Digital voltmeters can be classified in to the following broad categories.• Ramp type DVM.• Integrating DVM.• Continuous-balanced DVM

TYPES OF DMM’S

DIGITAL VOLTMETERSTHE SINGLE SLOPE PRINCIPLE

The DVM's outstanding qualities are (i) Input rangeInput range of DVM are from ± 1.000000 V to ± 1,000.000 V with automatic/Manual range selection and overload indication(ii) Absolute accuracyAccuracy of DVMs are as high as ± 0.005 per cent of the reading. (iii) StabilityFor short term Stability is 0.002 per cent of the reading for a 24 hr period. And for long term stability is 0.008 per cent of the reading for a 6 month period.(iv) Resolution1 part in 106.i.e 1 μV can be read on the 1-V input range. (v) Input Characteristics Input resistance typically 10 MΩ and input capacitance is typically 40 PF. (vi) Calibration (Not in all Models) Internal calibration standard allows calibration independent of the measuring circuit (vii) To measure current, resistance and voltage ratios an additional circuitry is needed

and other physical variable may be measured by using a suitable transducers.

Ramp-type Digital Voltmeter

Block diagram of a ramp-type digital voltmeter.

LINEAR RAMP TYPE DVM.1. TWO Comparators. 2. Input Voltage Comparator3. Zero Comparator4. Switch (gate), 5. Clock pulse generator 6. Ramp generator. An input voltage Vin is applied to one input of a voltage comparator. The second input being connected to the ramp voltage generator. When the ramp generator is first turned on, several things happen.

1. At the start of the RAMP counter is RESET to zero. 2. Downward Ramp coincides with the input voltage, a pulse is

generated which opens the gate. 3. Clock pulses from the clock generator are allowed through the gate to

the counter. 4. The counter then starts to count each clock pulse as it arrives. During the time the ramp voltage is decreasing, there is a relatively stable state in the circuit. 5. When zero is detected by zero comparator, a stop pulse is generated gate is opened and counter stops counting. The counter displays the

voltage to be measured.

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.

Counter Voltage-to-time conversion using gated clock pulses.

t1

t2

V0

The major disadvantage of this system 1. Very stable clock signal is required. 2. Noise in the signal causes errors. 3. Input Filters are required. 4. Stability is poor.

Advantages

1. It has excellent noise rejection because noise and superimposed ac are arranged and in the process of integration.

2. The speed and accuracy are readily varied according to the specific requirements.

3) Also accuracy of ± 0.05% in 100ms is available.

DUAL SLOPE DVM.

T=1000 clock pulses. The reference voltage is taken as half the input voltage.

Count

For 7106(LCD), 7107 (Common Anode LED)The clock frequency is set for 48KHz. It is divided by 4. So the clock period is 80msec. Integration period is 1000 clock pulses long. The analog period is integrated over a period of 80msec. This is for optimum hum frequency (50Hz) rejection. 3 samples per second are taken

t T

1. The system commences the measurement when the switch connects the Analog signal input to the integrator which commences to ramp down. At this point, the integration capacitor, C, falls linearly from the input, to a level decided by the average input signal value over the counter time period (T).

2. At the same time the counter begins from zero, to count the clock pulses.

3. When a predetermined number of pulses,( 1000 with the 7106), appear in the counter, the integrator is electronically switched over to the reference voltage.

As the switch changes to the reference, the counter is reset to zero and commences counting again. The reference, which is of opposite polarity to the input signal now causes the charged integration capacitor (C) to ramp upward with a fixed slope.

4. When the output of the integrator reaches the zero threshold, the counter is stopped and its contents displayed on the digital readout.

The count displayed is the ratio of the counts during the downward ramp (over time t) to the counts during the upward ramp (Over timeT). Thus, for a limit of 1000 counts during the downward ramp, a direct reading of input voltage is obtained if the reference voltage is chosen appropriately.

Advantages: 1. The absolute value of the integration capacitor and the clock frequency are of little significance provided they are stable for the duration of the conversion period.

2. The relatively long analogue to digital conversion period has an inherent advantage in that it ignores noise. When noise is integrated over an extended period, its amplitude tends to zero. Thus, dual-slope integration results in excellent accuracy.

S.No.

Ramp Type Dual Slope

1 Large errors are possible when noise is superimposed on the input signal

Has excellent noise rejection because noise and superimposed ac are averaged out in the process of integration.

2 Circuit complexity is low Circuit complexity is moderate3 Input filters are required Filters not required4 Low accuracy and

accuracy depends on the stability of oscillator and the linearity of the ramp slope

Accuracy of this DVM is high. Accuracy is independent of oscillator frequency

5 Poor stability High stable6 Operating speed slow Operating speed high

DPM USING 7107 CHIP.

To Generate -5V

Non-standard Voltage Input.Load cell of a weighing system may have an output voltage of 0.682V when it has 2.0 Kg weight on it. You want the meter to read the range 0 - 1.99 Kg directly.

It is an easy matter to adjust VREF to 0.341V (half the output voltage), put the decimal point in the correct position by moving the jumper and the panel meter now reads off 0 - 1.99 Kg directly from the display.

Current Measurement. Currents up to 2A can be easily measured using the space on the board for a 5W shunt resistor, R. The current is converted into a voltage by the shunt resistor. If R = 0.1 ohms then 200mV will be developed when the current through it is 2A. This voltage is applied to the meter which is set up for the 200mV range. (That is, VREF is set to 100mV.) Power dissipation at the maximum reading is I2R which is 0.4W, well within the 5W rating of the resistor.

To measure a full scale of 200mA then R should be 1.0 ohms in order to generate 200mV input to the meter. For a 20 mA meter then R = 10 ohms. Note that because of wide tolerances in the shunt resistors it may be necessary to adjust the reference voltage in order to get the correct reading. So further adjustment of VREF using a known current may be required.

Typical specification of DMM

General:

Maximum voltage between terminals:600VFuse protection :200mA/250VPower: 9V batteryDisplay: LCD 31/2digits, updates 2-3/ sec.Input impedance:10 MΩFrequency range : :40-400HzMeasuring method: Dual-slope integrationOver range indication: Only figure “1”on the displayPolarity indication “-”displayed for negative polarity

Example 1: 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 10ms. Find the conversion time for a 6.8volt input.

The total conversion time is then 10 ms + 6.8 ms = 16.8 ms

Accuracy of DMM

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 ½digit display and an accuracy of ±(0.6+1). Determine the measurement accuracy in each case.

Analog instrument: Voltage error = ±2%of 25V= ±0.5V Error = ±0.5 V/20 ×100% = ±2.5%

Digital instrument: For 20 V displayed on a 3½digit display1 Digit = 0.1 V. Voltage error=±(0.6%of reading + 1Digit)= ±(1.2V + 0.1V)= ±0.22VError = ±0.22 V/20V ×100% = ±1.1%

CONTINUOUS BALANCE DVM OR SERVO BALANCING POTENTIOMETER TYPE DVMThe basic block diagram of a servo balancing potentiometer type DVM is shown in Fig.5.15.The input voltage is applied to one side of a mechanical chopper comparator, the other side being connected to the variable arm of a precision potentiometer. The output of the chopper comparator, which is driven by the line voltage at the line frequency rate, is a square wave signal whose amplitude is a function of the difference in voltages connected to the opposite side of the chopper. The square wave signal is amplified and fed to a power amplifier, and the amplified square wave difference signal drives the arm of the potentiometer in the direction needed to make the difference voltage zero. The servo-motor also drives a mechanical readout, which is an indication of the magnitude of the input voltage.This DVM uses the principle of balancing, instead of sampling, because of mechanical movement. The average reading time is 2 s.

Fig. 5.15 Block Diagram of a Servo Balancing Potentiometer Type DVM

RESOLUTION AND SENSITIVITY OF DIGITAL METERSResolutionIf n = number of full digits, then Resolution (R) is 1 / 10n.The resolution of a DVM is determined by the number of full or active digits used,

Sensitivity of Digital MetersSensitivity is the smallest change in input which a digital meter is able to detect. Hence, it is the full scale value of the lowest voltage range multiplied by the meter's resolution.Sensitivity S = (fs)min x Rwhere (fs)min = lowest full scale of the meterR = resolution expressed as decimalExample: What is the resolution of a digit display on 1 V and 10 V ranges?Solution Number of full digits is 3. Therefore resolution is 1/10n where n = 3.

Hence the meter cannot distinguish between values that differ from each other by less than 0.001 of full scale.For full scale range reading of 1 V, the resolution is 1 x 0.001 = 0.001 V.For full scale reading of 10 V range, the resolution is 10V x 0.001 = 0.01 V.Hence on 10 V scale, the meter cannot distinguish between readings that differ by less than 0.01 V.

Example 5.4 A digit voltmeter is used for voltage measurements, (i) Find its resolution(ii) How would 12.98 V be displayed on a 10 V range? (iii) How would 0.6973 be displayed on 1 V and 10 V ranges.Solution: Resolution = 1/10n = 1/104.=0.0001 where the number of full digits is n = 4(ii) There are 5 digit places in 41/2 digits, therefore 12.98 would be displayed as

12.980.Resolution on 1 V range is 1 V x 0.0001 = 0.0001Any reading up to the 4th decimal can be displayed.Hence 0.6973 will be displayed as 0.6973. (iii) Resolution on 10 V range = 10 V x 0.0001 = 0.001 VHence decimals up to the 3rd decimal place can be displayed.Therefore on a 10 V range, the reading will be 0.697 instead of 0.6973.

Resolution R = 1/103 = 1/1000 = 0.001