Abstract: A method and apparatus are set forth which enables a gas chromatographic column output to be connected with a pulse discharge chamber in which chemically bound chlorine in volatile organic or inorganic samples is measured. A spark discharge is formed in the chamber to ionize and excite helium molecules to a metastable state. In turn, that transfers excitation to a trace of krypton gas in the chamber which is ionized, and the ionized krypton then preferentially binds with chemically bound chlorine. The latter binding occurs with the liberation of a photon centered at about 222 nanometers thereby defining a spectral region of interest which is measured by a photomultiplier tube to quantify chemically bound chlorine.

Abstract: The present invention is an apparatus and method for detecting differential mode signals in an environment where differential mode signals co-exist, and might be corrupted by, common mode signals. The most basic apparatus of the present invention essentially comprises first and second input leads through which both differential mode and common mode signals are input; a first amplifier block having a gain that is substantially one; and at least an inverting node and a non-inverting node connected to the first and second input leads. The output of the amplifier block is fed back to the input of the non-inverting node of the amplifier block in a manner to increase differential mode impedance while maintaining a low common mode impedance. Various embodiments of the basic presently claimed circuitry provides for additional methods of monitoring the level of common-mode signal introduced to the apparatus and other fault detection functions.

Abstract: The present invention is an apparatus and method for detecting differential mode signals in an environment where differential mode signals co-exist, and might be corrupted by, common mode signals. The most basic apparatus of the present invention essentially comprises first and second input leads through which both differential mode and common mode signals are input; a first amplifier block having a gain that is substantially one; and at least an inverting node and a non-inverting node connected to the first and second input leads. The output of the amplifier block is fed back to the input of the non-inverting node of the amplifier block in a manner to increase differential mode impedance while maintaining a low common mode impedance. Various embodiments of the basic presently claimed circuitry provides for additional methods of monitoring the level of common-mode signal introduced to the apparatus and other fault detection functions.

Abstract: A circuit for amplifying a weak direct voltage signal has a direct voltage amplifier that can be periodically switched from the direct voltage signal to a reference signal by a switching member connected to its input terminals. The circuit also has a measurement value processing device connected downstream of the direct voltage amplifiers. In order to obtain an amplified direct voltage signal closely approximating the weak direct voltage signal to be amplified, the switching member is arranged as a periodical shunt between the input terminals of the direct voltage amplifier.

Abstract: Electric apparatus responsive to currents and/or voltages having wide dynamic ranges incorporates ranging circuits for each input in which a multiplexer providing the ranging selection has a common input connected to the output of an operational amplifier providing the selective gain and has a plurality of outputs connecting selected ones of a plurality of series connected precision resistors in a feedback loop around the operational amplifier. A second operational amplifier is connected as a follower to the last output of the multiplexer so that the gain applied to the output signal is not distorted by the resistance of the switches in the multiplexer. For three-phase systems not having a neutral conductor, phase to neutral voltages with ranging are generated by subtracting a neutral-to-ground voltage produced from an artificial ground created by connecting three resistors in a wye across the phase conductors from the phase-to-ground voltages applied to the ranging circuit input.

Abstract: The disclosed electrometer is of the type including an integrating amplifier and an electronically controlled switch. When the switch is closed, the electrometer is reset or cleared, and when the switch is open, the amplifier operates as an integrator. The switch is configured to substantially eliminate leakage currents between the switch and the inputs of the amplifier. The electrometer is operative for very small input currents and is operative over a wide dynamic range. The disclosed electrometer may be used to implement a highly accurate pressure transducer.

Abstract: An output circuit in which both the signal type (i.e., current or voltage) as well as the signal level range (for example, 0 to 10 V, -10 V to +10 V) are based on user input parameters. The circuit is integrated in an application-specific integrated circuit (i.e., ASIC).

Abstract: An electronic circuit for measuring current in a working circuit is provided with a first operational amplifier having a positive input, a negative input, and an output connected to at least one in-line amplifier to form a first current path. A second operational amplifier having a positive input, a negative input, and an output connected to a differential amplifier is provided within the circuit to form a second current path. An input terminal is connected to the negative inputs of each of the operational amplifiers to provide a connection between the electronic circuit and the working circuit. A variable conductance component is connected in series between the input terminal and the negative input of the second operational amplifier to control the flow of variable currents through the two current paths.

Abstract: A current sensor includes first and second branches interconnected by a center conductor magnetically coupled to an annular current comparator for producing an output signal related, in magnitude and phase, to an input current. The input current divides, according to a predetermined relationship, between the first and second branches such that the current in the center conductor interconnecting the first and second branches is also related to the input current. A current is induced in the annular current comparator in relation to the current in the center conductor and, thus, in relation to the input current. Secondary windings are wound about and magnetically coupled to the annular current comparator for producing an output signal responsive to the current induced within the annular current comparator.

Abstract: Common mode error correction for differential amplifiers involves accurately measuring both the input and output of an amplifier using a low-leakage measurement path, and calculating common-mode gain. Two measurements are made at each node by appying two different common-mode voltages. Subtracting one set of measurements from the other eliminates voltage offset errors, and leaves a common-mode error term for gain calculation. The common-mode gain factor is stored, and thereafter, common mode error may be subtracted from measurements made by the differential amplifier.

Abstract: A circuit for measuring current flowing along a line including a resistance has two transistors with their emitters connected to the line on opposite sides of the resistance and with their collectors connected to a zero voltage rail via respective resistors. The bases of the transistors are connected together so that one transistor controls current flow through the second. The voltage across the resistor connected to the second transistor is representative of current flow in the line.

Abstract: A bidirectional, transconductance, DC amplifier exhibiting a compliance voltage exceeding either of the DC power supply voltages nearly double the compliance voltage of conventional circuits. The circuit uses a differential input operational amplifier. A voltage divider has its first resistor connected between the amplifier input and the noninverting input of the operational amplifier. The second resistor of the voltage divider is connected between the noninverting input and a common ground. A third resistor is connected between the amplifier input and the inverting input of the operational amplifier. A feedback circuit branch is connected between the inverting input and the output of the operational amplifier and includes a pair of output terminals for connecting the load in series in the feedback circuit branch.

Abstract: A high input impedance AC buffer for use in an instrument that does not require high frequency compensation or calibration. An op-amp receives an input signal that is to be measured via a high resistance input resistor and produces an output signal proportional to the input signal. A feedback resistor block and a feedback attenuator form the feedback path of the buffer and establish buffer gain. The low resistances of the feedback block and the feedback attenuator are substantially less than the resistance of the high resistance input resistor, precluding the need for compensation and calibration capacitors. A switching circuit operates to select resistors in the feedback resistance block and the feedback attenuator to provide predetermined gains for the buffer.

Abstract: A circuit for measuring current in a power MOS transistor (M0) comprises second (M1) and third (M2) transistors in series of the same type and same technology as, but having a smaller surface than, the power transistor and arranged in parallel on the latter. The two series transistors have their gates connected to the gate of the power transistor. The current in transistor (M2) which is connected to the reference electrode of the power transistor is measured.

Abstract: There is disclosed a monitoring circuit for a light emission device comprising; conversion circuit for converting a current flowing through a light emission device when a drive signal is applied thereto a voltage; shift adjustment circuit for producing shift adjustment signal of a variable output level; and a feedback type amplification unit for amplifying the output voltage of the conversion circuit to produce a monitor signal; wherein amplification unit includes gain control circuit for controlling gain of said amplification unit; the shift adjustment signal is supplied to a feedback loop of the amplification unit.

Abstract: An electrometer capable of measuring small currents without the use of a feedback resistor which tends to contribute a large noise factor to the measured data. The electrometer eliminates the feedback resistor through the use of a feedback capacitor located across the electrometer amplifier. The signal from the electrometer amplifier is transferred to a electrometer buffer amplifier which serves to transfer the signal to several receptors. If the electrometer amplifier is approaching saturation, the buffer amplifier signals a reset discriminator which energizes a coil whose magnetic field closes a magnetic relay switch which in turn resets or zeros the feedback capacitor. In turn, a reset complete discriminator restarts the measurement process when the electrometer amplifier approaches its initial condition. The buffer amplifier also transmits the voltage signal from the electrometer amplifier to a voltage-to-frequency converter.

Abstract: A spark detection apparatus is set forth and incorporates a closed chamber for receiving a carrier gas flowing therethrough between inlets and outlets, and the carrier gas is exposed to a pair of electrodes forming a spark across the chamber and through the carrier gas. Compounds of interest interact with the spark. The spark forms a diffusion of electrons or alternately ions during the spark distributed thereafter. The chamber includes a detector electrode spaced from the spark. High mobility particles (primarily electrons) are observed almost instantaneously with the spark while low mobility ionic particles diffuse more slowly after the termination of the spark. Using an inert carrier gas, high energy metastable molecules are dispersed and give up energy over time after the spark. The output is obtained from the electrode during, immediately after, or after a long delay relative to the spark. Another alternate output is obtained from the observed spectra during the spark and after the spark.

Abstract: Appropriately constructed differential amplifiers are suitable as wideband multiplexers. It is the object of the invention to use such amplifiers in conjunction with a measuring range switch. For this purpose, the differential amplifier is equipped with at least three differential amplifier stages, all of which have a common input and different gain due to different current feedback resistors. The switching is effected through blocking transistors, only one of which is optionally switched on in each case and connects a common current source to the associated differential amplifier stage. The invention can be used in measuring technology for multimeters and oscilloscopes.

Abstract: A circuit for providing a scaled sinusoidal voltage proportional to a sinusoidal current referenced to a first reference potential. The scaled sinusoidal voltage is referenced to a second reference potential. A scaling resistor is connected between an operational amplifier inverting input terminal and the operational amplifier output terminal. A transformer has a primary winding in series with the sinusoidal current. A secondary winding has a first terminal coupled to the second reference potential and a second terminal coupled to the operational amplifier inverting input terminal. An integrator has an inverting input terminal coupled to the operational amplifier output terminal, a noninverting input terminal coupled to the second reference potential and an output terminal coupled to the operational amplifier noninverting input terminal.

Abstract: A driver circuit for magnetic gages that electrically simulates the gage, both with and without a parallel resistor joined in conjunction therewith. An averaging circuit assists in preventing the circuit from responding to transient variations in the monitored parameter.

Abstract: A current measuring circuit for an integrated circuit comprises an operational amplifier (22), the non-inverting input terminal of which is grounded through a first resistor (23) having a value R.sub.B and a second resistor (21) having a value R.sub.2, the junction of those two resistors receiving the current (I.sub.NL) to be measured, and the output of which is connected by an inverter (26) to a MOS transistor (27). The biasing voltage of the operational amplifier and the characteristics of the inverter are chosen so that the MOS transistor is biased at its threshold in the absence of a current to be measured, whereby the output voltage V.sub.s is correlated with the current to be measured by a linear relation independent of the temperature.

Abstract: A non-contacting electrostatic voltage follower employs a vibrating capacitance detector coupled to a measured surface containing both wideband electrostatic A.C. data and D.C. potentials. A detector amplifier having an inverting input is connected to the detector, while its output is connected to the input of a high bandwidth amplifier. A first feedback path is provided from the output of the high bandwidth amplifier to the non-inverting input of the detector amplifier which provides both a signal which is equal to and follows the wideband electrostatic A.C. potentials on the measured surface, and a signal, at the vibrating capacitance modulation frequency, representative of the D.C. potential of the measured surface. A second feedback path is provided including a demodulator and integrator to monitor the first feedback path to provide a D.C.

Abstract: An automatic gain setting circuit for automatically adjusting the gain of an amplifier which drives an analog meter. The gain of this amplifier is adjusted automatically with the push of a button switch so that the meter will read mid-range for the signal input to the amplifier. In this circuit, the operational amplifier is connected between an input signal and the analog meter readout. This operational amplifier utilizes a feedback circuit having a variable resistance which changes the gain of the amplifier as the resistance is changed. The resistance in this feedback circuit is changed by an electrical comparison circuit which compares the output signal from the amplifier with a reference voltage upon the push of a button switch. The reference voltage is preset to cause the meter to read mid-range. Once the button switch is pushed, the comparison circuit provides an output control signal which is directed to the feedback circuit of the operational amplifier.

Abstract: Various combinations of three elements are disclosed for use in accurate measurement of power through current measurement over a wide dynamic range, namely, a current shunt which is preferably an isothermal current shunt, a current transformer, which is preferably a high initial permeability current transformer, and a low-impedance burden load, which in the preferred embodiment includes an active negative impedance element which causes the removal of the effects of excitation current by canceling secondary winding resistance of the current transformer. In a specific embodiment of an isothermal current shunt according to the invention, a very linear device is achieved by construction out of copper in such a manner that the poor resistance versus temperature coefficient of copper does not affect the accuracy of measurement. Specifically, the shunt is constructed in an unbalanced isothermal bridge configuration, so that heat-induced variations are suppressed.

Abstract: An atto amperemeter includes a shop tip (1) correctable to a voltage source (18) providing a known potential (U') via a sample conductor measured, the tip (1) faces a particle detector such as an electron multiplier (4). The count of the free electrons being emitted from the tip (1), multiplied by the electron multiplier (4) and counted by an electronic current (i) passing through the sample conductor (17). Calibration of the arrangement is performed by temporarily connecting the tip (1) to ground and applying a calibration potential (U cal) to a calibrated electrode (3) aligned with the tip (1) and the channel electron multiplier (4).

Abstract: A new sensor means for accurately measuring the current flowing in an electronically controlled switch is presented. An ac coupled current sensor is arranged in circuit with the electronically controlled power switch. The output of the sensor channel is zeroed (or restored to another fixed independent level) whenever the power switch is in the OFF condition. Using the natural modulation of the power switch itself to coordinate the reconciling of the sensor output level with the OFF state of the power switch helps restore to the dc level lost through ac coupling. This coordination of the sensor with the switch itself reduces dc offset and drift and allows ac coupling, which removes dc common mode and increases primary current sensor flexibility, all without adding complexity. A plurality of such sensors can be combined to measure indirectly the load current from a plurality of switches according to Kirchoff's Current Law.

Abstract: A circuit for providing DC bootstrapping to an AC amplifier integrates a sample of the output of the amplifier and feeds the result of that integration to the input of the amplifier, thereby driving the input of the amplifier to a zero potential level. The integration can be accomplished by an operator amplifier and a capacitor in a feedback circuit.

Abstract: Self contained radio transmitting state estimator modules are mounted on power conductors on both sides of power transformers in electrical substations and on power conductors at various places along electrical transmission lines. They are electrically isolated from ground and all other conductors. These modules are capable of measuring current, voltage, frequency and power factor (or the fourier components thereof) the temperature of the conductor, and the temperature of the ambient air. The modules transmit these parameters to local receivers. The receivers are connected by an appropriate data transmission link, to a power control center which allows determination of the state of the power system. Appropriate control signals are transmitted back to the electrical switchgear of the system to bring it to the appropriate optimum state. Direct local control may also be effected, for example, the prevention of overloading a transformer.

Abstract: A current sensing circuit senses current and signals if such current exceeds a predetermined level within an etched circuit board. The circuit comprises a conductive trace of a predetermined resistance within a metallization layer of the board and a reference voltage circuit and comparator coupled to the trace. The reference voltage circuit senses voltage at first sense point on the trace and offsets the voltage a predetermined amount to form a reference voltage. The comparator senses voltage at a second sense point on the trace and compares it to the reference voltage. So long as the second voltage exceeds the reference voltage, the comparator indicates that the current is below the predetermined level. If the second voltage drops below the reference voltage, the comparator generates an output signal indicating that the current has exceeded the predetermined level.

Abstract: A circuit for measuring the leakage current of an electrometer amplifier 10 determines the leakage currents at both amplifier input terminals simultaneously by a method that eliminates the effects of the amplifier offset voltage as well as of the offset voltages and leakage currents of the amplifiers (22 and 40) used in the measurement. With no amp. under test present in the circuit, measurements are made of the outputs of the two test-circuit amplifiers (22 and 40). This results in quantities that indicate the required compensation for the offset voltages and leakage currents of the test-circuit amplifiers. From then on, measurements are made of the amplifier output voltages with an amp. under test present and the leakage currents at both input terminals of said amplifier 10 under test can be inferred from these second measurements by using the information obtained from the first set of measurements.

Abstract: An apparatus for measuring low currents with high dynamics, comprising in series: a multi-range preamplifier, a logarithmic amplifier, a thresholding circuit, an alarm circuit and a reading circuit, the preamplifier comprising in series: a first differential amplifier receiving at its inverting input the low current to be measured and a second amplifier, the first amplifier having a linear feedback chain of very high impedance and the second amplifier having a linear feedback chain of much lower impedence, a zero point adjustment and a circuit for testing the gain of the apparatus being also provided on the first amplifier.

Abstract: Apparatus for measuring the difference between two voltages in which a first current proportional to the voltage difference is generated at a first point, and there is circuitry for delivering at a second point a second current that is based on the first current and is indicative of the voltage difference; the circuitry includes an uninterrupted current path from the first point to the second point and the current path has a circuit element across which at least a portion of the common mode voltage appears; and the circuit element provides an output current to the path which is independent of the voltage across the circuit element.

Abstract: A signal probe that does not require a direct ground contact is disclosed. The probe consists of a resistive measuring tip, an amplifier and a probe body which acts as an antenna providing a ground reference for the probe. The frequency of the signal measured may be compensated for by adjusting the resistive tip in the probe which varies the shunt capacitance between the resistor and the probe body.

Abstract: This field test instrument serves to home test the horizontal output circuits of television, so as to indicate if the circuit of the set is operating normally. Primarily, it consists of a housing with a circuit, which includes an ammeter, a pair of transistors, a circuit breaker, and a cable with an adapter jack, for placement into horizontal heat sink socket. The instrument further serves to indicate if the set is drawing too much current, and will let the technician know if it is safe to put in a replacement transistor, without having to take the set to the shop for repairs.

Abstract: In a gain control circuit, an electric power calculating circuit is connected to a variable gain amplifier and a gain setting circuit for generating a gain control signal applied to the variable gain amplifier to control the gain thereof.

Abstract: A signal quality monitor, and a protective relay system using the monitor, which utilizes both AC and DC signals from the receiver agc stage to monitor a plurality of different signal parameters. A signal responsive to the noise in the communication channel which transmits at least one of the protective relay signals used in a protective relay comparison function, is also used in the trip circuits of the associated circuit interrupter to continuously adapt or modify the trip level in response to channel noise.

Abstract: In a DC current detector, a current to be detected is applied to a current detecting resistor connected in series with a first diode, a voltage across the current detecting resistor is amplified by a transistor and the detected voltage is output from a collector resistor connected to the collector of the transistor. The voltage across the current detecting resistor is provided across the emitter and base of the transistor via an emitter resistor, and a second diode. A resistor the resistance value of which is selected so that the forward voltage of the second diode and the base-emitter voltage of the transistor may become equal to each other, is connected to the base of the transistor. The current detection accuracy is improved.

Abstract: A range switching device for an electric meter, e.g.

Abstract: A circuit for reducing the amplitude of a common mode voltage across the inputs of a differential measuring instrument. The circuit comprises a voltage divider between each of the differential input terminals and the instrument zero reference point on one side, and between the instrument's zero reference point and the common mode voltage reference (usually earth) on the other side. A potentiometer forms the divider junction at the instrument reference point and is used to compensate for resistance value tolerances in the circuit and any imbalance in the input path impedances of the two differential inputs. The value of the voltage divider resistors are selected so as to bring the instrument reference point as close as possible to the level of the common mode voltage without unduly reducing the instrument input impedance.

Abstract: An input terminal adapted to receive 0 to 300 volts AC or DC is connected through AC or DC connecting circuits, coupled into the path by switches, and attenuating apparatus including a plurality of ranges automatically switched into the path in response to the amplitude of an input signal, to a buffer which includes overvoltage protection and provides a relatively high input impedance with a relatively low output or source impedance. A frequency counter is connected through a hysteresis amplifier, a digital volt meter is connected through an amplifier including 60 cycle filtering and an oscilloscope is connected through an independent gain amplifier to the output of the buffer.

Abstract: A preamplifier circuit for a voltage measuring instrument which provides for noninteracting span and zero adjustments. Two voltage follower circuits are used. The first produces a current through the output resistor of the preamplifier proportional to the voltage being measured while the second produces a current through the output resistor proportional to a fixed reference voltage. For both amplifiers, the proportionality is adjustable by a potentiometer which modifies the feedback on the first amplifier and the part of the reference used as input to the second amplifier.

Abstract: A level indicating circuit includes a pair of transistors, arranged as a differential amplifier and to whose bases an input signal is applied, a level-detecting circuit to supply to a meter or the like an indicating current whose level is related to the magnitude of the input signal; and a network connecting the collector of at least one of the transistors of the differential amplifier to the level-detecting circuit and establishing a threshold for the input signal such that the level-detecting circuit supplies its indicating current only when the input signal exceeds the threshold. The level-detecting circuit can include a transistor which is biased so as to begin conducting only when the magnitude of the input signal exceeds the dynamic range of the differential amplifier and drives one or the other of the transistors thereto to cutoff or saturation.

Abstract: A circuit for sensing the magnitude of an AC signal applied to the circuit includes control means for reversing the polarity of the AC signal to thereby generate an output signal which is proportional in amplitude to the magnitude of the AC signal and which has a phase, with respect to the phase of the AC signal, determined by the control means. The control means also provides the capability of generating output pulses from the circuit which have an amplitude proportional to the magnitude of the AC signal and a pulse-width-spacing proportional to the duty cycle of a pulsating control signal applied to the control means.

Abstract: The disclosure describes an improved apparatus for measuring the flow of current through a conductor by means of a Hall generator. The Hall generator is biased by a constant current source employing feedback. In order to increase the accuracy of the current measurement, operational amplifiers having inputs which float in relationship to the power supply operating the amplifiers are used to amplify the Hall generator output voltage and to drive a meter. The current measuring apparatus can be used to detect defects in a vehicular alternator by measuring the fluctuating components of the alternator output current. The magnetic field generated by the current to be measured is concentrated in the area of the Hall generator by means of a unique hollow, cylindrical, coiled strip of metal made from nickel, molybdenum and iron. The apparatus includes a regulator in which the voltage drop is so low that a 3.25 volt regulated supply can be maintained from a poorly-charged 6-volt battery.

Abstract: An electrical meter has light-emissive display elements and a meter circuit featuring electrically floating input terminals relative to device ground, and offers a potentially significant cost advantage compared to available alternatives in low-accuracy measuring applications.A series string of the light-emissive display elements are physically arranged along a line in order of increasing turn-on current threshold of each element so that they are successively turned on along the line with increasing current to be measured. A control circuit included in the series string has first and second semiconductively-complementary active semiconductor devices, such as an NPN and PNP transistor, each with base, emitter, and collector. The input-sensing conductor, such as a base, of each active semiconductor is a respective floating input of the meter.

Abstract: Apparatus and method of compensating for a shift in the phase of the current applied to a current transformer terminated in a low impedance operational amplifier. The operational amplifier phase response is altered to minimize the phase angle between the transformer current and the amplifier output voltage to thus provide an output voltage directly proportional to the transformer current.

Abstract: The suspension of movable system instruments, for example, a galvanometer, introduces elastic reset torques upon deflection. To compensate for such torques and render the apparatus essentially independent of the deflection torques, an opto-electronic device provides an output signal representative of the deflection which is combined with the deflection voltage applied to the galvanometer to at least in part compensate for the reset torque introduced by the moving system suspension.

Abstract: An a.c. to d.c. voltage converter of this invention comprises an a.c. voltage amplifier whose inverting input is connected to a lead of an input resistor, and whose non-inverting input is grounded for alternating current. The converter further includes a non-linear detection unit having a separating capacitor, a diode with one electrode connected via the separating capacitor to the output of the a.c. voltage amplifier and the other electrode connected to the inverting input of the a.c. voltage amplifier, and a means for producing direct linear current through the input resistor connected in parallel with the diode and electrically coupled to the input of the converter. The a.c. to d.c. voltage converter of this invention provides for automatic filtering of detected signals and insures excellent time stability of the conversion coefficient.

Abstract: An arrangement for the determination of the effective value and/or the performance or power and, for example, also the cross-power of electrical signals, in particular those Doppler signals which are received in ultrasonic-Doppler blood flow measurement. The arrangement includes an amplifier for the present electrical signal regulatable in amplification which, on the one hand, is connected into a control loop or circuit for regulating the degree of amplification for the purpose of producing constant amplifier power outputs and, on the other hand, connected into a measuring circuit for determination of the degree of amplification which adjust itself at the current power constant-regulation of the amplifier.

Abstract: The meter is primarily adapted to replace a standard analog panel meter and is designed to selectively receive different ranges of input signals such as DC input signals in ranges extending from 50 mVDC to 500 VDC. In the disclosed embodiment there are four ranges also adapted to receive AC input signals in ranges extending from 100 mVAC to 480 VAC. Preferably, a jumper is used to select the different ranges. The jumper is readily accessible at the rear of the meter. The meter circuitry comprises an analog to digital converter having three digit, seven segment display and input circuitry including a range circuit, an operational amplifier and filter circuitry coupling serially to the converter.