Digital ohmmeter with electrical continuity tester

Abstract

In a digital test instrument such as a digital ohmmeter for measuring and digitally displaying the resistance of an unknown circuit element, there is included an electrical continuity tester coupled to the input of the digital ohmmeter for instantaneously and digitally indicating electrical continuity. Connection of the electrical continuity tester to the input of the digital ohmmeter is achieved in a manner such as to not overload or otherwise affect the accuracy of the resistance measurement.

I claim: 
     
       1. In a digital test instrument for measuring resistance including a pair of input terminals adapted to be connected across a circuit element of unknown resistance, a first reference current or voltage source operatively couple to said input terminals so that an analog voltage appears across said input terminals which is proportional to the resistance of said circuit element, a filter connected to one of said input terminals for filtering noise in said analog voltage, an analog-to-digital converter having an input connected to said filter and an output, a digital display, and driver circuit means connected between said output of said analog-to-digital converter and said digital display, the improvement comprising means for detecting and immediately signalling electrical continuity across said input terminals comprising: a single bit analog-to-digital converter having first and second input terminals, said first input terminal being connected directly to said one of said input terminals, said converter having a sufficiently high input impedance not to affect the operation of said resistance measuring circuitry of said digital test instrument;   a second source of reference voltage operatively connected to said second input terminal of said single bit analog-to-digital converter, said single bit analog-to-digital converter generating a logical true or logical false signal depending upon whether said analog voltage is higher or lower than said second reference voltage;   a digital continuity display; and   driver circuit means connected between said single bit analog-to-digital converter and said digital continuity display for instantaneously indicating continuity.   
     
     
       2. In a digital test instrument according to claim 1, the improvement wherein said single bit analog-to-digital converter comprises: a comparator.   
     
     
       3. In a digital test instrument according to claim 2, the improvement wherein said comparator has an inverting input connected directly to said one of said input terminals and a noninverting input connected to said second source of reference voltage, said comparator generating a logical false signal when said analog voltage is greater than said second reference voltage. 
     
     
       4. In a digital test instrument according to claim 3, the improvement wherein said comparator generates a logical true signal when said analog voltage is less than said second reference voltage. 
     
     
       5. In a digital test instrument according to claim 2, 3, or 4, the improvement wherein said comparator has a MOSFET input stage having an input impedance in excess of approximately 10 12  ohms. 
     
     
       6. In a digital test instrument according to claim 1, 2, 3, or 4, the improvement wherein said digital continuity display is part of said digital display. 
     
     
       7. In a digital test instrument according to claim 1, 2, 3, or 4, the improvement wherein said driver circuit means connected between said single bit analog-to-digital converter and said digital continuity display is the same as said driver circuit means connected between said analog-to-digital converter and said digital display. .Iadd. 8. A digital test instrument for measuring resistance comprising: a pair of input terminals adapted for connection across a circuit element of unknown resistance;   means for producing a voltage across the input terminals the value of which indicates the value of the unknown resistance;   a digital display and means for operating the display to indicate the value of the unknown resistance, the digital test instrument exhibiting a measuring time delay before the display indicates the value of the unknown resistance; and   means for substantially instantaneously detecting and signaling the presence of electrical continuity across the input terminals including:   a continuity indicator;   means for energizing the continuity indicator; and   means for selecting whether the continuity indicator is energized according to whether the voltage across the input terminals is greater or less than a reference voltage. .Iaddend..Iadd. 9. The digital test instrument according to claim 8, wherein said continuity indicator comprises means for displaying a continuity symbol. .Iaddend..Iadd. 10. The digital test instrument according to claim 9, wherein said means for displaying a continuity symbol is part of said digital display. .Iaddend. .Iadd. 11. The digital test instrument according to claim 9, wherein the means for selecting whether the continuity indicator is energized comprises a one-bit analog-to-digital converter. .Iaddend..Iadd. 12. The digital test instrument according to claim 11, wherein the one-bit converter has a first input connected directly to one of the input terminals of the instrument and a second input connected to a source of the reference voltage, the one-bit converter generating a logical signal of a first logic state when the voltage across the input terminals is greater than the reference voltage and of a second logic state when the voltage across the input terminals is less than the reference voltage, the logical signal being supplied to the continuity indicator energizing means. .Iaddend..Iadd. 13. A digital test instrument according to claim 12, wherein said first input is an inverting input and said second signal is a non-inverting input and wherein the one-bit converter generates a logical false signal when the voltage across the input terminals is greater than the reference voltage and a logical true signal when the voltage across the input terminals is less than the reference voltage. .Iaddend..Iadd. 14. The digital test instrument according to any of claims 11, 12 or 13, wherein the one-bit converter has an input impedance in excess of approximately 10 12  ohms. .Iaddend. .Iadd. 15. A digital test instrument for measuring resistance including:   a pair of instrument input terminals adapted to be connected across a circuit element of unknown resistance;   means for producing a voltage across said instrument input terminals the value of which indicates the value of the unknown resistance;   an analog-to-digital converter having an input connected to one of the instrument input terminals and having an output;   a digital display;   driver circuit means connected between the output of the analog-to-digital converter and the digital display for operating the display to indicate the value of the unknown resistance; and   means for detecting and immediately signaling electrical continuity across the instrument input terminals comprising:   a reference voltage source;   a one-bit analog-to-digital converter having first and second inputs and an output, the first input of the one-bit converter being connected to one of the instrument input terminals and the second input of the one-bit converter being connected to the reference voltage source, the one-bit analog-to-digital converter generating at its output a logical signal having either a logical true or a logical false value depending upon whether said analog voltage appearing across the instrument input terminals is higher or lower than the reference voltage;   continuity symbol display means for receiving an energizing signal and for displaying a continuity symbol in response thereto; and   driver circuit means connected between the output of the one-bit analog-to-digital converter and the continuity symbol display means for instantaneously indicating continuity by selectively supplying the energizing signal to the continuity symbol display depending on whether the logical value of the logical signal generated by the one-bit   
     
     
        analog-to-digital converter is true or false. .Iaddend..Iadd. 16.  The digital test instrument according to claim 15, wherein the continuity symbol display means is part of said digital display. .Iaddend..Iadd. 17. The digital test instrument of claim 15, wherein the one-bit converter has an input impedance in excess of approximately 10 12  ohms. .Iaddend.