Digital Multimeter (DMM)

Transcription

1 Digital Multimeter (DMM) Portable Analog Multimeter Hand-held DMM Bench-top DMM

2 Digital Voltmeter (DVM) DVM is essentially an Analog to digital converter (A/D) with a digital display Digital voltmeter Attenuator Amplifier Analog to Digital Converter Digital display Digital MultiMeter (DMM) = electronic Volt Ohm Millimeter with digital display

3 Comparison of Digital and Analog Meter Superior resolution and accuracy. (±0.5% or better) Digital meter Leaves no doubt about the measured quantity. Indicates a negative quantity when the terminal polarity is reversed No usually damaged by rough treatment Wrong scale might be used or might be read incorrectly. Inferior resolution and accuracy. (±3% in common) Analog meter Pointer attempts to deflect to the left when the polarity is reversed Can be damaged when dropped from bench level

4 Analog to Digital Conversion A/D converts an analog signal into the digital code which is proportional to the magnitude of the coming signal. k Digital output Where k is step size or resolution Ex. Signal from mv may be converted to 8-bit binary codes starting from (80 10 ) to ( ). In this case, the step size k is equal to 10 mv. Quantization error or Conversion error of a A/D Quantization error = step size full scale 100 = 2 N % Where N is the number of bit

5 Analog to Digital Conversion Conversion time, T c time requires to convert an analog signal to the corresponding digital code Digital output step size = 1 V A/D will give 010 digital code. Quantization error = 1/7 100 =14.3% Ex. An analog inputs V will be represented by digital output (Full scale) Analog input (V) An example of 3 bit ADC

6 Ramp signal Ramp-type Digital Voltmeter (also called single slope) Operation principle: The measurement of the time it takes for a linear ramp voltage to rise from O V to the level of the input voltage, or to decrease from the level of the input voltage to zero. This time interval is measured with an electronic time-interval counter. Start measurement Coincindence V ramp (t) = V o m t Where m is the ramp rate Time V ramp (t 1 ) = = V o m t 1 Gating time interval t V ramp (t 2 ) = 0 = V o mt 2 t = t 2 - t 1 = /m If the period of the clock is T, then during the time interval t 1, the number of pulses is Clock pulses to counter n pulse Voltage-to-time conversion using gated clock pulses. t nt or nmt Accuracy depends on both the ramp rate and clock period.

7 Comparator V+>V-; Vo = V(1) Logic high V+<V-; Vo = V(0) Logic low Vo V(1) + Vo V ref V(0) V ref Vo V ref + Vo V(1) V(0) V ref

8 Ramp-type Digital Voltmeter Voltage-to-time conversion Time measurement unit DC input voltage Ranging and Attenuator v in - + input comparator clk start count Digital display Oscillator Gate Counter start ramp Ramp Generator v ramp 0 V - + ground comparator stop count Sample Rate MV reset Block diagram of a ramp-type digital voltmeter.

9 Staircase Ramp Digital Voltmeter (also called digital ramp) V + in Comp. - V AX D/A Compare the input voltage to the internally generated stair case ramp. The most simple A/D Slow conversion and conversion time depends on the magnitude of input signal. T N C, max = (2 1) Clock period Digital output lock Counter D/A output V AX Control circuit clock period time Block diagram

10 Successive Approximation Digital Voltmeter Ex. To determine a number between (9 bit binary), given, the number to be determined is 301 No Estimate = = = = = = = = Results > V AX < < > < > > < Finished

11 Successive Approximation Digital Voltmeter V + in Comp. - V AX D/A Compare the input voltage to the internally generated voltage The most common A/D for general applications Conversion time is fixed (not depend on the signal magnitude) and relatively fast T C = N Clock where N is the number of bits period Digital output Full scale lock Succesive Approximation Register Control circuit D/A output Full scale Full scale Full scale V AX Block diagram Clock period

12 Digital Ramp VS Successive approximation Digital Ramp method Successive approximation method

13 Dual-slope Digital Voltmeter C C V ref R - + V out V ref R - + V out Phase 1: charging C with the unknown input for a given time. Assume V c (0) = 0 VinT Vout1 = RC where T is the charging time Charge Discharge time Phase 2: discharging C with the reference voltage until the output voltage goes to zero. Vref Tx V out = + V RC out1 find T x at which V out becomes zero T = x V V in T ref V out Phase 1 Phase 2

14 Dual-slope Digital Voltmeter 0 Conversion time Charge Discharge time Small input voltage Large input voltage Accuracy does not depend on R C and Clock (high accuracy) Relatively slow Capable to reject noise V out T = T + C const T variable R - + V ref C V out Zero crossing detector - + count Display Counter reset Clock generator Control logic

15 Ex A dual slope A/D has R= 100 kω and C = 0.01 µf. The reference voltage is 10 volts and the fixed integration time is 10 ms. Find the conversion time for a 6.8 volt input. T V T (6.8 V)(10 ms) (10 V) in x = = = Vref 6.8 ms The total conversion time is then 10 ms ms = 16.8 ms Ans Ex Find the successive approximation A/D output for a 4-bit converter to a volt input if the reference is 5 volts. (1) Set D 3 = 1 V AX = 5/2 = 2.5 Volts > V AX leave D 3 = 1 (2) Set D 2 = 1 V AX = 5/2 + 5/4 = 3.75 Volts < V AX reset D 2 = 0 (3) Set D 1 = 1 V AX = 5/2 +5/8= Volts > V AX leave D 1 = 1 (4) Set D 0 = 1 V AX = 5/2+5/8+5/16 = Volts < V AX reset D 0 = 0 By this procedure, we find the output is a binary word of Ans

16 Typical specification of DMM General Accuracy of DMM Maximum voltage between terminals Fuse protection Power Display Input impedance Frequency range Measuring method Over range indication Polarity indication Indicate as ± (% of reading + No. of digits) :600 V :200mA/250V :9V battery :LCD 31/2 digits, updates 2-3/ sec. :10 MΩ : Hz Dual-slope integration Only figure 1 on the display - displayed for negative polarity Ex. ± (0.5% of rdg + 1 digits) sometimes simplify as ± ( ) Ex. For an accuracy of ± ( ), calculate the maximum error of in the V reading error = ± (0.5% of V) = ± ( V) = ± 0.01 V or ± 0.56% of reading

17 Ex A 20 V dc voltage is measured by analog and digital multimeters. The analog instrument is on its 25 V range, and its specified accuracy is ± 2%. The digital meter has 3 ½ digit display and an accuracy of ±(0.6+1). Determine the measurement accuracy in each case. Analog instrument: Voltage error = ± 2% of 25 V = ± 0.5 V error = ± 0.5 V 100% 20 V = ± 2.5% Digital instrument: For 20 V displayed on a 3 ½ digit display 1 Digit = 0.1 V Voltage error = ± (0.6% of reading + 1 Digit) = ± (1.2 V V) = ± 0.22 V error = ± 0.22 V 100% 20 V = ± 1.1% ½digit 3½ digit display