Abstract:
A clock-controlled voltage-to-frequency converter comprising a reference voltage source, an integrator for receiving the reference voltage and a variable input voltage and producing an output having a triangular waveform, the integrator including a capacitor which is charged at a rate proportional to the input voltage and discharged at a rate proportional to the difference between the reference voltage and the input voltage, the charging and discharging occurring during a time interval T o , a clock pulse source for generating a continuous series of clock pulses, a counter responsive to the clock pulses for terminating the discharging of the capacitor and initiating the charging of the capacitor at a fixed time interval following termination of the charging of the capacitor, a logic circuit responsive to the output of the integrator for terminating the charging of the capacitor and initiating the discharging of the capacitor when the integrator output rises above a preselected threshold level, the time required for the capacitor to charge to the threshold level varying in accordance with the magnitude of the input voltage.

Description:
FIELD OF THE INVENTION 
     The present invention relates to clock-controlled voltage-to-frequency converters suitable for use in analog-to-digital converters and, more particularly, to converters of the type which use an integrator to produce a triangular waveform, which in turn is used to produce a series of pulses having periods which are proportional to the magnitude of a variable input voltage. 
     SUMMARY OF THE INVENTION 
     It is a primary object of the present invention to provide a clock-controlled voltage-to-frequency converter which is capable of producing high-resolution analog-to-digital conversion within a short conversion time. In this connection, a related object is to provide a high-resolution, direct-counting, analog-to-digital converter which is clock-controlled. 
     It is another important object of this invention to provide such a clock-controlled voltage-to-frequency converter whose output is, to the first order, substantially independent of circuit component values. 
     Another object of this invention is to provide such an improved clock-controlled voltage-to-frequency converter which has low power consumption and a low parts count. 
     It is a further object of this invention to provide such an improved clock-controlled voltage-to-frequency converter which is readily adaptable to either single or dual power-supply operation, voltage gain and linearization, and/or use with resistance-transducers. 
     Other objects and advantages of the invention will be apparent from the following detailed description and the accompanying drawings. 
     In accordance with the present invention, the foregoing objectives are realized by providing a clock-controlled voltage-to-frequency converter comprising a reference voltage source; an integrator for receiving the reference voltage and a variable input voltage and producing an output having a triangular waveform, the integrator including a capacitor which is charged at a rate proportional to the input voltage and discharged at a rate proportional to the difference between the reference voltage and the input voltage, the charging and discharging occurring during a time interval T o  ; a clock pulse source for generating a continuous series of clock pulses; counting means responsive to the clock pulses for terminating the discharging of the capacitor and initiating the charging of the capacitor at a fixed time interval following termination of the charging of the capacitor; and logic means responsive to the output of the integrator for terminating the charging of the capacitor and initiating the discharging of the capacitor when the integrator output rises above a preselected threshold level, the time required for the capacitor to charge to the threshold level varying in accordance with the magnitude of the input voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a clock-controlled voltage-to-frequency converter embodying the present invention; 
     FIG. 2 is a series of waveforms produced at various points in the system of FIG. 1; 
     FIG. 3 is a diagram of the system of FIG. 1 as implemented with commercially available components; 
     FIG. 4 is a schematic diagram of an inverse counting analog-to-digital converter utilizing the voltage-to-frequency converter of FIG. 1; and 
     FIG. 5 is a series of waveforms produced at various points in the converter of FIG. 4. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that it is not intended to limit the invention to the particular forms disclosed but, on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims. 
     Turning now to the drawings and referring first to FIG. 1, the illustrative system will be described with reference to the waveforms of FIG. 2 as well as the system diagram of FIG. 1; the location of each of the waveforms of FIG. 2 in the system of FIG. 1 is identified by the addition to FIG. 1 of the same reference characters that appear in the left margin of FIG. 2. All the active components in the system of FIG. 1 are powered from a single supply voltage V ref  so that the high level of any binary signal is at V ref  volts and the low level is at zero volts. 
     The input signal for the system of FIG. 1 is a variable analog voltage V t  applied to the non-inverting input of an operational amplifier 10. The amplifier 10 cooperates with a resistor R and a capacitor C to form an integrator. The capacitor C is connected between the inverting input and the output of the operational amplifier 10, while the resistor R is connected between the inverting input of the amplifier 10 and the output of the entire system. 
     The system of FIG. 1 uses the output of the integrator to produce a binary output signal V out  comprising a series of pulses having a period T o  which varies in proportion to the magnitude of the variable input voltage V t . As will be described in detail below, the width T p  of the output pulses is fixed by a clock and counter, and the average ratio of the pulse width T p  to the pulse period T o  over a sufficiently long time is proportional only to the ratio of the input voltage V t  to the fixed reference voltage V ref . Because T p  is fixed, the average output frequency f oave  =1/T oave  is dependent only on the ratio of V t  to V ref  and the clock frequency. This ratio is also independent, to first order, of the values of the capacitor C and the resistor R. 
     The entire system of this invention is synchronized with a continuous series of high-frequency clock pulses. Both edges of the output pulses at V out  are controlled by positive edges of the clock pulses. Consequently, the converter is capable of providing high-resolution analog-to-digital conversion. 
     During the portion of each cycle of operation of the illustrative system when the binary output signal V out  is low, the capacitor C charges at a rate proportional to the magnitude of the variable input voltage V t . Thus, the integrator output voltage V 1  ramps up during this portion of each cycle (see FIG. 2). The charging of the capacitor C and the resultant ramping up of V 1  continues as long as the converter output V out  remains low. 
     The time at which V out  goes high is determined by the rate at which the integrator output voltage V 1  ramps up due to the charging of the capacitor C. This ramp-up rate determines when V 1  reaches a fixed threshold level V th , i.e., the switching point, of a set-reset flip flop 12 which receives the voltage V 1  at its &#34;set&#34; input. The Q output of the flip flop 12 is a binary signal V 2  which goes low each time V 1  reaches the threshold level V th  (see FIG. 2). This signal V 2  is applied to the D input of a D-type flip flop 13 whose clock input receives clock pules from the source 11. Thus, whenever V 2  goes low, the positive edge of the next clock pulse received by the flip flop 13 causes the Q output V out  to go high (see FIG. 2). This Q output V out  is the binary output of the converter, and is also the voltage which is applied to the resistor R. 
     When the Q output V out  of the flip flop 13 goes high, the Q output goes low, thereby triggering the master reset of a timer 14 to initiate the counting of a preselected number m of clock pulses. When m clock pulses have been counted, the Q output of the timer 14 produces an output pulse V 3  which is supplied to the reset input of the flip flop 12. This causes the Q output V 2  of the flip flop 12 to go high. This signal V 2  is the signal supplied to the D input of the flip flop 13, so V out  goes low in response to the positive edge of the next clock pulse received by the flip flop 13. This terminates the discharging, and initiates the charging of the capacitor C, thereby completing a single cycle of operation. 
     It will be recalled that the capacitor C charges whenever V out  is low. When V out  goes high, the charging of the capacitor C terminates and the capacitor immediately starts to discharge at a rate proportional to the difference between the input voltage V t  and the reference voltage V ref  (see FIG. 2). It will be appreciated that the time required for the capacitor C to charge to the fixed threshold level V th  (which causes V out  to go high) is determined by the rate at which the capacitor was charged, which in turn depends on the magnitude of the variable input voltage V t . The time required to discharge the capacitor C in any given cycle is essentially constant, but the overall period T o  of the charge-discharge cycle in any given cycle is a function of the magnitude of the variable input voltage V t  that determines the rate at which the capacitor is charged. 
     The effect of the magnitude of V t  on the pulse width T p  can be illustrated by a second example represented by the broken-line waveform for V 1  in FIG. 2. This broken-line waveform is the result of an input voltage V t  having a magnitude greater than that which produces the solid-line waveform for V 1 . Because of the increase in V t , the capacitor C charges at a faster rate and thus reaches the threshold level V th  in a shorter time period (T o  -T p ). The capacitor discharges at a slower rate, but the discharge time T p  remains constant because it is fixed by the timer 14. The end result is a shorter period T o , as illustrated in FIG. 2. 
     For single-ended power supply operation, the charge balance for the capacitor C for a time NT c  corresponding to n periods is: ##EQU1## where T p  is the capacitor discharge time, T c  is the clock period, and N is the number of clock periods in the n periods (maximum error will be T c  V t  /R). It then follows that the average output frequency f oave  is dependent only on the ratio of V t  to V ref  and the clock frequency: ##EQU2## To produce a digital output representing V t  /V ref , an inverse counting A/D conversion scheme may be utilized. In this inverse counting scheme, ##EQU3## 
     The maximum error in N will be ±1 count, and as a result V t  /V ref  can be resolved to within ±1 count in N. For a fixed conversion time and a large n, the resolution is governed by the length of T convert  described below. As an alternative, the conversion can be carried out by fixing the number of periods n, in which case only one counter is required. In this case V t  /V ref  is still given by ##EQU4## The resolution of V t  /V ref  will still be ±1 part in N, and the minimum value of N will be n(m+1) at full scale. The conversion time will be directly dependent on T oave  and can be as large as (n+1)T oave . 
     A practical implementation of the system of FIG. 1 is shown in FIG. 3, using commercially available chips. The operational amplifier 10 is an ICL7650, the D flip flops 12 and 13 are an MC14013, and the timer 14 is an MC14541 programmable timer. Because this converter is clock-controlled, it has a high degree of precision and resolution. As can be appreciated from FIGS. 1 and 3, the converter also has a low parts count, can be operated from a single-ended power supply, and requires only a small amount of power. Furthermore, the accuracy of the system is, to first order, dependent only on the reference voltage and not on the values of the other components in the system. 
     To implement the inverse counting scheme described above, the A/D conversion system of FIGS. 4 and 5 may be utilized. In this system, a switch 20 applies the reference voltage V ref  to the D input of a flip flop 21 for a time T switch . When V ref  is not connected to the D input, the switch 20 connects that input to ground. The output signal C out  is supplied to clock input of the flip flop 21 and the D input of a second flip flop 22. Clock pulses from the source 11 are supplied to the clock input of the flip flop 22 and to a NOR gate 23 whose other input is the Q output of the flip flop 21. 
     The Q output of the flip flop 21 goes low in response to the first pulse in V out  after the switch 21 has been moved to its low (ground) position, and the Q output remains low until the first pulse in V out  after the switch has restored a high D input to the flip flop 21. As a result, the gate 23 passes clock pulses to a counter 24 for n cycles, thereby accumulating N counts in the  counter 24. These n cycles occur within a time period T convert  which is up to one period longer than the interval T switch  and is synchronized with the positive edges of the pulses in V out . The Q output of the counter 22 produces the same number of output pulses appearing in V out  during the time interval T convert  when the gate 23 is open, thereby accumulating n pulses in a counter 25 connected to the Q output of the flip flop 22. The ratio of n/N of the counts accumulated in the two counters 25, 24 corresponds to the ratio V t  /V ref . 
     Gain can be provided in the illustrative system by simply connecting a resistor between ground and the inverting input of the operational amplifier 10, although this makes the output dependent on the ratio of the two resistors. Linearization can also be added to the illustrative system by connecting a further resistor between ground and the inverting input of the operational amplifier only when V out  is low or only when V out  is high. The illustrative system is also particularly well suited for use with resistance bridge transducers, e.g., by connecting such a transducer across the inputs of the operational amplifier 10.