Abstract:
A method for generating a ramp comprises providing a voltage reference source, providing a summing amplifier, providing n switched capacitor elements coupled in parallel between the voltage reference source and the summing amplifier, and selectively activating a predetermined number of the switched capacitor elements to first store charge on each activated switched capacitor element and then to measure the sum of the charges on the activated capacitor switch elements in each of a fixed-integer number of time slots in a cyclical manner, the predetermined number being between 0 and n.

Description:
BACKGROUND 
     Field of the Invention 
     The Prior Art 
     The present invention relates to analog-to-digital converters (“ADC&#39;s”) and to companding analog-to-digital converters, an architecture commonly known as “single slope ADC&#39;s”. More particularly, the present invention relates to apparatus and methods to minimize nonlinearity in analog-to-digital converters. 
     “Single slope ADC&#39;s” is the common name given to a family of analog-to-digital converters employing a ramp voltage generator, a digital counter, an analog front end sampling section, a comparator that compares the analog input voltage with the generated ramp voltage, and a digital latch. 
     In a simple well-known case, the ramp voltage follows a linear function. To reduce the conversion time, the ramp voltage may be “accelerated” by using a segmented ramp function as shown in  FIG. 1A . Initially, in SEGMENT(1) the ramp in  FIG. 1A  runs with a unit step size (STEP(1)=1*LSB or 1× ramp rate). After a specified number of clock pulses, in SEGMENT(2) the ramp rate is increased to twice the unit step size (STEP(2) or 2× ramp rate). The count at which the transition from STEP(1) to STEP(2) occurs is may be referred to as Knee(1). After counting for a certain number of steps at a 2× ramp rate, the rate is doubled again in SEGMENT(3) to STEP(4) or a 4× ramp rate. This occurs at a count that may be referred to as Knee(2). Further doubling in SEGMENT(4) results in STEP(8) at count Knee(3). At the same knee points the ramp counter increases the count steps size by 2× so that the overall ADC transfer function is linear. Companding is also done in the prior art to take advantage of the fact that the absolute level of the noise in most natural source signals increases with the signal value so that increased ADC quantization noise at larger input signal values is masked by input noise and thus the “quality” (SNR) of the ADC conversion does not decrease at higher levels. Image sensors are an example of such an application. 
     Persons of ordinary skill in the art will readily appreciate that the acceleration of the ramp voltage need not be limited to integer multiples (e.g., 2×) but may be configured in virtually any manner as warranted by the particular application. Such an alternative scheme is shown in  FIG. 1B , in which the individual ramp segments SEGMENT(1) through SEGMENT(4) may have slopes that are not integer multiples of one another. 
     In real world implementations, non-idealities such as charge injection, amplifier offset, finite amplifier gain, and component mismatch cause each of the SEGMENT(N) sections to have unpredictable ramp rates. In the general case, the difference from the intended step size may be independent for each SEGMENT so that the composite transfer function (digital number out vs. V in ) may be non-linear. In addition, these circuit non-idealities (such as amplifier offset) may drift over the lifetime of the circuit. STEP(1) through STEP(4) and Knee(1) through Knee(3) are illustrated in SEGMENT(1) through SEGMENT(4) in  FIG. 1A . A non-ideal gain for SEGMENT(3) is shown in  FIG. 1A  along with an ideal SEGMENT(3). It can be seen from  FIG. 1A  that a non-ideal gain for a single section can cause integral non-linearity (INL). Correct linearity for the ADC assumes that the gain for STEP(4) is half of that of STEP(8) and twice that of STEP(2). However, a sampled voltage during SEGMENT(4) will not have a count that corresponds linearly to a voltage sampled during STEP(2) or STEP(8). Because only part of the voltage versus count curve has non-ideal gain, integral non-linearity results. If all sections were affected in the same way, an overall gain error would occur but the transfer function would be linear. 
     In order to have low integral non-linearity over the entire ramp, the ramp gains (expressed in Volts/digital number, Amps/digital number or other measured quantity) for each section must be accurate. Actually, for applications such as imaging or in the general case, applications including some form of AGC function in the system, only the ratios of the gains need to be accurate for a low integral non-linearity. If the overall gain is also of interest, accurate gain for each section is desired. 
     As shown in  FIG. 2 , a typical ramp generator circuit  10  may be implemented as a switched capacitor integrator. Ramp generator circuit  10  includes a switched capacitor network shown inside of dashed line  12  that includes capacitor  14 . A switch  16  couples capacitor  14  to V ref  source  18 . Switch  20  couples the first plate of capacitor  14  to ground and switch  22  couples the second plate of capacitor  14  to ground. Switch  24  couples the second plate of capacitor  14  to the inverting input of operational amplifier  26 . Feedback capacitor (C fb )  28  is coupled between the output  30  and the inverting input of operational amplifier  26 . Switches  16  and  24  allow the unit element  12  to be selectively switched in and out of the ramp generator circuit  10 . In addition, other methods may be used to deliver quanta of charge to the input of an amplifier with capacitor feedback. 
     The switched capacitor network  12  at the input of the amplifier  26  delivers discrete packets of charge to the amplifier  26 . The amplifier  26  has a capacitive feedback network  28  configured to provide negative feedback. The feedback forces the amplifier  26  to move the ramp output voltage in order to re-balance the inputs after each packet of charge is delivered. The size of the ramp step is proportional to the input voltage from a voltage source, such as a resistor ladder, and the ratio of the input capacitance to the output capacitance. One or both of the input voltage and the size of the input capacitance may be programmable. Persons of ordinary skill in the art will understand that other methods may be used to implement the ramp generator, such as a DAC or a continuous integrator driven by a constant current source. In addition, other methods may be used to define quanta of charge to the input of the amplifier with capacitive feedback. 
     Referring now to  FIG. 3 , a timing diagram shows the operation of the switches in the ramp generator circuit of  FIG. 2 . Two clock signals, φ1 and φ2, having opposing phase relationship are used to drive the switches in switched capacitor network  12 . The φ1 signal drives switches  16  and  22 , and the φ2 signal drives switches  20  and  24  as is known in the art. During φ1, switches  16  and  22  are closed, charging capacitor  14  to V ref . During φ2, switches  20  and  24  are closed, transferring the charge from capacitor  14  to the inverting input of amplifier  26 . 
     Referring now to  FIG. 4 , a schematic diagram shows how the ramp generator  10  of  FIG. 2  may be used in an analog-to-digital converter  40 , in which the ramp generator  10  is associated with a counter  42  driven from the same clock source  44  as the ramp generator  10 . The output count of the counter  42  has a known relationship to the ramp voltage. An analog input voltage and the ramp voltage are compared in a comparator  46  and the output of the comparator  46  is used to trigger latch  48  to latch the output of the counter  42  when the ramp voltage equals the analog input voltage. The latched count, which has a known relationship to the ramp voltage, is thus a digital representation of the measured analog input quantity. 
     The major sources of error in the ramp gain of circuits such as the one depicted in  FIG. 4  arise from the input offset of the ramp amplifier  26  and a differential charge injection error. The step size of the ramp generator (and therefore the gain in V/digital number) is proportional to the amount of charge injected at each step. For each possible setting, the step size is given a constant error by the charge injection. The offset of the amplifier gives an error that is proportional to the amount of capacitance used for the particular setting. The error due to capacitance mismatch will result in an error that is proportional to the input voltage. There may also be also errors in the reference voltages V in  and V ref  used by the ramp generator that must be considered. Lastly, the finite amplifier gain will cause an error that is proportional to the output voltage, which creates a non-linearity in the ramp voltage. The total error in the ramp gain of circuits such as the one depicted in  FIG. 2  is Q err =Q inj +Q offset +Q gain . 
     There are other sources of gain error such as the relative size of the feedback capacitance. These errors will be the same for all settings. The gain errors common to all settings will result in an overall gain error, but will not result in integral non-linearity due to the accelerated ramp. The inaccuracies and non-idealities described above result in circuit area and/or power and/or cost constraints which mean this approach to Analog-Digital conversion is unattractive for modern integrated circuit implementation. 
     BRIEF DESCRIPTION 
     A new unit element approach to generating an accelerated ramp is proposed. The approach uses rotating unit elements to eliminate charge injection errors described above from causing non-linearity during an accelerated ramp. The approach also eliminates the input voltage source as a cause of non-linearity. 
     According to one aspect of the present invention, the step sizes are made to be linearly related by using unit cells and a single reference voltage. To achieve different step sizes, a different number of unit cells are used for each step size. Since the step size is equal to the amount of charge injected at each step, either the capacitance or voltage can be varied. It will be shown that the charge injection error makes the previous approach, stepping the voltage, impractical. 
     The unit element should not only contain a unit capacitor, but it should also contain its own unique unit switches. Switches may not be shared among the different unit cells, because the charge injection from the switch elements must have a unit size. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         FIG. 1A  is a representation of a multi-slope output from a ramp generator in which the slopes of successive ramp segments double. 
         FIG. 1B  is a representation of a more general multi-slope output from a ramp generator in which the slopes of successive ramp segments are different from one another. 
         FIG. 2  is a simplified schematic diagram of a typical prior-art ramp generator circuit. 
         FIG. 3  is a timing diagram showing the operation of the switches in the ramp generator circuit of  FIG. 2 . 
         FIG. 4  is a block diagram of a prior-art analog-to-digital converter. 
         FIG. 5  is a representation of a ramp generator circuit according to one aspect of the present invention. 
         FIG. 6  is a graph illustrating the ramp slope obtained by using 1×, 2×, 4×, 8×, and 16× step sizes. 
         FIG. 7  is a block diagram showing a ramp generator system for generating fractional step sizes. 
         FIG. 8  is a graph showing the effects of low pass filtering on the ramp waveform. 
     
    
    
     DETAILED DESCRIPTION 
     Persons of ordinary skill in the art will realize that the following description of the present invention is illustrative only and not in any way limiting. Other embodiments of the invention will readily suggest themselves to such skilled persons. 
     Referring now to  FIG. 5 , a block diagram shows an illustrative ramp generator circuit  60  including a plurality of unit elements  62 ,  64 ,  66 ,  68 ,  70 , and  72  in accordance with another aspect of the present invention. While  FIG. 5  shows six unit elements for purposes of illustration only, persons of ordinary skill in the art actual embodiments of the invention may include differing numbers of unit elements according to the needs of the particular application. Each of unit elements  62 ,  64 ,  66 ,  68 ,  70 , and  72  may be configured as shown in the illustrative unit element  12  of  FIG. 2 . 
     The unit elements  62 ,  64 ,  66 ,  68 ,  70 , and  72  are all coupled to V ref  source  74  and to the inverting input of operational amplifier  76 . Feedback capacitor (C fb )  78  is coupled between the output  80  and the inverting input of operational amplifier  76 . 
     In an illustrative embodiment of the present invention that will be used to show the operation of the present invention, sixteen unit elements are employed. A 50 fF capacitor and a 24 mV input voltage are used order to achieve a 1.2 fC step size. Since sixteen unit elements are used in this example, the step sizes for a given ramp may vary by a 16:1 ratio. 
     In order to generate a ramp having a unit step size, the circuit will operate by using one unit element at a time, but rotating through all sixteen of the unit elements. For example, on the first step, the capacitor in the first unit element  62  is charged, then that charge is deposited onto the input of the amplifier. The next step uses only the second unit element  64 . This is followed by using one at a time the succeeding unit elements  66 ,  68 ,  70 , up to and including the last (sixteenth) unit element  72 . This completes a single charge cycle. After the last unit element  72  is used, the next step starts back with the first unit element  62 . The foregoing description in which the unit elements  62  through  72  are used in order is illustrative only, and persons of ordinary skill in the art will appreciate that any algorithm that uses all unit elements equally within each charge cycle could be employed. Decoding and control of switches to place the unit elements in and out of the circuit is well known in the art. 
     Following the preceding example, the charge from the first step is Q step =V in *C 1 +Q err1 . After sixteen steps, the total charge is Q total =V in *(C 1 +C 2 + . . . +C 16 )+Q err1 +Q err2 + . . . +Q err16 , where Q err1  to Q err16  represent the total error in the charge for steps 1 to 16, respectively. 
     The circuit of  FIG. 5  may be operated to produce different step sizes. For example, a step size of 2 units may be produced by operating the unity elements in simultaneous groups of two. This step size is double the size of the unit step size described above, and employs two unit elements at a time using the same input voltage V ref . This eliminates the error due to voltage mismatch caused by imprecise reference voltage scaling. 
     As in the generation of the unit step size, each of the unit elements  62  through  72  are used in a rotating fashion. In one non-limiting example, the first step may be generated using the first two unit elements together. The next step could use the third and fourth, and so on. It should be noted that after eight steps, all elements are used, whereas generating the unit step size uses all unit elements after sixteen steps. 
     Following this example, the charge from the first step is Q step =V in *(C 1 +C 2 )+Q err1 +Q err2 . After eight steps, the total charge is Q total =V in *(C 1 +C 2 + . . . +C 16 )+Q err1 +Q err2 + . . . +Q err16 . 
     The step size may be doubled again by using four unit elements at a time, again by using eight unit elements at a time, and doubled one more by using all sixteen unit elements simultaneously. Each time, the same input voltage V ref  is employed. In this example, the charge from the first step (assuming a perfect amplifier) is Q step =V in *(C 1 +C 2 + . . . +C 16 )+Q err1 +Q err2 + . . . +Q err16 . This sum is exactly equal to the Q total  for the unit step size after sixteen steps, the 2× step size after eight steps, and the 4× step size after four steps. 
     Referring now to  FIG. 6 , a graph illustrates the ramp slope obtained by using 1×, 2×, 4×, 8×, and 16× step sizes in operating the ramp generator  60  according to the present invention. 
     The overall slope of the ramp may be changed by changing the input voltage V ref . This may be done by configuring the V ref  source as a variable voltage source. No accurate method of scaling the input voltage is proposed here. However, it will be understood by persons of ordinary skill in the art that, even with an accurate scaled voltage, the charge errors described earlier will prevent linear scaling of the step size using a voltage-based scaling. Fortunately, the system of the present invention is tolerant to such errors between scaled ramps but is not tolerant of scale inaccuracies within an accelerated ramp. Therefore, the system of the present invention may be advantageously employed using voltage scaling to vary the ramp shape in where the system can tolerate error and using unit element scaling to scale the step size where extreme scale accuracy is required. 
     A major deleterious effect of charge injection is circumvented by using the charge injection method of the present invention. After 16 unit steps, the total charge injection is Q inj1 +Q inj2 + . . . Q inj16 . This is identical to the charge injection expected if all unit elements switch at once, creating a single step. By using the same voltage for both cases, the effects of reference voltage scaling errors are also eliminated. 
     According to another aspect of the present invention, fractional gains may be achieved. Fractional gains can be achieved by dithering the step size. In the proposed implementation, a delta sigma modulator precedes the unit element ramp generator to facilitate fractional gains. A low pass filter follows the modulator in order to smooth out the fractional steps. A block diagram for a ramp generator system  90  for producing fractional gains is shown in  FIG. 7 . N integer bits and M fractional bits are input to delta-sigma modulator  92 . The output of delta-sigma modulator  92  selects/enables unit elements in the ramp generator  94 , which outputs a raw analog ramp signal. Low pass filter  96  smoothes the modulated ramp signal. 
     Table 1 illustrates this aspect of the present invention. Each box in the table represents one time slot. For simplicity, sixteen time slots are shown, but persons of ordinary skill I the art will appreciate that this aspect of the present invention is not so limited. Any number of unit elements A, B, C, and D can be turned on during each time slot. In the illustrative embodiment shown in Table 1, zero, 1, or 2 elements are turned on during each time slot. All unit elements should be used exactly once per charge cycle to preserve precise gain ratios. 
     
       
         
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 No. of  
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 elements 
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 on/ 
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 No. of 
                   
               
             
          
           
               
                 Time Slots 
                 time 
                   
               
             
          
           
               
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
                 9 
                 10 
                 11 
                 12 
                 13 
                 14 
                 15 
                 16 
                 slots 
                 Gain 
               
               
                   
               
             
          
           
               
                 A 
                 — 
                 — 
                 — 
                 B 
                 — 
                 — 
                 — 
                 C 
                 — 
                 — 
                 — 
                 D 
                 — 
                 — 
                 — 
                  4/16 
                 0.25 
               
               
                 A 
                 — 
                 B 
                 — 
                 C 
                 — 
                 D 
                 — 
                 A 
                 — 
                 B 
                 — 
                 C 
                 — 
                 D 
                 — 
                  8/16 
                 0.5 
               
               
                 A 
                 B 
                 C 
                 — 
                 D 
                 A 
                 B 
                 — 
                 C 
                 D 
                 A 
                 — 
                 B 
                 C 
                 D 
                 — 
                 12/16 
                 0.75 
               
               
                 A 
                 B 
                 C 
                 D 
                 A 
                 B 
                 C 
                 D 
                 A 
                 B 
                 C 
                 D 
                 A 
                 B 
                 C 
                 D 
                 16/16 
                 1 
               
               
                 A, B 
                 C 
                 D 
                 A 
                 B, C 
                 D 
                 A 
                 B 
                 C 
                 D, A 
                 B 
                 C 
                 D 
                 A, B 
                 C 
                 D 
                 20/16 
                 1.25 
               
               
                 A, B 
                 C 
                 D, A 
                 B 
                 C, D 
                 A 
                 B, C 
                 D 
                 A, B 
                 C 
                 D, A 
                 B 
                 C, D 
                 A 
                 B, C 
                 D 
                 24/16 
                 1.5 
               
               
                 A, B 
                 C, D 
                 A, B 
                 C 
                 D, A 
                 B, C 
                 D, A 
                 B 
                 C, D 
                 A, B 
                 C, D 
                 A 
                 B, C 
                 D, A 
                 B, C 
                 D 
                 28/16 
                 1.75 
               
               
                 A, B 
                 C, D 
                 A, B 
                 C, D 
                 A, B 
                 C, D 
                 A, B 
                 C, D 
                 A, B 
                 C, D 
                 A, B 
                 C, D 
                 A, B 
                 C, D 
                 A, B 
                 C, D 
                 32/16 
                 2 
               
               
                   
               
             
          
         
       
     
     The low pass filter  96  smoothes the modulated ramp signal.  FIG. 8  is a graph showing the effects of the low pass filter  96  on the ramp waveform.  FIG. 8  shows a fractional step size of 0+¼. The ramp generator  94  would output a stepped waveform. After the low pass filter  96 , the waveform is smoothed. 
     If the ratio of the ramp clock frequency to the low pass filter bandwidth is high compared to the fractional pattern length, then the error caused by the truncation in the modulator and finite number of unit elements becomes less than the desired fractional step size. In this way, the resolution of the system is increased from the nominal step size of the raw ramp generator to the fractional resolution (from N bits to N+M bits). 
     If the bandwidth of the low pass filter is around 3 MHz and the ramp clock frequency is greater than 100 MHz, the ratio of bandwidth to sample clock frequency is 3/100. The maximum pattern length for the 4-bit word is 16. Therefore, the ratio of the sample clock frequency to low pass filter is about 33:1. This is greater than the maximum pattern length of 16 samples. Therefore, the ramp system should operate with a resolution that approaches the fractional resolution, 16 times greater than the nominal step size. 
     The low pass filter may be implemented by a bandwidth-limited voltage buffer between the raw ramp generator output and the comparator  46  in  FIG. 4 . Implementing an amplifier with a bandwidth equal to the ramp clock frequency would actually be prohibitive due to the power required. So, the amplifier as low pass filter is advantageous to low power ramp designs. 
     While embodiments and applications of this invention have been shown and described, it would be apparent to those skilled in the art that many more modifications than mentioned above are possible without departing from the inventive concepts herein. The invention, therefore, is not to be restricted except in the spirit of the appended claims.