1. PCAR Precision In Practice 1 Precision In Practice Achieving the best results with precision Digital Multimeter measurements Paul Roberts Fluke Precision Measurement Ltd.

2. PCAR Precision In Practice 2 Introduction Precision dmms are sophisticated instruments –comprehensive capability can appear complex Modern precision dmms can offer efficiency & usability advantages over traditional methods –at similar performance levels Objective: Explore precision dmm operation & provide guidance on using advanced features –particular focus on resistance measurement Illustrated with examples from a new design

3. PCAR Precision In Practice 3 Multimeter Architecture Analog to Digital Converter (ADC) is key element in a precision multimeter –multi-slope integrating ADC –8 1 / 2 digit resolution, 2 x 10 8 counts (1 999 999 99 fsd) –better than 0.1ppm full scale linearity DC PreAmp ADC RMS Converter Current Converter Ohms Converter Input Switching AC PreAmp Front Input Rear Input

4. PCAR Precision In Practice 4 ADC Operation Multi-slope integrating ADC –higher resolution  longer integration time –longer integration  more averaging, lower noise –multiple ADC cycles digitally averaged *Eg: 7.5 Normal, 8.5 Fast & Normal ADC Frequency Response 20dB/ decade roll-off f = 1 T Integration f2f 3f4f 5.5 Resolution 6.57.58.5 Integration Time ADC Resolution and Integration Time 10ms 100ms 1s 10s = Fast mode = Normal mode (3.3/3.3) (16.7/20) (267/320) (1.07/1.28) (17.1/20.5) (4.25/5.12) (60Hz/50Hz) Can also reject unwanted power line pickup –if integration time exact number of line periods –requires correct instrument line frequency setting (50Hz/60Hz)

5. PCAR Precision In Practice 5 Resistance Measurement Topology Current source sinks current from Input Lo Low Follower maintains Sense Lo at 0V Resulting potential difference measured via Sense Hi by DC Voltage sub-system DC Voltage Pre-Amp Sense Lo Sense Hi Input Hi RxRx Input Lo Constant Current Sink Lo Follower Ohms Range Control

6. PCAR Precision In Practice 6 Avoiding Thermal Emf errors Cancelling static & dynamic thermal emfs –True Ohms –Offset Compensated Ohms V 1 = Current Off = S1 Open V 2 = Current On = S1 Closed V Rx S1 I R x = V 2 - V 1 I Effectively measures and removes thermal offsets –current ON & OFF measurements But, modulates stimulus current at read rate –can lead to errors if UUT resistor sensitive to power dissipation changes

7. PCAR Precision In Practice 7 Current Reversal True Ohms Sense Lo Sense Hi Input Hi Input Lo Reversal Switching PD Measurement (V) Current Source (I) UUT Resistor (R) Thermal Emf (V th ) Sense path reversal ensures V 1 & V 2 same polarity for ADC Offsets in PD measurement path after reversal not cancelled –removed by zero calibration and input zero operations With forward current: V 1 = I x R + V th With reverse current: V 2 = -(-I x R + V th ) Averaging V 1 and V 2 : = 0.5( 2 x I x R +V th –V th ) = I x R

8. PCAR Precision In Practice 8 Experimental confirmation Experimental procedure (R Std = 10  ): Note: Thermal emf magnitude & rate of change greatly exaggerated…. Allow setup to stabilise (V th <100  V) Plunge thermocouple into water bath at ~35  C Readings taken & stored automatically by PC Compare Normal & Tru Ohms results Precision Dmm Thermocouple Standard Resistor Sense Hi Sense Lo Input Hi Input Lo True Ohms Dmm V

9. PCAR Precision In Practice 9 Results - Normal Ohms 20  range stimulus current = 10mA  100  V  10m  Measured resistance value tracks thermal emf Precision Dmm Thermocouple Standard Resistor Sense Hi Sense Lo Input Hi Input Lo True Ohms Dmm V

10. PCAR Precision In Practice 10 Results - Tru Ohms Effect of changing thermal emf eliminated –almost! Thermal emf initial rate of change extremely fast –initial cancellation less effective due to comparatively long integration time Precision Dmm Thermocouple Standard Resistor Sense Hi Sense Lo Input Hi Input Lo True Ohms Dmm V

11. PCAR Precision In Practice 11 Results - Tru Ohms, shorter  time Shorter integration time, more rapid reversal More effective cancellation of rapidly changing V th

12. PCAR Precision In Practice 12 Ensuring effective V th cancellation at the higher resolutions Multiple ADC cycles used at higher resolutions –to achieve required integration time V th Time ADC Cycles Current Reversal per Reading V th Time Reversal per ADC Cycle ADC Cycles Current Reverse current for each ADC cycle –more effective tracking and cancellation of changing thermal emf (V th )

13. PCAR Precision In Practice 13 Ratio measurements Two input channels –front & rear terminals Typical application: Comparing resistance standards V RxRx V R std Front Input Rear Input Stimulus current & potential difference measurement scanned between inputs –each resistor connected separately to measurement circuits But… resistor power dissipation modulated at scan rate –can lead to errors due to resistor temperature changes

14. PCAR Precision In Practice 14 Resistance ‘Voltage’ Ratio Stimulus current passes continuously though both resistors in series Potential difference measurement scanned between the two (front & rear) channels Front Input Rear Input INPUT Lo SENSE Lo SENSE Hi INPUT Hi INPUT Lo SENSE Lo SENSE Hi INPUT Hi Potential Difference Measurement Stimulus Current Source (Reversing)

15. PCAR Precision In Practice 15 Elimination of measurement errors Continuous current flow eliminates power modulation in UUT resistors –avoids dynamic & average power changes in the resistors –useful if resistors have significant power coefficient –particularly useful for PRT/RTD measurements Current reversal eliminates thermal emf errors –using the Tru Ohms resistance function Helpful for low resistance measurement –measurement currents are higher, Eg: 100mA on 2  range –measurement voltages lower, Eg: 200mV fs

16. PCAR Precision In Practice 16 High Voltage Ohms measurement Stimulus currents from 10  A to 10nA Resistance ranges from 20M  to 20G  DC Voltage Pre-Amp Sense Lo Sense Hi Input Hi RxRx Input Lo Constant Current Sink Lo Follower Ohms Range Control 200V output compliance 200V range Z in = 10M  I in Increased Lo Follower compliance voltage –From 20V to 200V Voltage measurement uses 200V DCV range –Measurement unaffected by DC Preamp input current I in, supplied by Lo Follower

17. PCAR Precision In Practice 17 Using High-Voltage Ohms For increased high resistance measurement capability –20M  to 20G  ranges Higher measurement current reduces effects of leakages, etc –better accuracy –improved noise performance –faster settling Allows evaluation of resistor voltage coefficients –measure in normal and high voltage ohms modes

18. PCAR Precision In Practice 18 Conclusions An appreciation of precision multimeter operation allows users to identify and configure the most appropriate measurement configurations for their application…... Many sources of error can be avoided –excessive noise, thermal emfs, thermal modulation, etc Particularly in resistance and PRT/RTD applications