Abstract:
A two-terminal multimeter capable of both volt/ohm and current measurement employs an input protection circuit with a positive temperature coefficient impedance and a reference impedance, across which an A-D converter takes measurements. A current measurement signal activates a switch to ground one side of the reference impedance for taking current measurements. A controllable clamping switch grounds the positive temperature coefficient impedance in an input overload situation to provide input protection against damage to the meter components.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to digital multimeters capable of performing multiple functions such as voltage, resistance and current measurement, and more particularly to a two-terminal digital meter that provides microamp measurement capabilities. 
     Multimeters in general and digital multimeters (DMMs) in particular, employ a common and a volt/ohms input terminal. Meters that provide microamp current measurement employ a third input terminal that is separate from the volt/ohms input terminal. Therefore, is to measure microamps, a meter user must pull the test probe out of the volt/ohms terminal and insert it into the microamps terminal, before taking the measurement. 
     Especially with measuring microamps, it is desirable to provide protection circuitry, to avoid damaging a meter in case the user accidentally touches the probe tips to the wrong place and overloads the meter. Such protection typically comprises a fuse that is chosen to blow before the rest of the meter circuitry can be damaged. However, having to change fuses is annoying, especially when it results from an accidental slip of a test probe into an unintended test point. 
     Referring to FIG. 1, which is a diagram of input circuitry of a typical prior art digital multimeter, the meter  10  comprises a common input terminal  12 , a volts/ohms input terminal  14 , and a microamp input terminal  16 . A shunt resistor  20  is placed between the common and microamp input terminals, with a fuse (F 1 )  18  in line with the microamp input terminal. The fuse is chosen to blow if the current exceeds a predetermined amount. Further provided across the microamp and common terminals is diode bridge circuitry  22 , which provides further input protection. A protection resistor (100 k ohm in the illustration) is used to limit current into a single chip analog to digital (A-D) converter  26  when there is an overload. The measurement system  26 , which ,as noted, suitably comprises a single chip. analog to digital (A-D) converter, receives the output of resistor  24  and passes it through a switch  28 , whereupon an A-D converter circuit is provided the input signal for measurement. Switch  28  suitably comprises a field effect transistor (FET) switch. 
     Other details of the prior art system include a switch  30  and resistor  32  in series in the common input line, the output thereof being provided to FET switch  34 , which then is supplied to the A-D converter. The volts/ohms input terminal has a series resistor  36 . which supplies the input signal to FET switch  38 , the output thereof being connected to the junction of resistor  32  and FET switch  34 . The various switches are actuated in accordance with the measurement to be taken. However, the multimeter user must physically move the input probe from the volts/ohms input terminal to the microamp input terminal in order to perform a microamp measurement. Once the current measurements are completed, then the probe must be moved back to the volts/ohms terminal if non-current type measurements are to be taken. Another drawback to the prior art system is that if the wrong input terminals are selected for a particular measurement, the multimeter can be damaged. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, a two-terminal multimeter is provided that enables voltage/ohm/microamp measurements without need of a third terminal. A first input and a common input are provided. A reference resistor is provided at either side thereof with a switch to ground, and these switches are activated to either ground the first or second end of the resistor. The first and second ends are also suitably connected to plus and minus sides of an A-D converter for obtaining appropriate measurement it values. 
     Accordingly, it is an object of the present invention to provide an improved digital multimeter that enables microamp measurements without need of a third input terminal. 
     It is a further object of the present invention to provide an improved multimeter that employs existing input protection and measurement components within a meter to enable microamp or volt/ohm measurements via only two terminals. 
     It is yet another object of the present invention to provide an improved system for volt/ohm/microamp measurement in a two terminal input meter. 
     A further object of the present invention is to provide an improved meter that is capable of measuring DC or AC microamps in addition to volts/ohms or other typical multimeter measurement functions with a two terminal input. 
     The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of input circuitry of a typical prior art digital multimeter; 
     FIG. 2 is a diagram of input circuitry of a multimeter in accordance with the present invention; and 
     FIG. 3 is a perspective view of an exemplary multimeter incorporating the invention. 
    
    
     DETAILED DESCRIPTION 
     The system according to a preferred embodiment of. the present invention comprises a two terminal multimeter that measures volts/ohms as well as microamps via two input terminals. A third input terminal normally required for microamp measurements in accordance with the prior art is dispensed with. 
     Referring to FIG. 2, a diagram of input circuitry of a multimeter in accordance with the present invention, the front end input circuit  40  comprises a common input terminal  42 , connected to ground, and a volts/ohms/microamp input terminal  44 . A positive temperature coefficient resistor (PTC)  48  and a protection resistor (R p ) 46  are connected in series between input terminal  44  and a junction point  50 , connected to ground via clamp switch  53 . The meter&#39;s control processor  41  includes a good portion of the. operational components of the meter. An op-amp  49  receives a voltage reference signal VREFH as an input, the output of the op-amp also being connected to junction point  50 . In input ISRC Control  75  to the op-amp governs whether the op-amp is set to a high-impedance. A switch  51  connects point  50  to a switch  52 , the other side of which is connected to ground. The common junction of switch  51  and  52  is also connected to input impedance Z 1   54  (suitably comprising a reference resistor R ref ), and the opposite end of reference resistor Z 1   54  provides sense inputs  56  and  56 ′ to the plus inputs AHI and BHI of A-D converter  58 . Switches are suitably provided to connect or disconnect the sense inputs from the A-D converter (switch  55  governing connection of the reference resistor  54  to sense lines  56  and  56 ′, and additional switches  57  and  59  governing connection to AHI and BHI respectively). Further, an ohms source  61  is provided through a switch  63  to the reference resistor. The end of: resistor  54  that provides input to the sense line also connects through a FET switch  60  to ground, while the control of the FET switch  60  is directed by a microamp enable signal (μA enable)  62 . The junction point of switches  51  and  52  also is provided as input to the minus side of A-D converter  58  as ALO and BLO, through switches  65  and  67  respectively. Additional reference resistors  69 ,  71  and  73  provide inputs to the control processor, to enable different range inputs, for example. In the illustrated embodiment, these additional resistors are not employed for microamp measurement functions. In a preferred embodiment, clamp  53  suitably comprises a pair of switching transistors. 
     Switches  51 ,  52 ,  55  and  63  are suitably Kelvin switches. Switches  59 ,  57 ,  65 ,  67  are switches in a matrix of switches within the instrument. The state of the switches depends upon the measurement mode. For example, in Ohms measurement mode, Kelvin switches  51 ,  55  and  63  are on, and matrix switches  67  and  59  are also on. Kelvin switch  52  is off, matrix switches  65  and  57  are off, and switch  60  is off. For voltage measurement mode, Kelvin switches  52  and  55  are on, matrix switches  65  and  57  are also on. Kelvin switches  51  and  63  are off, as are matrix switches  67  and  59 . Switch  60  is also off. 
     For microamp measurement mode, Kelvin switches  51  and  55  are on, as are matrix switches  57  and  65 . Switch  60  is on, while Kelvin switches  52  and  63  and matrix switches  59  and  67  are off. The ISRC control signal  75  sets op-amp  49  to a high impedance state, so it does not affect the current measurement. The AHI and ALO inputs to the A-D converter sense the voltage directly across reference resistor  54 . Because of this, the on resistance of switches  51  and  60  are eliminated from the measurement. 
     In normal operation (V check mode) for voltage measurement, clamp  53  conducts if input voltage peak levels are exceeded, to protect other meter circuitry from an overvoltage input. In addition, R p    46  and PTC  48  present a low impedance to the input signal. 
     In accordance with certain style multimeters manufactured by the assignee of the present invention, R p    46 , PTC  48 , R ref    54  and clamp switch  53  are already provided as input protection and measurement components. In an overvoltage situation, the clamp automatically connects the right end of the PTC/R p  resistor to ground. The impedance of the PTC resistor rises in this condition, providing a high impedance between the input terminal and ground, effectively protecting the rest of the meter from damage. Resistor R p  is present to limit the surge current to avoid damage to the PTC resistor. 
     In voltage measurement mode, for example, the voltage drop across R ref  is measured by the A-D converter. In accordance with the invention, these existing components are employed, and FET switch  60  is further utilized to enable dual use of R ref    54 . For micro amp measurement, (μA enable)  62  is activated (by the multimeter&#39;s control processor, for example) which sets the right end of R ref    54  at ground. The left end of R ref    54  is supplied to the minus input of A-D converter  58  (the plus input being set to ground by operation of FET switch  60 ). Current measurement is thereby accomplished. The actuation of FET  60  is accomplished by the multimeter&#39;s control circuitry as is understood by those of skill in the art. The positive threshold for the clamp may be set by the controlling processor. The CLAMP  53  signal levels are +2.6 volts and +1.2 volts. The positive clamp threshold is approximately 0.5 volts above these values. The negative clamp voltage is fixed at −1.2 volts. When in microamp mode, the circuitry is automatically protected by Rp  46 , PTC  48 , and clamp  53 . When a full scale current (400 uA, for example) is applied, the voltage at node  50  is below the clamp threshold. 
     With the embodiment illustrated, it will be noted that in microamp mode, the polarity of the connections to the A-D converter is reversed from the volt/ohm mode connections. This is accommodated by inverting the polarity of the measured value in further processing by the multimeter. The system according to the invention is also suitable for measurement of AC microamps, in addition to DC microamps. Such AC measurement is accomplished by re-routing of the measurement signals within the microprocessor by operation of the internal switching matrix thereof, to pass the signals through an AC signal source path. 
     Referring to FIG. 3, a perspective view of an exemplary multimeter incorporating the invention, common input.  42  and volt/ohm/current input terminal  44  are provided. A display  64  shows the measured value as well as the particular units of measurement. A selector region  66  includes plural switches  68  (Volts),  70  (ohms),  72  (microamps) and  74  (other function, e.g. continuity), which enable selection of the desired measurement mode. Multiple functions may also selected by a single switch. 
     Accordingly, the present invention adds the capability of selectable voltage/ohm and current measurement with only a two input multimeter. The existing overvoltage protection components are advantageously employed to work either as volt/ohm measurement systems or as current measurement components. 
     While a preferred embodiment of the present invention has been shown and described, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the invention in its broader aspects. For example, while the preferred embodiment is shown for microamp measurement, larger ranges can be provided with selection of appropriate current rating for the multimeter&#39;s components. The appended claims are therefore intended to cover all such changes and modifications as fall within the true spirit and scope of the invention.