2. Digital Instrumentation

3. Topics Multimeters Digital Multimeters (DMMs) Specifications (Display,Accuracy, Resolution) Meter Applications

4. Moving-Coil Meter Two Types of Multimeters VOM (analog) DMM (digital) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

5. 8-1: Moving-Coil Meter Types of Meters Analog meter: Uses a moving pointer and a printed scale to indicate values of voltage, current, or resistance. Volt-Ohm-Milliammeter (VOM): Allows all three kinds of measurements on a single scale or readout. Digital multimeter: Uses a numerical readout to indicate the measured value of voltage, current or resistance.

6. 8-1: Moving-Coil Meter Direct Current Meters Direct current in a moving-coil meter deflects the pointer in proportion to the amount of current. A current meter must be connected in series with the part of the circuit where the current is to be measured. A dc current meter must be connected with the correct polarity.

10. Susanta K Pal PMD Collaboration Meeting, December 26th 2005 10 Analog to Digital Conversion Techniques 1. Flash ADC 2 n comparators for n bits (8 bit resolution requires 256 comparators) Feasible in monolithic ICs since the absolute value of the resistors in the reference divider chain is not critical, only the relative matching. Advantage: short conversion time (<10 ns available) Drawbacks: limited accuracy (many comparators) power consumption more Differential non-linearity ~ 1% High input capacitance (speed is often limited by the analog driver feeding the input) The input signal is applied to n comparators in parallel. The switching thresholds are set by a resistor chain, such that the voltage difference between individual taps is equal to the desired measurement resolution.

11. Susanta K Pal PMD Collaboration Meeting, December 26th 2005 11 Analog to Digital Conversion Techniques 1. Flash ADC 2 n comparators for n bits (8 bit resolution requires 256 comparators) Feasible in monolithic ICs since the absolute value of the resistors in the reference divider chain is not critical, only the relative matching. Advantage: short conversion time (<10 ns available) Drawbacks: limited accuracy (many comparators) power consumption more Differential non-linearity ~ 1% High input capacitance (speed is often limited by the analog driver feeding the input) The input signal is applied to n comparators in parallel. The switching thresholds are set by a resistor chain, such that the voltage difference between individual taps is equal to the desired measurement resolution.

12. Multimeters Multimeters are also called multitesters. Multimeters are used to measure voltage, current, or resistance. Main types of multimeters are: Volt-ohm-milliammeter (VOM) Digital multimeter (DMM)

13. Multimeters Table 8-3 VOM Compared to DMM VOMDMM Analog pointer readingDigital readout DC voltmeter R V changes with rangeR V is 10 or 22 MΩ, the same on all ranges Zero-ohms adjustment changed for each range No zero-ohms adjustment Ohm ranges up to R x 10,000 Ω, as a multiplying factor Ohm ranges up to 20 MΩ; each range is the maximum

14. Multimeters Fig. 8-13: Analog VOM that combines a function selector and range switch. Fig. 8-14: Portable digital multimeter (DMM). Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

15. Multimeters Fig. 8-15: DMM with amp clamp accessory. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The problem of opening a circuit to measure current can be eliminated by using a probe with a clamp that fits around the current-carrying wire. The clamp probe measures only ac, generally for the 60-Hz ac power line.

16. Digital Multimeters (DMMs) The digital multimeter has become a very popular test instrument. The digital value of the measurement is displayed automatically with decimal point, polarity, and the unit for V, A, or Ω.

17. Digital Multimeters (DMMs) Fig. 8-16: Typical digital multimeter (DMM). Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Digital multimeters are generally easier to use.  They eliminate the human error that often occurs in reading different scales on an analog meter with a pointer.

18. Electronics Chapter 3 DMM specifications Display Accuracy Range and resolution

19. Understanding DMM display specs Display is specified as digits or as count Digits: 3 1/2, 4 1/2, etc. Example: 3 1/2: starting from the least significant digit, 3 “full” digits from 0-9, 1 “half” digit at less than 9. Ex: 1999 Can be confusing: how do you specify 3999? Count: 6000 5000 4000 3200 etc 4000 count display reads from 0-3999 3200 count display reads from 0-3199 Hands-on: 6000 count display Select V, measure battery 5000 count

20. Understanding DMM accuracy specs Accuracy is specified in percentage Closeness with which an instrument reading approaches the true value being measured; largest allowable error Percentage of reading (digital multimeters) vs. percentage of scale or range (analog meters): Example: 1 % scale vs. 1 % reading % scale: If scale or range is 1000 V, an accuracy of 1 % is equal to +/- 10 V. 120 V reading could = 110-130 V % reading: 1 % accuracy with 120 V reading = 118.8-121.2 V Least significant digit unstable: Example: Accuracy spec = +/-(1 % +2) Reading of 200.0 mV= 197.8 - 202.2 mV

21. Range and resolution Resolution is the smallest change in measured value to which the instrument will respond As the range increases, the resolution decreases: Turn Fluke 179 to Vac and hit Range button (Auto disappears): Range: Resolution: 600.0 mV.1 mV (=1/10 mV) 6.000 V.001 V (=1 mV) 60.00 V.01 V (=10 mV) 600.0 V0.1 V (=100 mV) 1000 V 1 V (=1000 mV) (To exit Manual Range, hold Range button for 2 secs) For maximum resolution, choose the lowest possible range Understanding DMM specs

22. Meter Applications Table 8-4 (next slide) summarizes the main points to remember when using a voltmeter, ohmmeter, or milliammeter.

23. Meter Applications VoltmeterMilliammeter or Ammeter Ohmmeter Power on in circuit Power off in circuit Connect in parallelConnect in seriesConnect in parallel High internal RLow internal RHas internal battery Has internal series multipliers; higher R for higher ranges Has internal shunts; lower resistance for higher current ratings Higher battery voltage and more sensitive meter for higher ohms ranges