed to connect the left hand side of input sampling capacitor C1  306  (node Y) to Vin  302 , and switch S2  310  is open. Thus the voltage of node Y is equal to Vin. In the second clock phase Phi2 switch S3  316  is opened, switch S1  308  is opened and switch S2  310  is closed, causing the voltage at node Y  317  to step up by ΔV=Vref−Vin. This in turn causes the voltage at node X  318  to step up by the same amount from Vgnd, causing an output current of Gm x (Vin−Vref), the inversion arising because node X is coupled to the inverting input of transconductance amplifier  312 . In practical implementations there is additional capacitance associated with node X, such as the input capacitance of transconductance amplifier  312 , and thus the voltage step and consequent output current are attenuated. However this is merely equivalent to a reduction in Gm so that the output current is nonetheless proportional to the difference between Vin and Vref.  
         [0049]    As illustrated in FIG. 3 b  it is preferable that switch S3  316  is turned off slightly before S1  308 , to improve charge injection performance. Preferably there is also a slight underlap between S1 and S2 being closed, to avoid a momentary conductive path between Vin and Vref, which could give avoidable load transients on the preceding circuitry generating Vin and Vref.  
         [0050]    [0050]FIG. 3 c  shows one example of a differential implementation  370  of the differential transconductance amplifier  300  of FIG. 3 a , and FIG. 3 d  shows timing waveforms for the circuit of FIG. 3 c.    
         [0051]    Broadly speaking the differential circuit of FIG. 3 c  corresponds to two matched circuits of the type shown in FIG. 3 a , although a single clock/timing generator  372  can be used to control both halves of the differential implementation  370 . Apart from this, two circuits of the type shown in FIG. 3 a  are used, one to provide a positive Iout  314   a  and one to provide a negative Iout− 314   b  from respective positive and negative differential inputs Vin  302   a, b  and Vref  304   a, b . In FIG. 3 c  like elements to those of FIG. 3 a  are indicated by like reference numerals, “a” and “b” denoting positive and negative signal processing portions of the circuit respectively.  
         [0052]    In mathematical terms, taking the transconductance of amplifier  312   a  to be Gm +  and the transconductance of amplifier  312   b  to be Gm − ,  
           Iout   +   =Gm   + ( Vin   +   −Vref   + )  
           Iout   −   =Gm   − ( Vin   −   −Vref   − )              dIout   =              Iout   +     -     Iout   -                   =                Gm   +          (       Vin   +     -     Vref   +       )       -       Gm   -          (       Vin   -     -     Vref   -       )                                       
         [0053]    and where Gm + =Gm − =Gm, say, (implying that differential transconductance amplifiers G1  312   a  and G2  312   b  are substantially matched):  
           dIout=Gm ( dVin−dVref )  
         where  dVin=Vin   +   −Vin   −   
         and  dVref=Vref   +   −Vref   −   
         [0054]    In this way, providing that the positive and negative signal processing circuit portions of the differential difference amplifier implementation  370  are substantially symmetrical and matched, common mode changes to Vin + , Vin −  and/or Vref + , Vref −  are substantially rejected.  
         [0055]    The timing diagram shown in FIG. 3 d  for the circuit of FIG. 3 c  is very similar to the diagram of FIG. 3 b  for the circuit of FIG. 3 a , waveforms  380 ,  382 ,  384 ,  386  and  388  corresponding to waveforms  350 ,  352 ,  354 ,  356  and  358  respectively. Attention is drawn, however, to waveforms  380 ,  382  and  384  which refer to switches  1  and  4 , switches  2  and  5  and switches  3  and  6  respectively rather than simply to switches  1 ,  2  and  3  as in FIG. 3 b . Apart from this difference, which takes account of the two matched versions of the circuit of FIG. 3 a  providing a differential implementation of the circuit, the explanation of FIG. 3 d  corresponds to that already given for FIG. 3 b  and, for brevity, will not be repeated.  
         [0056]    The skilled person will appreciate that variants on the circuit described with reference to FIGS. 3 c  and  3   d  are possible. For example transconductance amplifiers  312   a  and  312   b  may be implemented as a single fully differential transconductance amplifier.  
         [0057]    [0057]FIG. 4 shows a second difference amplifier  400 , generalizing the switched input sampling capacitor concept illustrated in FIG. 3 a . Like elements to those of FIG. 3 a  are indicated by like reference numerals. In FIG. 4 an additional input sampling capacitor  406  is provided, also coupled to node X  318 . The other plate of capacitor  406  is coupled via a first switch S1a  408  to Vin2  402  and via S2a  410  to Vref2  404 . Switches S1a  408  and S2a  410  are operated in synchrony with switches S1  308  and S2  310  respectively; for simplicity the clock generator is not shown in FIG. 4. FIG. 4 does show the total parasitic capacitance (including the input capacitance of transconductance amplifier  312 ) on the inverting input of the transconductance amplifier explicitly as a lumped capacitor Cp  414 . The dashed line  412  connected to node X  318  indicates that the circuit of FIG. 4 may be extended by adding further input sampling capacitors and associated switches. Ignoring, for the time being, the parasitic capacitance Cp, the total capacitance is C1+C2. The left hand plate of C1 steps up by a voltage Vref−Vin and the left hand plate of C2 steps up by a voltage Vref2−Vin2. The total charge stored on the two capacitors is  
         ( Vref−Vin ) C 1+( Vref 2− Vin 2)C2  
         [0058]    which when shared over the total capacitance C1+C2 gives rise to a voltage change ΔV given by  
         ( C 1 +C 2) Δ V =( Vref−Vin ) C 1+( Vref 2− Vin 2) C 2.  
         or  
         Δ V =[( Vref−Vin ) C 1+( Vref 2 −Vin 2) C 2]/( C 1 +C 2)  
         [0059]    Where the parasitic capacitance Cp is non-zero the charge is also shared over Cp so that the total effective capacitance is C1+C2+Cp. In this case the voltage change in Phi2, dVin, is given by  
                           -   dVin     =            -     (     Vx   -   Vgnd     )                   =              [         (     Vin   -   Vref     )        C1     +       (     Vin2   -   Vref2     )        C2       ]     /                          (     C1   +   C2   +   Cp     )                     and             Iout   =     -     G   .   dVin                         Iout   =              G        [         (     Vin   -   Vref     )        C1     +       (     Vin2   -   Vref2     )        C2       ]       /                          (     C1   +   C2   +   Cp     )                         (     Equation                 1     )                               
 
         [0060]    Often Cp is voltage dependent, however in practical designs this dependence has proven small enough not to be a significant constraint on linearity.  
         [0061]    Referring now to FIG. 5, this shows a third difference amplifier  500 , generalising on the arrangement of FIG. 4. Like elements to those of FIG. 4 are indicated by like reference numerals. In FIG. 5 a plurality of transconductance amplifiers is provided, illustrated by second differential transconductance amplifier G2  502  and Nth differential transconductance amplifier GN  508 . Transconductance amplifiers  502  and  508  have their inverting inputs connected to node X  318  and, in the illustrated embodiments, their non-inverting inputs connected to Vgnd  320 . Each of these additional transconductance amplifiers has a respective current output  506 ,  512  and, optionally, a closed loop feedback switch S32  504 , S3N  510  to establish node X  318  as a virtual earth. It will be appreciated, however, that only one of switches S31  316 , S32  504  and S3N  510  is required.  
         [0062]    Each of the differential transconductance amplifiers G1, G2, . . . , GN may have a different transconductance to provide, in effect, a difference amplifier with a plurality of transconductances and a plurality of output, currents in fixed scales or ratios to one another. In the arrangement of FIG. 6 the output  314  of transconductance amplifier  312  is mirrored by a current mirror  602  to provide multiple outputs. The current mirror comprises a plurality of current sinks  602   a - d  (alternatively current sources may be employed) to provide up to N current outputs of which three  604   a - c  are shown in FIG. 6. Each of current outputs is in a fixed ratio to or multiple of current Iout on transconductance amplifier output  314  and in this way the difference amplifier  600  may be provided with a plurality of outputs with different (overall) transconductances. The multiple output current mirror  602  may be constructed in any conventional manner using FET or bipolar transistors of different sizes or using multiple transistors to provide current ratios other than 1:1. As illustrated in FIG. 6 switch S3  316  may be coupled either directly to the current output  314  of transconductance amplifier  312  or to one of current outputs  604   a - c  to provide feedback either from the first stage output or from one (or more or all) of the mirror outputs. However this depends upon the polarity of the first stage transconductance in that there should be an overall inversion to allow the closure of S3  316  to create a virtual earth at node X  318 .  
         [0063]    [0063]FIG. 7 shows a fifth difference amplifier  700 , again generalising on the arrangement of FIG. 4, and again in which like elements of those of FIG. 4 are indicated by like reference numerals.  
         [0064]    In the arrangement of FIG. 7 the output current Iout on output  314  of transconductance amplifier  312  is split by a plurality of transistors  702   a - c  into a plurality of output currents Iout 1, Iout 2, . . . . . Iout N  704   a - c . Transistors  702   a - c  are connected in parallel and have a common control connection  706 ; in the illustrated embodiment the transistors comprise field effect transistors, preferably NMOS devices, with a common source connection to current output  314  and a common gate connection to a bias voltage on line  706 . The drain connections of the transistors provide current outputs  704   a - c.    
         [0065]    To allow bi-directional output currents Iout 1, Iout 2, . . . , Iout N a constant current sink  708  may be connected to the common source connection of bias transistors  702   a - c , and companion constant current sources (not shown in FIG. 7) connected to the current outputs to subtract off the added constant current before the output Iout1, Iout2, . . . IoutN.  
         [0066]    As with the arrangement of FIG. 6, the sizes (or other parameters) of transistors  702   a -c may be in fixed ratios to generate output currents in fixed ratios to one another. For this reason transistor  702   a - c  are preferably matched, for example by fabrication on a common substrate. In this way again a plurality of different overall transconductances may be provided for difference amplifier  700 . The current introduced by constant current sink  708  will also be scaled in the same ratios, so the above-mentioned companion current sources should also be ratioed in the same ratios.  
         [0067]    A differential output equivalent to each of the above-described difference amplifiers, in particular the difference amplifier circuits of FIGS.  4  to  7  and the difference amplifiers of the interpolating ADC stage of FIG. 8 below, may be implemented by replacing all the signal paths in each amplifier by differential equivalents. This has been already been described with reference to FIG. 3 and will be readily understood by the skilled addressee.  
         [0068]    [0068]FIG. 8 shows a current mode interpolating ADC  800  incorporating a plurality of the above-described difference amplifiers. The switched input sampling capacitor in each of these difference amplifiers provides a relatively cheap and simple alternative to the sample-and-hold circuit which is conventionally employed.  
         [0069]    The ADC of FIG. 8 provides a 6-bit conversion accuracy and may be employed, for example, as a 6-bit back end to follow a 10-bit pipeline, to produce a 16-bit analogue-to-digital converter.  
         [0070]    Referring to FIG. 8 an analogue input voltage Vin on line  802  is provided to a flash ADC  804  and to one input of each of a plurality of transconductance difference amplifiers  816   a - d , four in the illustrated example. Flash ADC  804  has eight coarse (MSB) comparators with thresholds at −7/8, −5/8, −3/8, −1/8, +1/8, +3/8, +5/8 and +7/8 of the reference range. These thresholds determine the input voltage to be in one of nine regions, centred on −8/8, −6/8, −4/8, −2/8, 0, +2/8, +4/8, +6/8, +8/8. The flash ADC  804  provides a digital output on line  806  which is provided to a combiner  808  and to a digital-to-analogue converter  812 . This digital output comprises a sign bit plus three additional bits to define a 2&#39;s complement number in the range [−4, +4] (not all of the 16 codes defined by the 4 bits are used). Combiner  808  adds the MSB information from the flash ADC to LSB information on bus  836  with appropriate delays to provide a 6-bit output on bus  810 .  
         [0071]    The most significant bits received by DAC  812  are converted back to an analogue voltage VrefM on line  814 , which is provided to a second input of each of transconductance difference amplifiers  816   a - d . The DAC  812  may comprise, for example, a multiplexer constructed from a plurality of transmission gates configured to select taps on a resistor string.  
         [0072]    The possible VretM voltages on line  814  are −8/8, −6/8, −4/8, −2/8, 0, +2/8, +4/8, +6/8 and +8/8 or, equivalently, −4/4, −3/4, −2/4, −1/4, 0, +1/4, +2/4, +3/4 and +4/4 of the reference range. It will be appreciated that the value of VrefM output by DAC  812  will lie in the middle of the region of the reference range in which flash ADC  804  determines the input voltage on line  802  to lie. The voltage VrefM is fed to the reference inputs of the four difference amplifiers  816   a - d , that is the input corresponding to input  304  of FIG. 4, and the input voltage Vin is fed to the associated input of difference amplifiers  816   a - d , that is to the input corresponding to input  302  of the difference amplifier illustrated in FIG. 4.  
         [0073]    The pair of inputs  818   a - d  of each of the four difference amplifiers  816   a - d  thus corresponds to inputs  302  and  304  of the difference amplifier of FIG. 4. Each of difference amplifiers  816   a - d  also has a second pair of inputs  820   a - d  corresponding to inputs  402  and  404  of the difference amplifier of FIG. 4. Input pair  818  is connected to an input sampling capacitor  826  (C1 in FIG. 4) and input pair  820  is connected to a second input sampling capacitor  828  (C2 in FIG. 4). The transconductance amplifier sides of both these capacitors are connected together and to the inverting input of transconductance amplifier  830  (as shown in FIG. 4). The output of transconductance amplifier  830  is mirrored or split as illustrated in FIG. 6 or  7 , or a plurality of transconductance amplifiers may be employed as illustrated in FIG. 5, to provide a plurality of current outputs  832  for each of difference amplifiers  816   a - d . For example difference amplifier  816   a  provides current outputs in the ratio 1.0:0.8:0.6:0.4:0.2. Current outputs from difference amplifiers  816   a - d  are summed as illustrated in FIG. 8 in a conventional manner for an interpolating ADC, for example as described with reference to FIG. 1. Multiple current outputs at a given scaling factor, for example 0.2, may be provided where the current output at that scaling factor is used more than once. This may be implemented by a conventional current mirror with one output transistor for each of outputs  832  along the lines illustrated in FIG. 6. Alternatively arrangements such as those shown in FIG. 5 and/or  7  may be employed. For simplicity the clock generator controlling difference amplifiers  816   a - d  is not shown in FIG. 8.  
         [0074]    Each pair of inputs  820   a - d  has a first input connected to a common reference voltage line  822 , as illustrated at the 0 volts, and a second input connected to a respective second reference voltage line  824   a - d . The second reference voltages on lines  824   a - d  form a ladder of reference voltages, in the illustrated example spaced apart by 5/16 of the reference range. Accordingly the reference voltage applied to each of difference amplifiers  816   d - a  is −8/16, −3/16, +2/16 and +7/16 of the reference range and varies linearly. Capacitor  826  of each difference amplifier (C1 in FIG. 4) has a value twice that of capacitor  828  (C2 in FIG. 4). Choosing C1 larger than C2 reduces the attenuation in the signal path from Vin to the transconductance amplifier input. This capacitor ratio of 1/2 sets the LSB conversion range to be from −8/32 to +7/32 of the reference range and, due to the double subtraction (i.e. because the difference amplifier output is also dependent upon Vin−VrefM), this LSB conversion range is centred around the approximation to the input voltage VrefM. The input range extrema are +31/32 and −32/32 to give  64  codes, so that the LSB step size is 1/32=1/2 5  even though the illustrated converter is a 6-bit converter.  
         [0075]    Current interpolation between the outputs  832  of difference amplifiers  816   a - d  generates a range of currents with zero crossing thresholds equivalent to one LSB apart. To achieve this there are five weighted current mirrors between adjacent difference amplifiers, giving  16  output taps including endpoints. The threshold determined by the current output of difference amplifier  816   a  alone is +7/32 of the reference range; the threshold determined by difference amplifier  816   b  alone is +2/32 of the reference range; the threshold determined by difference amplifier  816   c  is −3/32; and the threshold determined by difference amplifier  816   d  is −8/32 of the reference range. There are four intermediate thresholds between each of these formed by summing fifths of the output currents, in a similar manner to the way in which, in the arrangement of FIG. 1, three intermediate thresholds are defined by summing quarters of the output currents. Zero-crossing comparators  834  on these 16 taps determine the difference (VrefM−Vin) to lie within one of 17 regions, and provide a thermometer code output on bus  836 . This thermometer code is converted to a conventional binary code for combining with the MSBs on bus  806  by combiner  808 . The use of current interpolation rather than resistive interpolation improves the linearity of the ADC by removing end effects due to mismatched impedances at the top and bottom of a resistive interpolation string.  
         [0076]    The foregoing discussion may be clarified by an example. Difference amplifier  816   a  has capacitor C2  828  switched between +7/16 and 0 volts. Inspection of equation 1 shows that when capacitor C2 is half the value of C1 (capacitor  826 ) the voltage difference on inputs  820   a  is effectively given half the weight of the voltage difference on inputs  818   a  of difference amplifier  816   a . Thus the zero crossing threshold for difference amplifier  816   a  is shifted by +7/32. Since the voltage difference on input pair  820   a  is, in effect, of opposite sign to the voltage difference on input pair  818   a  the voltage Vin on line  802  must be+7/32 of the reference range higher than VrefM on line  814  for zero output current. Thus it can be seen that the “1.0 times” output of difference amplifier  816   a  defines a threshold at +7/32 of the reference range. The “1.0 times” output of difference amplifier  816   b  defines a threshold at +2/32 of the reference range in a similar way. The +6/32 threshold is defined by summing 0.8 of the current output of difference amplifier  816   a  and 0.2 of the current output of difference amplifier  816   b , according to the equation 0.8×(7/32)+0.2×(2/32)=6/32. The other thresholds are defined similarly at 2/5, 3/5 and 4/5 of the difference between +7/32 and +2/32. The thresholds from +1/32 to −8/32 are defined in a corresponding manner.  
         [0077]    Strictly speaking only 8 comparators are need to cover the full range between each VrefM, the additional comparators in effect providing an extra MSB. However it is preferable to provide additional comparators since these provide a margin for error in the comparator thresholds of flash ADC  804 . The 16 thresholds define  17  regions (compare the description of Flash ADC  804  above) and the 16 outputs from comparators  834  are preferably encoded as a twos complement LSB value in the range [−8, +8], although in other embodiments other schemes and ranges may be employed. The complete digital output value may then be calculated simply by adding the 2&#39;s Complement MSB&#39;s from flash ADC  804  to the 2&#39;s complement LSBs from the second, interpolating ADC stage, appropriately shifting the MSBs left, according to the equation:  
         Output=8 ×MSB+LSB.    
         [0078]    Adding the digital output signals from the two stages in this way automatically provides correction for conversion errors made by flash ADC  804  for example because of offset voltages in the flash ADC comparators. Even if the digital output from flash ADC  804  is “incorrect”, the output from the DAC will still be consistent with this digital output, which will alter the reported second stage region by an amount which automatically corrects for the error in VrefM once the respective digital outputs are combined. This LSB over-range can accommodate an offset in the flash ADC comparators of approximately 1/16 of the (flash) reference range.  
         [0079]    A further benefit of providing additional comparators in excess of the number required for the desired number of LSBs accuracy is to provide the two-stage ADC  800 , considered as a whole, with an over-range capability. This is because the overall digital output extends over the range [−40, +40], a total of 81 codes, because in effect there are 8 additional codes provided by the second-stage, difference amplifier-based ADC at either end of the range. This is because the extreme codes from the flash correspond to regions centred on+/−8/8 Vref, giving nominally 0/64 at the LSB stage for nominal full-scale inputs, thus giving  8  extra codes at either end. Of these codes,  64  can be considered “in range”, corresponding to inputs that lie within the reference range and the remainder can be considered “over-range” codes. Where, as mentioned above, the ADC is used to provide a backend to a pipeline or other converter, this over-range capability may be employed to correct comparator offset voltages or other errors in a preceding converter or converters. The arrangement of FIG. 8 shows a single-ended rather than a differential analogue-to-digital converter but the skilled person will readily recognise the architecture may be modified to provide a differential implementation in a routine manner as indicated in FIG. 9 a , which shows a portion of a differential version 900 of the interpolating ADC of FIG. 8, in which like elements to those of FIG. 8 are indicated by like reference numerals. A single-ended implementation has been described to facilitate understanding of the invention but in many instances a differential implementation is preferred as it enables an improvement in signal-to-noise ratio, as internal signal swings may be doubled for a fixed supply headroom and extraneous interfering signals are rejected.  
         [0080]    Referring to FIG. 9 a , this illustrates a portion of one embodiment of a differential version 900 of the single-ended circuit of FIG. 8. The interpolating ADC  900  has a differential voltage input Vin+802, Vin− 802 ′ which is provided to both a differential input Flash ADC  804 ′ and to a plurality of pairs of transconductance difference amplifiers of which one pair  816   a ,  816   a ′ is shown. Flash ADC  804 ′ drives a differential output DAC  812 ′ which in turn drives the pairs of difference amplifiers.  
         [0081]    The positive voltage input Vin+802 goes to a first  816   a  transconductance difference amplifier of the pair and the negative voltage input Vin− 802 ′ goes to the second  816   a ′ transconductance difference amplifier of the pair. The transconductance difference amplifiers  816   a ,  816   a ′ of the pair are substantially identical, one outputting a positive current Iout+, the other  816   a ′ outputting a negative output current Iout−. This pair of output currents, and each other pair of output currents from the remaining pairs of transconductance difference amplifiers (not shown in FIG. 9 a ) are provided, scaled in a similar way to that described with respect to FIG. 8, to differential inputs of a set of differential input current comparators  834 ′. Positive and negative versions of the difference amplifier reference voltages are provided, one to each of the difference amplifiers of each pair. Thus, for example, a reference  824   a  at +7/16 of the reference range is provided to difference amplifier  816   a  and a reference  824 ′ at −7/16 of the reference range is provided to difference amplifier  816   a ′. Differential versions of each of the other reference voltages are provided to corresponding ones of the other pairs of difference amplifiers. The common reference voltage  822  is provided to both the positive  816   a  and negative  816   a ′ voltage processing difference amplifiers and, in embodiments, may be left floating as its voltage is “common mode” to each of the pairs of difference amplifiers. With this arrangement the reference voltages of the single-ended version of the ADC of FIG. 8, Vref, are replaced by differential pairs of reference voltages Vref+, Vref− each of the same amplitude as the previous single-ended Vref. This has the effect of doubling the “full scale” input range (i.e. ignoring over-range capability) since full scale is reached at Vin+=+2.0 and Vin−=−2.0 rather than at Vin=1.0, −1.0. However the same full scale range may be obtained by halving the reference voltages.  
         [0082]    [0082]FIG. 9 b  shows one implementation of a differential input current comparator  910  suitable for use in comparator bank  834 ′ of the differential interpolating ADC  900 . Current comparator  910  has a pair of differential current inputs Iin+ 912, Iin− 912 ′, each connected to a respective resistor  914 ,  914 ′, the other ends of these resistors being connected together to a common-mode voltage source VR  916 . The inputs  912 ,  912 ′ are also connected to respective non-inverting and inverting inputs of a conventional differential input voltage comparator  918  which in turn provides an output  920 . The skilled person will recognise that the arrangement of FIG. 9 b  may be modified to suit a particular application and/or to meet bias constraints on the transconductance amplifier stages. For example one or more common gate stages may be added in series with each input, or MOS resistors may be employed for resistors  914 ,  914 ′, or these resistors may be replaced by a cross-coupled MOS transistors to increase the voltage gain of the comparator.  
         [0083]    [0083]FIG. 10 shows one example of a differential difference amplifier suitable for use for the pair of positive  816   a , and negative  816   a ′ difference amplifiers described above with reference to FIG. 9 a . Although the differential difference amplifier of FIG. 10 may be considered as a pair of differential amplifiers it is perhaps better viewed as a combined differential difference amplifier  1000 .  
         [0084]    Broadly speaking the differential difference amplifier  1000  of FIG. 10 is a combination of two single-ended differential amplifiers of the type shown in FIG. 4 with a differential transconductance amplifier similar to that shown to that in FIG. 1 b . Thus in FIG. 10 the circuitry to the left of line A′-A′ corresponds to the input circuitry of the FIG. 4 difference amplifier embodiment, as does the circuitry to the right of line B′-B′, these two sets of input circuitry providing differential inputs A and B to a differential transconductance amplifier. The reference numerals for the circuit portions correspond to those of FIG. 4 and, for convenience, reference numerals for the lines to which the differential difference amplifier is attached when used in the analogue-to-digital converter of FIG. 9 a  are indicated in brackets. Broadly speaking elements  1002 ,  1004 ,  1006 , and the combination of  1014  and  1016  of FIG. 10 correspond to elements  136 ,  132 ,  134  and  138  of FIG. 1 b  respectively.  
         [0085]    The differential transconductance amplifier comprises a current sink  1002  coupled to the source connections of a differential pair of input transistors  1004 ,  1006 , each of which has a drain connected to a respective (active load FET) transistor  1014 ,  1016 .  
         [0086]    The current through FETs  1014  and  1016  is mirrored by respective sets of scaled PMOS transistors  1018 ,  1020  to provide scaled sets of differential (current) outputs, in a broadly similar manner to the NMOS devices  602  of FIG. 6. Respective switches  1008 ,  1010  are provided for transistors  1004 ,  1006 , each corresponding to switch S3  316  of FIG. 4, and connected to a voltage preset line  1012  (to preset the voltages on the transconductance amplifier side plates of capacitors C1 and C2), in the illustrated embodiment connected to ground. In FIG. 10 switches S1, S2, S1a, S2a,  1008  and  1010  are controlled by a controller or clock generator (not shown in FIG. 10, for clarity) in a similar manner to that described above with reference to FIGS. 3 b  and  3   d.    
         [0087]    In a first state or clock phase switches S1 and S1a are closed to couple first voltage inputs on lines  302  and  402  to one side of the input sampling capacitors C1 and C2 and switches  1008  and  1010  are closed to allow the other sides of these capacitors to come to an initial voltage either by sharing charge or by having their plates connected to a bias or virtual earth voltage. Then in a subsequent state or clock phase these switches are all opened and switches S2 and S2a are closed to apply second input voltages to capacitors C1 and C2 to provide differential input voltage changes dependent upon the differences between the first and second input voltages of each set to the differential inputs of the transconductance amplifier.  
         [0088]    In operation the input stages of differential difference amplifier  1000  operate similarly to the input stages of the difference amplifier of FIG. 54. Thus input A to the differential transconductance amplifier receives a change in voltage dependent upon the difference between Vin +  and VrefM +  plus an offset (scaled by capacitor C2) dependent upon the difference between the voltage on line  824   a  and that on line  822 . Likewise input B receives a change in voltage dependent upon the difference between Vin −  and VrefM −  plus an offset (scaled by capacitor C2) dependent upon the voltage difference between lines  824   a ′ and  822 . The transconductance amplifier provides an output current dependent upon the difference between the voltages at differential input nodes A and B and provides a differential output (in other embodiments this could be a single-ended output) across load FETs  1014  and  1016 , these output currents being mirrored by sets of FETs  1018  and  1020  respectively. In other words, one of nodes A and B may be considered as a positive differential input and the other considered as a negative differential input and the differential transconductance amplifier provides a differential output comprising output currents of substantially the same magnitude but of opposite sign.  
         [0089]    The skilled person will recognise that a number of variations on the circuit of FIG. 10 are possible. For example the current mirrors made up of transistors  1014 ,  1018 ,  1016 ,  1020  could be replaced an output stage similar to that shown in FIG. 7 comprising FETs  702 .  
         [0090]    Additionally or alternatively elements  1002 ,  1004  and  1006  could be replicated along the lines indicated in FIG. 5 and the PMOS current mirrors could be reduced in number or removed. Switches  1008  and  1010 , which are shown connected to a bias voltage such as a ground voltage, could be connected instead to a virtual earth, for example to the respective gates of transistors  1014  and  1016  (switches  1008  and  1010  then being separate rather than connected together). The skilled person will further recognise that the common reference voltage line  822  may be any convenient bias voltage, and that in embodiments this line may be allowed to float (that is not connected to any specific bias voltage) in which case capacitors C2  406  would share charge when switches S2a  410  were closed (i.e. on).  
         [0091]    The requirements on a transconductance amplifier used in an interpolating ADC, such as ADC  800  of FIG. 8, are relatively modest. The accuracy of the converter depends upon the accuracy of the two difference amplifiers that have the smallest input signals. For the example of FIG. 8 the only amplifiers participating in any decision will at most have about +/−4/64 input voltage, and the resulting output need only be correct to say 0.5LSB=1/64. For this reason in many such applications the difference amplifiers do not require any feedback, nor must they be particularly high gain, or low offset, or accurate settling difference amplifiers. Settling time problems are unlikely to arise with the output of the most critical difference amplifier as the output of this amplifier only has to settle to a small voltage, since the accuracy is only critical near the threshold points, where the amplifier sees only small differential voltages. The difference amplifiers can therefore be very small and simple and the transconductance amplifier need only comprise a simple long-tailed bipolar or MOS pair, as shown for example in FIG. 1 b . Nonetheless the difference amplifiers are still the most sensitive part of the design.  
         [0092]    The gain from the ADC signal input line  802  to the inputs to comparators  834  is typically of the order of Gm/Gout, where Gout is the output impedance at the scaled current outputs (reduced by the capacitive attenuation at the differential amplifier input capacitive network). This is equivalent to the preamplifier often included before a comparator in a conventional ADC, so provides a sufficiently large comparator overdrive to allow the use of a simple comparator architecture, such as a simple differential latch, without a separate preamplification stage.  
         [0093]    The reference voltages for lines  824   a - d  may be generated by a resistor string and preferably this is arranged to have sufficiently low impedance to be able to provide the current required by the switched input sampling capacitors (such as capacitor  828  in FIG. 8) without suffering significant loading effects. It is further preferable that the second stage LSB over-range provision is sufficient to overcome offset errors in the MSB (flash ADC) comparators.  
         [0094]    No doubt many effective variants of the above-described circuits will occur to the skilled person. For example switch S3  316  in the circuit of FIG. 3 a  may be connected to a reference voltage to set a charge on capacitor C1  306  rather than to the output  314  of transconductance amplifier  312  and, optionally, an additional switch may be interposed between node X  318  and the inverting input of transconductance amplifier  312  (although this would effectively remove the auto-zero function of the circuit). Likewise it is generally possible to exchange current sources for sinks and vice versa. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.