Analog-to-digital converter apparatus for condition responsive transducer
This is a continuation of application Ser. No. 939,315, filed Sept. 5, 1978 now abandoned. 
    
    
     TECHNICAL FIELD 
     The technical field to which the invention pertains relates to the art of dual slope analog-to-digital converters for effecting a digital output signal of a measured analog signal being supplied to the converter. 
     BACKGROUND OF THE INVENTION 
     The dual slope type analog-to-digital slope converter for effecting a digital output signal from a measured analog input signal is well known as disclosed, for example, in U.S. Pat. Nos. 3,061,939; 3,316,546; 3,458,803, 3,660,834 and 3,566,397. Briefly, the method of conversion involves integrating a current directly related to an unknown voltage for a fixed period of time, followed by the integration of a standard current related to a reference voltage of opposite polarity until the integrator output returns to zero. The total time period required to null the integrator is directly proportional to the ratio of the measured current to the standard current, and, therefore, to the measured voltage. The integrator, therefore, is a circuit producing a linearly changing output with time (usually a ramp) when the input is some constant voltage and the rate of integrator output voltage increase is directly proportional to the magnitude of input voltage. When input voltage is zero, output voltage is not subject to change but remains zero at whatever output value was achieved at the beginning of the time period. 
     Standard operation for such prior art converters has included integration of the unknown in the same direction of polarity as the input signal, e.g. positive-to-positive which is then switched to a reference signal of opposite polarity that is integrated to zero. This is then detected by a comparator of the integrated signals and for large analog inputs prolonged time periods are required to effect the zero integration. The digital counts are then accumulated in a register proportional to the time factor associated with the unknown integration. 
     While this basic arrangement has functioned well with a high order of accuracy, it requires reference switching and polarity detection which becomes difficult at very low inputs leading to switching uncertainties. Also, bias currents associated with these prior devices have added and subtracted from the slopes in each of its changed directions. For overcoming these deficiences, it has been necessary to utilize precision low offset amplifiers. Despite recognition of the foregoing, means for effecting their elimination has not heretofore been known. 
     SUMMARY OF THE INVENTION 
     This invention relates to analog-to-digital conversion apparatus for effecting a digital output signal correlated to an analog signal being emitted from a condition responsive transducer. More specifically, the invention relates to an A/D converter of the dual slope type able to eliminate the reference switching and bias current problems characteristic of such prior art converters whereby to enhance the overall accuracy and reliability of such systems. Optionally, a further feature also affords enhanced linearization of the output signal. 
     The foregoing is achieved in accordance with the invention by always integrating the unknown analog input signal in a direction polar opposite to a reference signal enabling one of the two prior used reference signals to be eliminated. At the same time there is afforded the ability to both measure and verify polarity of the input signal without regard to the signal level. By using the integrator in a non-inverting mode, the A/D converter integrates positively all the analog signal values below a maximum negative input voltage while accumulating digital counts proportional to the differences therebetween. A digital display or other appropriate digital device is then operated by the count signals received from the converter. Optionally, digital linearization of the count signals can be provided for correcting any encountered deviation from ideal linearity. This is achieved by a programmed microcomputer reading a PROM to run a variable frequency clock via the PROM programming. The clock frequency is regulated to effect a first phase converter time that corresponds with an even multiple of the power line frequency. Accumulator counts will then vary directly with clock frequency. 
     It is therefore an object of the invention to provide novel modifications to an analog-to-digital converter of the dual slope type for effecting a digital output signal correlated to a transducer provided analog signal. 
     It is a further object of the invention to effect the previous object with modifications that effectively eliminate the reference switching and bias current problems characteristic of similar purpose prior art A/D converters. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a system apparatus embodying the invention; 
     FIG. 2 is an electronic schematic of the modified dual slope A/D converter hereof; 
     FIG. 3 is a timing diagram for the dual slope converter of FIG. 2 at zero and non-zero signal levels; and 
     FIG. 4 is an optional variation for the dual slope converter of FIG. 2 for affording digital linearization. 
    
    
     Referring first to FIG. 1, there is disclosed a transducer 10 adapted to continuously emit an analog signal Va correlated to the measured value of a condition being measured by the transducer. Transducer 10 can typically be any suitable type measuring instrument from which an analog output can be obtained and in a preferred use may constitute a pressure or temperature transducer as disclosed, for example, in U.S. Pat. Nos. 3,742,233 or 4,109,147. Analog signal Va from the transducer, along with a reference signal Vr of a known regulated voltage as will be explained are both supplied to A/D converter 12 in accordance herewith to in turn emit a count signal Vd for any of a variety of applications such as for operating a digital display 14. 
     Referring now to FIG. 2, the modified A/D dual slope converter 12 hereof is comprised of a summing amplifier 16, an integrator 18, a comparator and auto-zero amplifier 20, a sample/hold amplifier 22 and a logic and accumulator module 24 which for the embodiment being described is of a commercially available type similar to National Semiconductor MM5330 A/D Building Block operative as will be explained below. Analog signal Va, being supplied from transducer 10 in correlation to its measured parameter, is of negative polarity for connection directly through switch SW-1 to summing amplifier 16, while reference voltage Vr, being supplied from a regulated voltage source, is of negative polarity for connection through switch SW-2 to comparator and auto-zero amplifier 20. The output of amplifier 16 is provided through switch SW-3 to the positive terminal of integrator 18 while a V max  voltage is derived from Vr through dividers R 1  and R 2  for feeding the negative terminal of integrator 18. By way of example, if maximum positive voltage at Va=1.9999 volts, V max  would arbitrarily be set at 2.2000 volts and at Va=zero measurement would be based on 2.2000 volts. Logic and accumulator module 24 provides the logic for sequencing the aforementioned switches. 
     With reference also to FIG. 3, the complete A/D conversion cycle of transducer signal Va is shown in which VA=0 volts is the solid slope and Va≠0 volts for a measured value not equal to zero has a dashed slope. The cycle from left to right (FIG. 3) consists of three phases being auto-zero phase III, the integrator reference phase I for a predetermined fixed time period T r  and the integrated unknown phase II for a variable time period Tx. During phase III, comparator and auto-zero amplifier 20 functions as a high gain zeroing amplifier that with switch SW-4 closed, drives sample/hold amplifier 22, capacitor C-1 and inverting summing amplifier 16 through switch SW-3 to restore the integrator capacitor C-2 voltage to zero volts. Switches SW-1, SW-2 and SW-5 during this phase are all in grounded position. Also, because of the characteristics of amplifier 22, all amplifier voltage offsets are stored on capacitor C-1 where they remain through the integration cycle serving to eliminate even large offset voltages. 
     During phase I, in response to a received power line sync signal, switches SW-4 and SW-5 are opened. Reference voltage Vr is applied through switch SW-2 to amplifier 20 for simultaneously establishing V max  via divider R 1  and R 2 . V max  is chosen based on the full scale counts desired. At the same time, Va is connected to the input integrator 18 via summing amplifier 16 and switch SW-3. The output of integrator 18 then slews to VA for integration to begin for a reference period Tr during which time integration occurs for a signal value based on the differential between (V max  -Va). Time Tr is preferably selected to be of some even multiple of the local power line frequency for increasing the normal mode noise rejection of the converter. 
     On completion of phase I, phase II is initiated during which reference voltage Vr is applied to integrator 18 for the unknown period Tx until the threshold of comparator 20 is crossed in its comparison between the signal levels of Vr and the integrator output. At such time, coincident with the comparator output, the digital counts which accumulated during integration period Tx are transferred from the counter to the latches, the parallel output signal of which V d  is applied to display unit 14 or transmitted elsewhere as desired. The latter can be understood by the following example in which: ##EQU1## For any positive Va less than V max  the code converter 9&#39;s complements the counter to display the measured value. In this manner, display 14 yields a linear conversion for Va less than V max . 
     For effecting digital linearization usable with A/D converter 12 or with any dual slope standard system, reference is made to FIG. 4 in which there is shown a schematic circuit therefor including a microcomputer 26, a PROM 28 and a variable frequency clock source 30 connected to logic and accumulator module 24. During phase I (FIG. 3), microcomputer 26 reads PROM 28 for receipt of programmed information to drive clock 30 at a frequency such that Tr is an even multiple of the power line frequency. Each of microcomputer 26, PROM 28 and clock 30 are commercially available from various manufacturers. Microcomputer 26 can, for example, comprise a single chip microcomputer manufactured by Fairchild Camera and Instrument or Mostek Corp. under the designation &#34;F3870&#34; while PROM 28 can be obtained from either Signetics under their commercial designation &#34;DM74S188&#34; or from Texas Instruments under their commercial designation &#34;SN74188&#34;. Oscillator 30 is available as a 74LSO4 gate with crystal feedback while the counters are 74LS193 types, both manufactured by various integrated circuit manufacturers such as National Semiconductor. 
     Phase II is sensed by the microcomputer on the convert line Co and based on information received from PROM 28 drives the frequency of clock 30 at various rates per unit time as programmed into PROM 28. In this manner, clocking frequencies that are different between phases II and I do not affect the conversion time which instead remains constant for a given input voltage Va. Integrator 18 functions in the same manner as described above while the counts accumulated during time period Tx will vary more directly dependent on the frequency of clock 30. Should Va be a non-linear response of some linear function being measured, the final accumulated counts are therefore adjusted in that manner to yield a linear digital representation of the measurement. 
     If, for example, different clocking frequencies are utilized during phase II than Phase I, the conversion time nevertheless remains the same for a given input voltage Va since integrator 18 makes the same excursion. The counts accumulated during Tx will be more or less depending on the frequency of clock 30 such that the accumulated counts over the course of Tx can be varied in direct relation to the value of Va. Using the previous example and varying the clocking frequency a number of times (n) then when n=5 ##EQU2## Nt=21997 and if the dual slope converter hereof reaches crossover Tx=0.0066666 seconds, three corrections have occurred and the display will read: 
     
         22000-Tn (F.sub.1 +F.sub.2 +F.sub.3)=9902 counts 
    
     In the same example with a constant clocking frequency 
     
         Tx=0.0611111 Tn=0.0122222 
    
     giving an Nt=21999 and Linear Display=22000-13200=8800 counts. 
     Comparison of the above examples shows that in both cases the total number was essentially the same but the non-linear accumulation resulted in a 1102 count increase for a given Va=Tx. 
     By the above description there is disclosed a novel modification of an analog-to-digital dual slope type converter able to give a linear digital representation of a non-linear analog signal representing the measured output of a condition responsive element. By use of relatively inexpensive non-critical components, the precision elements required for eliminating the inherent problems of standard dual slope systems are thereby avoided. When employing linearization in accordance herewith, greater flexibility is afforded as compared to analog summing techniques previously utilized. Moreover, the linearization attains the precision of a digital correction that is highly repeatable and inherently temperature stable with the ability to tailor the response from one instrument to the other. Whereas the linearization has been described in combination with the modified dual slope converter hereof, it is not intended to be so limited since it could be readily utilized with such standard unmodified dual slope converters of the prior art. 
     Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the drawings and specification shall be interpreted as illustrative and not in a limiting sense.