Fallstricke präziser DC- Messungen

Transcription

1 Fallstricke präziser DC- Messungen Sascha Egger, Applications Engineer Group Leader National Instruments Switzerland GmbH

2 Agenda Overview of Precision Test Systems Techniques for: Low-voltage measurements Low-resistance measurements Low-current measurements High-impedance measurements Sweeps Conclusion: Measurement Considerations 2

3 What are Precision Measurements? Precision explicitly means: Reproducibility/repeatability Yield same results each time measurement is taken To man people, it also implies: Accuracy the overall uncertainty of a measurement Resolution number of bits used by the ADC (or DAC) to represent a signal Sensitivity the smallest signal detectable by a particular instrument on its most sensitive range 3

4 What are Precision Measurements? With modern instrumentation, these parameters usually come into question for the following: Voltage under 10 µv Current under 10 µa Resistance under 10 mω A precision measurement system should offer good accuracy, resolution, sensitivity, and precision for measurements under/of? these values. 4

5 Precision Measurement Examples Dielectric absorption Electrochemical Leakage Low-current semiconductor Light measurements Connector testing Surface insulation resistance Resistivity Charge measurements Temperature Contact resistance Superconductor resistance NI CONFIDENTIAL 5

6 Challenges in Precision Measurements Cost/Specialty of measurement equipment Environmental challenges Test times Cabling Channel count This presentation will provide a case study for building a precision DC test system in PXI and point out some pitfalls. 6

7 Case Study Precision Device Under Test (DUT) Reed relay SRC devices form a relay Tests to be performed: Contact resistance Coil resistance Thermal offset Coil pickup voltage, current Coil dropout voltage, current Open contact resistance Settling time 7

8 Relay Specifications 150 mω 10 GΩ-1 TΩ 500 us Ω 3.75 V, 0.8 V 8

9 Equipment PXI-1042 chassis and PXI-8119 controller: Measurements: o PXI ½-digit DMM o PXI wire guard and current amplifier Source: o PXI-4110 programmable DC power supply Switch: o PXI x16 reed relay matrix o PXI channel SPST relays External 100V DC-DC converter or SMU PXI

10 System Architecture 10

11 Low-Voltage Measurement Devices Data acquisition modules: X Series DAQ: 16 bits, sensitivity as low as 15 µv Absolute accuracy as low as 291 ppm (1 V range) Digital multimeters (DMM): PXI-407x FlexDMMs: bits, sensitivity as low as 10 nv Absolute accuracy as low as 28 ppm (wide temperature? range, 2-year calibration, 100 mv range) 11

12 Calibration Instruments must be calibrated at regular intervals. Benefits of calibration: Reduced measurement errors Consistency between measurements Assurance you are making accurate measurements Calibration procedures: Self-calibration Manual calibration procedures NI calibration services 12

13 NI-DMM Driver API, examples and help will be installed with NI-DMM driver. Supported platforms: LabVIEW & LabVIEW Real-Time LabWindows CVI & LabWindows CVI Real-Time Microsoft Visual C++ Microsoft Visual Basic

14 Soft Front Panels for Modular Instruments DMM Soft Front Panel will be installed with NI-DMM Driver. 14

15 Low-Voltage Measurements Major concern #1: Offset voltages Thermal offsets develop from junction points in the system (similar to a thermocouple). Voltmeter or DMM input offsets 15

16 Low-Voltage Measurements Major concern #2: Noise Johnson noise Magnetic fields Ground loops Johnson noise e nr = 1.28 x10-10 * Rf Twisted-pair wires to reduce noise 16

17 Thermal Offset 17

18 Low-Resistance and In-Circuit Measurement Devices Digital multimeters: PXI-407x FlexDMMs: bits, sensitivity as low as 10 µω Absolute accuracy as low as 60 ppm (wide temperature? range, 2-year calibration, 100 Ω range) 2-wire and 4-wire capability In-circuit guard amplifier accessory: PXI wire guard and current amplifier: enables 6-wire resistance measurements ±200 µv guard accuracy, typical 18

19 Low-Resistance Measurements Major concern #1: Lead resistance 2-wire measurement 4-wire measurement 19

20 Low-Resistance Measurements Major concern #2: Thermal EMFs Current reversal method Offset-compensated ohms V OCO = V M1 - V M2 = (I S R X + V THERMAL ) - V THERMAL = I S R X therefore: R X = V OCO /I S 20

21 Contact Resistance 21

22 Low-Level Resistance Measurements with DMM (4-Wire Mode) Internal current source (1mA) External current source (100mA) 22

23 Low-Level Resistance Measurements with a DMM and Additional Current Source External current source 23

24 Low-Current Measurement Devices Digital multimeters (shunt ammeters): PXI-407x FlexDMMs: bits, sensitivity as low as 1 pa Absolute accuracy as low as 390 ppm (2-year calibration, 1 µa range) Max burden voltage: <50 mv on 1 µa range, <700 mv on 3 A range PXI-4022 as a feedback ammeter: PXI-4022 current amplifier: I-V converter used with voltmeter, sensitivity as low as 0.5 pa Absolute accuracy as low as 7,000 ppm Max burden voltage: <20 µv on 100 na range 24

25 Low-Current Measurements Major concern #1: Generated currents Offset currents due to measurement equipment Triboelectric effects cable movements induce charge Piezoelectric effects mechanical strain induces charge Contamination/humidity reduce resistance 25

26 Low-Current Measurements Major concern #2: Leakage currents and guarding i test - i leakage HI Sense DUT HI DMM i test Guard + - i leakage Test Fixture LO Sense LO 26

27 Low-Current Measurements Major concern #3: Burden voltage I measured = (1.5 V V)/(5 Ω) I measured = 0.2 A I actual = 1.5 V/5 Ω I actual = 0.3 A 27

28 High-Resistance Measurements Low-current measurement + high-voltage source PXI-4022 feedback ammeter with sensitivity to 0.5 pa 100 V power supply 100 V / 0.5 pa = 100 TΩ Am 100 V R measured 28

29 Insulation Resistance 29

30 Low Level Current Measurement with a DMM and an Additional Preamplifier Connection diagram for using PXI-4022 with PXI-4071 as picoammeter 30

31 Voltage/Current Sweeps Necessary for device characterization: Example: sweep voltage to measure diode operation Key parameters: Voltage/current programming resolution Voltage/current readback resolution Programming speed (up and down) For our relay DUT, we measure the voltage or current required to operate and release the contacts. 31

32 Operate-Release Voltage-Current 32

33 Settling Time Settling time is the amount of time required for the contacts to close (including bouncing). Use PXI-4071 DMM with digitizer mode to measure microseconds. Apply voltage across contacts. Begin digitizer mode acquisition on contacts. Apply voltage across coil. Voltage across contacts will be shorted, so power supply will current limit. Amount of time to settle at limited voltage is settling time. 33

34 Settling Time 34

35 Testing Scalability Switching/cabling considerations dominate precision measurement scalability. Avoid testing the test equipment instead of the DUT. Perform system calibrations. Software plays a key role in productivity. TestStand, Switch Executive, and LabVIEW enable test system deployment to multiple DUTs. Maximize code reuse and performance with PXI hardware. 35

36 Conclusion: Measurement Considerations When taking measurements with a DMM, consider the following: Normal/Common/Effective -mode rejection ratio (NMRR/CMRR/ECMRR) Input resistance Burden voltage Thermal voltages Settling time Resistor self-heating Dielectric absorption Residual impedance Stray admittance 36

37 For more information Digital multimeter measurement tutorial Modular instruments product demonstrations Understanding instrument specifications Calibration FAQs 37

38 Questions? 38