Model34904

SectionIV

SECTIONIV

T H E O ROYF O P E R A T I O N

4I . INTBODUCTION .

42.This section describes the methods and circuits used in the Model 3490A Multimeter to make dc voltage, ac voltage, and resistancemeasurements.The circuits neededfor ratio measurement,sample-and-hold measurements,remote control, and data output arealso describedA. generaltheory ofoperation is followed by a more detailedexplanationof the circuitsused.

+3 . GENERALTHEORYOFOPERATION .

44.The Model 34904 Multimeter usesthe dual-slope integrationtechniquefor measurement(seeFigure4-1), in which an integrator chargesfor a fixed length of time to a voltageproportional to the input sigral,and then is discharged at a fixed rate determined by a known reference voltage. The measurement display is deter- mined by the dischargetime, which is proportional to the input sigral. The integrator is part of the Analog-to- Digital Converter shown in the Basic Block Dagram in Figare 4-2. A description of the basic operation of the 3490A is contained in Figure 4-2 and Paragraphs4-5 through 4-l 8.

45.SignalConditioningGircuits.

46.The signal conditioning circuits include the DC

Input Attenuator, the AC Converter, and the Ohms Convertercircuits. The output ofone ofthese circuitsis applied to the DC Amplifier for the run-up portion of the measurementsequence.

47. ReferenceVoltages.

4-8. One of three referencevoltagesis applied to the DC Amplifier input for the run-down portion of the

GREATERINPUT_

SMALLERINPUT- - - - - . r\?

ßüy

measurementsequenceThe. proper referenceis selected by the Logic circuits accordingto the function selected and/or the polarity of the input signal.

+9. DCAmplifier.

z1-10.The DC Amplifier output is l0 Vdc for a full- range input on any range in any function. For any measurementexcept Sample/Hold,this output goesto the Analog-to-Digital Convertercircuits. In Sample/Hold measurements,the DC Amplifier output is appliedto the Sample/Holdcircuits (seeFigure 421), nd the Sample/ Hold output is appliedto the A-to-D Converter.

4l 1. Analogto-DigitalConverter.

412.The Analog-to-Digital (A-to-D) conversioncircuits consist of an Integrator, followed by a x20 Amplifier and,a Zero Detect Amplifier. If the Integratorinput is positive during run-up, the A-to-D output goes HIGH (near + 5 V) during run-up and retums to LOW (near 0 V) when the Integrator is dischargedto zero. If the input is negative,the A-to-D output goesLOW during

run-up and HIGH at the zero detectpoint. Input signal polarity, as well as "end of measurement"information, is derived from this output signal. The length of time betweenthe start ofrun-down and the zerodetectpoint determinesthe numericalvalueof the display.

413. Logic.

4-14. The timing of the measurementsequence is

. .qontrolled by the logic circuits. This timing may be influencödby the rangeand function selectedThe. basic

'"' a crystal-controlledoscillator, from which a clock is

number of timing signalsare derived through dividing counters. The sample rate is controlled by the logic

I N T E G R A T O R C H A R G E S

F O R F I X E D T I M E .

3 4 9 0 - 8 - 3 5 8 4

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R O

I N T € G R A T O R D I S C H A R G E S A T F I X E D R A T E .

Figure4-1. Dual-Slopelntegration.

4-1