1. Digital instrumentation – Unit 1
-Digital voltmeters
-Digital multimeters
-Measurement

2. The magnitude of voltage signals can be measured by various electrical indicating and test instruments, such as meters (both analogue and digital), the cathode ray oscilloscope and the digital storage oscilloscope.
As well as signal-level voltages, many of these instruments can also measure higher-magnitude voltages, and this is indicated where appropriate.

3. All types of digital meter are basically modified forms of the digital voltmeter (DVM), irrespective of the quantity that they are designed to measure.
Digital meters designed to measure quantities other than voltage are in fact digital voltmeters that contain appropriate electrical circuits to convert current or resistance measurement signals into voltage signals.
The binary nature of the output reading from a digital instrument can be readily applied to a display that is in the form of discrete numerals

4. Comparison of Digital and Analog Meter
ADVANTAGES:
Higher accuracy and resolution
Greater speed
No parallax
Reduced human error
Compatibility with other digital equipment for further processing and recording
Comparison of Digital and Analog Meter 4

5. VARIETIES OF DVM
Digital voltmeters differ mainly in the technique used to effect the analogue-to-digital conversion between the measured analogue voltage and the output digital reading.
As a general rule, the more expensive and complicated conversion methods achieve a faster conversion speed
DVM differ in the following ways
Number of digits
Number of measurements
Accuracy
Speed of reading
Digital output of several types

6. CHARACTERISTICS FEATURES OF DVMs
Input Range: From ±1.000 to ±1000V with automatic range selection and overload indication
Absolute accuracy: As high as ±0.005% of the reading
Resolution: 1 part in 10^6
Stability: Short term – 0.002% of the reading for a 24-hr period
Long term – 0.008% of the reading for a 6 month period
Input resistance: Typically 10 MΩ
Input capacitance: 40 pF
Output signals: BCD form 6

7. CLASSIFICATION OF DVMs
Voltage to Time - Ramp type DVM
Voltage to Frequency - Integrating type DVM
Dual slope integrating type DVM
Direct - Successive approximation DVM
Staircase ramp DVM 7

8. Ramp-type Digital Voltmeter8

9. Ramp-type Digital Voltmeter9

10. Vi = Vref (T1 /T2) 10

11. 11

12. Staircase Ramp Digital Voltmeter12

13. 13

14. 14

15. POTENTIOMETRIC DIGITAL VOLTMETER
Uses a servo principle,
The error between the unknown input voltage level and a reference voltage is applied to a servo-driven potentiometer that adjusts the reference voltage until it balances the unknown voltage.
The output reading is produced by a mechanical drum-type digital display driven by the potentiometer. This is also a relatively cheap form of DVM that gives excellent performance for its price

16. Auto polarity indication
Integrating type - polarity measured at end of integrating period
Flip-flop to store till next measurement

17. Automatic Ranging
For a Standard 3-1/2 digit display
201mV as 0201
195mV as 195.0

18. Automatic zeroing
Optimum accuracy for low valued readings – zero adjustment is necessary
i.e. short circuit may read 0001
Zero error before real measurement and stored – in integrating type i9t is stored in capacitor

23. Time measurement

24. Time period measurement

25. Time period measurement

26. Frequency measurement

27. Frequency measurement

28. Reference : Electronic Instrumentation and Measurement
INC 331 Electronic and Digital Measurement
Department of Control System and Instrumentation Engineering, Faculty of Engineering, King Mongkut’s University of Technology Thonburi
PAGE : 1
Introduction to Electronic and Digital Measurement
Range Selection Error
Time period and decimal point selection for a digital frequency meter. When the time period (or gate time) is 1 s, a count of 1999 pulses is displayed as kHz. The decimal point and units display are selected according to the time period.
Reference : Electronic Instrumentation and Measurement
(Figure 6.10)

29. Time measurement
Time interval measurement

30. Time measurement

31. Pulse width measurement

32. Digital IC tester