TUP-8

AUTOMATED MEASUREMENT OF CAPACITANCE FROM 500pF to 4omF

Geoffrey R Ives, Peter B Crisp and Paul C A Roberts Wavetek Ltd.

Datron Division Hurricane Way

Norwich Airport, Norwich, NR6 6JB.

Abstract

This paper describes a method of deriving capacitance from Current, Voltage and Time measurement with high accllliicy over a wide range. The method was developed to provide automated traceable measurement of the continuously vaxiable capacitance function of a newly developed calibrator.

Summarv of PaDer

Introduction:

Modem multifimction measuring standards make it relatively easy to automatically caliirate voltage, current, resistance and frequency. A new multifunction calibrator with a function providing continuously variable capacitance from 50OpF to 40mF prompted the development of an automatic measurement system to m e r this range of capacitance using a single method. Total system measurement uncertainties in the order of 0.01% were required.

Various methods of automating the calibration of capacitance were investigated but only one technique provided sufticient accuracy over the whole range. In the method chosen, capacitance is derived from the change of voltage with time caused by charging the capacitor with a known, constant current 111. The technique is not new as the majority of Hand Held DMM's use some form of this method, but significantly better accuracy was needed to achieve the specifications required for the new calibrator (Wavetek model 9000).

One of the initial difficulties was how to determine the time taken to measure a voltage using a stand alone voltmeter. This was overcome by initiating measurements with a trigger pulse of known repetition rate.

The main elements of the system are:

1. A programmable constant current sowee with 6 ranges Erom l O O n A to lmA, whose output is constant over the required voltage span.

2. A linear, high speed DC digital voltmeter with sufficient scale length to enable small increments of a voltage ramp to be resolved to the required acnuacy. Readings are retained within the voltmeter memory until required.

3. A crystal controlled timing source linked to current sou~ce on/& control providing triggers to the digital voltmeter.

4. Computer control via a standard GPJB interface.

The unlcnown capacitance is connected across the current source terminals. The voltmeter is connected in parallel with the capacitor to measure the voltage ramp caused by the current in the capacitor. In the quiescent state, the current output is off and a shunting resistor ensures that all components are discharged so the voltage ramp may start at close to zero volts.

At the start of the measurement, the selected current is turned on and begins to charge the capacitor heady. One trigger period later, the first trigger pulse initiates a voltmeter reading. Reading triggers continue for a predetermined period then the analogue system is reset to the quiescent state.

Accumulated readings are extracted Erom the voltmeter in a single string containing a series of absolute voltage measurements from which step sizes are calculated. Step sizes are used in a l l calculations rather than total ramp voltage to facilitate analysis of the quahty of the

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result (as steps are coherent, the average step size may be calculated to the full resolution ofthe voltmeter).

Capacitance is calculatedfiom C = 1.TNwhere I is the known stimulus current, T is the known time between voltmeter trigger pulses and V is the average step size. Generally the results of several ramps are averaged to reduce the effect of random noise.

Potential errors:

There are many possible sources of error in this type of measurement but most can be eliminated or reduced to acceptable levels. Errors which are constant are removed by system calibration. Other sources of error are reduced by careful design. The major sources of error and solutions are:

Resistance in the capacitor charging path cause3 steps in the voltage ramp as the current is turned on and off. This error is avoided by starting and stopping the measurement within the steps.

Self capacitance within the system. This has the effect of an offset error and is removed during calibration by a 'zero' measurement.

Capacitor charging current not accurately knownbecause ofleakage paths such as voltmeter input current. Effectively removed during system caliiration as the same voltmeter is used during voltage ramp measurements and current source calibration.

Leakage resistance in the unknown capacitance. This manifests itselfas a non- linearity in the voltage ramp. Linear@ can be determined from the step size variation in the ramp. This is not a measurement system error (other than in system self capacitance) and

Resalts:

although not removed, the error can be quantified and thus allowed for in UILCertainty calculations.

Dielectric absorption in the unknown capacitance. Tbis error is frequency depembt and affects all methods of capacitance measurement. Inthe system d e s c r i i theefkctivecalibrationfrequencyis close to that applied when the calibrator is in n o d use to minimise dielectric effects.

Inaccuracies due to the input voltage slew rate (maximum 67Vfsecond) and variable delay in the digital voltmeter between receipt ofthe trigger and start ofthe A to D conversion. These have both been evaluated and found to be negligible in the type ofvollmeter used.

Other errors due to the accuracy of the current source and voltmeter, and Electromagnetic errors due to coupling via power supplies and control circuits are minimised by selection of component parts and circuit design.

The capacitance measurement system has been operating m x e d d l y for 6 months. The ammicy of the system has been checked independentl YWing various ratio techniques and known capacitors. A certified LCR Bridge has also been used where the specificaton of the Bridge allows accurate measurements to be made. Consistency of results has been very good.

Reference

[ 13 Geoff R Ives "Automatic Realisation of C = &I&", IEE Colloquium on Automation in Electrical Measurements, 30th November 1993.

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