Abstract:
An analog to digital converter includes an array of differential input amplifiers. Each amplifier inputs an input voltage and a corresponding voltage reference, and outputs a differential signal representing a comparison of the input voltage and the corresponding voltage reference. A plurality of latches stores the differential signal from each of the differential input amplifiers. A decoder converts the stored differential signals to N-bit digital output. A first interface amplifier is connected to a first edge amplifier of the array through a first cross point. A second interface amplifier is connected to a second edge amplifier of the array through a second cross point. The first interface amplifier and the second interface amplifier are connected to each other through a third cross point.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority to commonly assigned U.S. Provisional Patent Application No. 60/407,312, filed on Sep. 3, 2002, which is incorporated by reference herein. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    Field of the Invention  
           [0003]    Applications such as disk drive read channels, fiber optical receivers, front end and data communication links using multi-level signaling (e.g. PAM and QAM) require high speed analog-to-digital converters (ADCs). It is of particular interest that such ADCs be fabricated in a standard CMOS technology. The main challenges in ADC performance are as follows: static and dynamic offset, low supply operation, design trade-offs between power, speed and chip area, time interleaving, gain-bandwidth optimization, high-speed input signal feed through and interpolation.  
           [0004]    [0004]FIG. 1 shows a generic, conventional analog-to-digital converter. As shown in FIG. 1, the conventional analog-to-digital converter includes a resistor ladder  104  comprised of equal resistors R, an amplifier array  101 , a corresponding array of latches  102 , and a decode circuit  103 .  
           [0005]    The transistor performance in 0.18 μm CMOS process is ruled by threshold mismatch. Averaging, proposed by Kattman and Barrow, see “A Technique for Reducing Differential Nonlinearity Errors in Flash A/D Converters,”  ISSCC Digest of Technical Papers , pp. 170-171, February 1991, is a proven technique to reduce offsets of the amplifiers array. See also K. Bult, A. Buchwald et al., “A 170 mW 10b 50 MSample/s CMOS ADC in 1 mm 2   ,” ISSCC Digest of Technical Papers , pp. 136-137, Feb.1997; H. Pan, et al., “A 12b 65 MSample/s CMOS ADC with 82 dB SFDR at 120 MHz,”  ISSCC Digest of Technical Papers , pp. 38-39, Feb.2000; M. Choi, A. A. Abidi, “A 6b 1.3 GSample/s A/D Converter in 0.35 μm CMOS,”  ISSCC Digest of Technical Papers , pp. 126-127, February 2001; and P. Scholtens and M. Vertregt, “A 6b 1.6 GSamples/s ADC in 0.18 μm CMOS Using Averaging Termination,”  ISSCC Digest of Technical Papers , pp. 168-169, February 2002, which are incorporated by reference herein. However, an effective method to fix the boundary bending caused by offset averaging needs to be developed.  
           [0006]    In general, there are two issues at the boundaries of the averaging network. First, the zero crossing points drift from the input reference voltage level due to the symmetrical nature is destroyed at the boundaries. This cause systematic non-linearity error. FIG. 2 illustrates zero-crossing shift versus amplifier position. Amplifier  101 A is a “left edge” amplifier (see top portion of FIG. 2.) the zero crossings at the edges shift from their ideal values, causing distortion and nonlinearity. However, as may be seen from the middle portion of FIG. 2, the zero crossings in the center of the array  101  are located at the “ideal” position.  
           [0007]    Additionally, the bottom portion of FIG. 2 shows another distorting edge effect, which is referred to as “offset distribution.” As may be seen from the figure, the closer to the edge of the array  101  one gets, the greater the offset distribution, resulting in additional nonlinearity.  
           [0008]    Thus, the downstream averaging network collects fewer random components for averaging at the boundaries. This causes less differential nonlinearity/integral nonlinearity (DNL/INL) improvement. State-of-the-art ADCs either use a partial of the amplifier array as dummies to preserve the electrical behavior of an infinite array of amplifiers (see, e.g., K. Bult, A. Buchwald et al., supra and H. Pan, et al., supra), or use a termination method to suppress the zero-crossing shifts at the edge (see P. Scholtens and M. Vertregt, supra). The termination method can effectively restore the systematic error when R 0 &lt;3R 1 , namely the averaging window is narrow and boundary issue is not serious. Yet the edges with termination have less averaging.  
           [0009]    For the dummy approach (i.e., adding additional “dummy” amplifiers at the edges of array  101 ), the wider the averaging window, the more dummies are needed. For example, M. Choi, A. A. Abidi, indicate that 18 dummies are needed for an averaging window covering  18  amplifiers. Furthermore, the more dummy amplifiers that are added to the array  101 , the greater the reduction in voltage headroom available for the analog-to-digital conversion, and the greater the power consumption of the ADC. For low voltage operation, this becomes unacceptable.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention is directed to a high-speed analog-to-digital converter with low distortion that substantially obviates one or more of the problems and disadvantages of the related art.  
           [0011]    There is provided an analog to digital converter including an array of differential input amplifiers. Each amplifier inputs an input voltage and a corresponding voltage reference, and outputs a differential signal representing a comparison of the input voltage and the corresponding voltage reference. A plurality of latches store the differential signal from each of the differential input amplifiers. A decoder converts the stored differential signals to N-bit digital output. A first interface amplifier is connected to a first edge amplifier of the array through a first cross point. A second interface amplifier is connected to a second edge amplifier of the array through a second cross point. The first interface amplifier and the second interface amplifier are connected to each other through a third cross point.  
           [0012]    In another aspect there is provided an analog to digital converter including a reference ladder with a plurality of resistors and taps providing voltage references. At least one controllable current source in series with each tap adjusts the voltage reference of each tap. An array of differential input amplifiers, where each amplifier inputs an input voltage and a corresponding voltage reference, and outputs a differential signal representing a comparison of the input voltage and the corresponding voltage reference. A plurality of latches store the differential signal from each of the differential input amplifiers. A decoder converts the stored differential signals to N-bit digital output.  
           [0013]    Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.  
           [0014]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings:  
         [0016]    [0016]FIG. 1 illustrates a conventional analog-to-digital converter.  
         [0017]    [0017]FIG. 2 is a graphic illustration of the problems inherent with the conventional analog-to-digital converter.  
         [0018]    [0018]FIG. 3 illustrates an embodiment of the present invention using a triple cross connection.  
         [0019]    [0019]FIG. 4A illustrates the problem of shifting of reference voltage values in a conventional voltage reference ladder.  
         [0020]    [0020]FIG. 4B illustrates the use of programmable current sources to remedy reference voltage distortion.  
         [0021]    [0021]FIG. 5 illustrates an alternative track and hold amplifier structure of the present invention.  
         [0022]    [0022]FIG. 6 illustrates the nonlinearity performance when using the approaches of FIGS. 3 and 4A.  
         [0023]    [0023]FIG. 7 illustrates dynamic performance using the track and hold amplifier of FIG. 5.  
         [0024]    [0024]FIG. 8 illustrates the improvement and the edge effect when using the circuits illustrated in FIGS. 3 and 4B.  
         [0025]    [0025]FIG. 9 illustrates dynamic performance of an ADC with the circuit of FIG. 5.  
         [0026]    [0026]FIG. 10 illustrates overall system performance using the circuits shown in FIGS. 3, 4B and  5 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]    Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings.  
         [0028]    [0028]FIG. 3 illustrates an amplifier array of an analog-to-digital converter according to the present invention. As may be seen in FIG. 3, the amplifier array includes a plurality of amplifiers  101  (amplifier array  101 ), similar to FIG. 1. The outputs of the amplifier array  101  are fed to a resistor network comprising resistors R 0  and R 1 , as shown in the figure (the latches and decoder of FIG. 1 are not shown for clarity). In addition to the amplifier array  101 , on the edges of the amplifier array  101  there are two interface amplifiers  301 A and  301 B, as shown in the figure. The interface amplifiers  301 A,  301 B replace the dummy amplifiers of conventional analog-to-digital converter circuits. The interface amplifiers  301 A,  301 B are connected to the amplifier array  101  through two cross-points  302 B,  302 C, and to each other through a cross-point  302 A, as shown in FIG. 3. This circuit allows the elimination of the dummy amplifiers on each edge of conventional amplifier arrays, dramatically reducing power consumption, and substantially solving the voltage headroom problem.  
         [0029]    [0029]FIG. 4A illustrates the problem of reference voltage shift from a reference ladder  404 . As shown in FIG. 4A, conventional analog-to-digital converter circuits include a reference ladder  404 , with two current sources  401  on each side of the reference ladder  404 , and taps leading to a zero crossing generator and averaging network  403 . (Note that the “dots” at the V REF1  and V REF2  lines illustrate the shift in the zero crossing points that occurs due to the edge effect, as described above, and not two wires being connected together). As shown in FIG. 4A, and as discussed above with reference to FIG. 2, the zero crossing point shifts further and further from its ideal position as one gets closer to the edge of the amplifier array  101 .  
         [0030]    As an alternative, reference voltages can be pre-distorted to compensate for the zero-crossing shifts at the array boundary. The reference voltage is usually generated by a constant current flow through a resistor ladder  404 . Since it is difficult to change the resistor values, the current flowing through resistor taps can be programmed instead, as shown in FIG. 4B.  
         [0031]    [0031]FIG. 4B illustrates a solution to the problem of reference voltage shifts. As may be seen from FIG. 4B, in addition to the constant current sources  401 , each tap from the reference ladder  404  includes two programmable current sources  402 . Thus, through the use of programmable current sources  402 , the reference ladder taps at the V REF1  can be shifted back to their ideal positions at V REF2 . In other words, V REF1  is predistorted to shift V REF2  to its ideal position. Programmable current sources are well known in the art, and any number of circuits may be used. Furthermore, the programmable current sources  402  can be controlled through the use of relatively simple control logic, and may be reprogrammed “on the fly.” Thus, through the use of programmable current sources  402 , there is no longer a need for expensive trimming of resistors in the reference ladder  404 , particularly since it is not so much the absolute values of the resistors that matter, but their values relative to each other.  
         [0032]    The triple cross-connection averaging arrangement of FIG. 3, and the use of pre-distortion of the reference voltages (FIG. 4B) can maintain even averaging for a wide averaging window. The later method helps to relax the strict requirement of interface amplifiers  301 A,  301 B in the triple cross-connection averaging scheme of FIG. 3. The translational symmetry for the impulse response is maintained through triple cross-connections  302 A,  302 B,  302 C, forming a circular resistor network. The input linear region of the transconductance (interface) amplifiers  301 A,  301 B should be wide enough to cover one edge of the original amplifiers array  101 . The transconductance (interface) amplifiers  301 A,  301 B should satisfy:  
             g     m   ,              int     2     ·       h   2          (   1   )       ·     1       W   int          L   int           =       ∑     n   =     -   N         -   1                           g   m   2          (   n   )       ·       h   2          (   n   )       ·     1   WL           ,                         
 
         [0033]    where g m,int , W int  and L int  are the transconductance, transistor width and length of the interface amplifier  301 , g m , W and L correspond to original amplifier array  101 , and h(n) is the impulse response of the post-amplifier resistor network (comprised of R 0 , R 1 , see FIG. 3). This ensures a good integral non-linearity/differential non-linearity (INL/DNL) enhancement in the full range.  
         [0034]    For the reference voltages, biasing currents for the interface amplifiers  301 A,  301 B are determined such that output zero-crossings of amplifier array  101  are as even as possible. The remaining nonlinearity can be remedied by a digitally controlled reference voltage of FIG. 4B. The reference voltages are traditionally tapped from a resistor ladder  404 . The subtractors use these reference voltages to generate zero-crossings for a digitizer to make decisions.  
         [0035]    [0035]FIG. 5 illustrates an improved track and hold (T/H) amplifier that may be used in an analog-to-digital converter of the present invention. To enable the high speed and low supply operation, only open-loop and common mode logic (CML) type circuits are incorporated in this design. For Gigahertz sampling rate, using a T/H becomes essential to achieve the desired dynamic performance with a wide-band input signal.  
         [0036]    As may be seen in FIG. 5, the track and hold amplifier includes two transistors MIA, MIB driven by the CLK input, two transistors M 2 A, M 2 B driven by the CLKB input, two current sources  501 A,  501 B connected to sources of transistors M 3 A, M 3 B, respectively, and two input capacitors C+, C− (which are typically about 0.5 pF).  
         [0037]    The source follower&#39;s size (M 3 A, M 3 B) is quite large in order to drive the preamplifier bank  101  (array  101 ). The big non-linear well capacitance of M 3 A, M 3 B limits the T/H output bandwidth and adds distortion.  
         [0038]    To address the distortion due to the wells of M 3 A, M 3 B, a replica circuit of a main source follower, designated  502 , is added to the conventional track and hold amplifier. The replica circuit  502  includes two current sources  503 A,  503 B, and two transistors M 4 A, M 4 B biased by the current sources  503 A,  503 B. The addition of the replica circuit  502  provides bias to the wells of the transistors M 3 A, M 3 B, substantially reducing the nonlinearity effects that exist due to the bulk (substrate) connections of the transistors M 3 A, M 3 B. Replica circuit  502  tracks the source voltage of the main source follower and provides biasing to the wells of M 3 A, M 3 B. The replica circuit is about 5% of the main source follower size.  
         [0039]    [0039]FIG. 6 illustrates the advantages of the embodiments shown in FIGS. 3 and 4B. As may be seen from the differential nonlinearity (DNL) graph of FIG. 6, the differential nonlinearity is under 0.3 LSB (least significant bit). Without the use of the circuits of FIGS. 3 and 4B, the DNL would be on the order of 5 LSB. Similarly, the integral nonlinearity (INL) is generally under 0.5 LSB, substantially lower than the numbers seen in conventional circuits. (The X axis represents to the digital codes, i.e., for a six-bit analog-to-digital converter, there are 64 digital codes, or 0-63.) In other words, through the use of the circuits illustrated in FIGS. 3 and 4B, the nonlinearity is reduced by an order of magnitude.  
         [0040]    [0040]FIG. 7 illustrates dynamic performance achievable with the track and hold amplifier of FIG. 5. As may be seen from FIG. 7, the spurious free dynamic range (SFDR) and the signal-to-noise distortion ratio (SNDR) are extremely good all the way up to input frequencies of 1 GHz. Without the track and hold amplifier of FIG. 4, the performance would be about 15-20 dB worse than that shown in FIG. 7.  
         [0041]    [0041]FIG. 8 shows the improvement in the integral nonlinearity (INL) at the edges of the amplifier array  101  using the circuits illustrated in FIGS. 3 and 4B. As may be seen in FIG. 8, while the performance is only somewhat better in the center of the array (i.e., digital codes ranging from about 7 to about 60), there is a dramatic improvement at the edges, i.e., codes 61-63 and codes 0-6.  
         [0042]    [0042]FIG. 9 illustrates the dynamic performance of an analog-to-digital converter using the track and hold amplifier of FIG. 5. As may be seen from FIG. 9, which shows SFDR and SNDR as a function of sampling frequency f s , the dynamic performance is dramatically better, by about 15-20 dB, all the way up to approximately 2 Giga-samples per second.  
         [0043]    [0043]FIG. 10 illustrates overall system performance using the track and hold amplifier of FIG. 5, together with the interface amplifiers of FIG. 3 and the controllable current sources  402  of FIG. 4B. As may be seen in FIG. 10, the overall system performance is extremely good all the way to signal frequencies of close to 1 GHz.  
         [0044]    The simulated INL profiles of the amplifier array  101  reduce peak INL from 5 LSB before fixing boundary problem to 0.3 LSB after fixing it. Three hundred times Monte-Carlo simulations show sigmas of the input referred offset at the array center and array edge are 1.37 mV and 1.41 mV respectively, which implies even averaging is achieved with the method of triple cross-connections and pre-distorting reference voltage.  
         [0045]    Time-interleaving technology is often used to reduce the bandwidth requirements on individual amplifiers (except the front-end track and hold (T/H) still need full tracking bandwidth), and increases the data throughput with lower speed clocking. One advantage is that it can also save the power consumption in the system. The required gain to suppress the dynamic offset from the amplifier array is evenly distributed between preamplifier stages in a multi-stage ADC to maximize the bandwidth.  
         [0046]    Fabricated in a 0.18 μm 1P6M CMOS technology, the prototype ADC occupies an active area of 0.5 mm 2 . A clock amplifier and high speed digital buffers are integrated. A 50-Ohm on-chip termination for a 0.8 Volt differential clock helps to reduce the reflection. CMOS level data and clock directly drive Agilent logic analyzer 16702B, which has 1 GHz state mode and 4 GHz timing mode.  
         [0047]    Instead of being mounted in a conventional package, the prototype of A/D converter was mounted on a printed circuit board with chip-on-board technology. Including front-end T/H and clock buffers, the circuits consume 310 mW from 1.8V at a 2-GHz conversion rate and 996 MHz input. The measured linearities are shown in FIG. 6. The peak DNL and INL are 0.32 LSB and 0.5 LSB respectively. FIG. 7 shows the measured SNDR and SFDR as function of input signal frequency. The effective resolution bandwidth reaches 1 GHz. This ADC achieves a state-of-the-art Figure-of-Merit of 3.5 pJ defined by (Power)/2 ENOB ·2·ERBW), where ENOB is effective number of bits, and ERBW is effective resolution bandwidth.  
         [0048]    It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.