Block Diagram of Digital Meter

Block Diagram of Digital Meter Points : Block Diagram of Digital Meter, Signal preparation, Analog-to-digital conversion, Display, Analog to Digital (AID) Conversion Techniques, Dual slope integration A/D converter A digital meter consists of following three parts, as shown in the block diagram.
Signal preparation
Analog-to-digital conversion
Display

Signal preparation The function of signal preparation or signal converter block is to reduce or increase the input signal so that it should be suitable for input of the analog-to-digital converter. In other words the input signal are brought to the level of analog-to-digital converter. In this signal preparation step the signal amplitude is kept in view as well as the source is protected from loading.

When the level of the signal to be measured is high, it is brought low with the help of “attenuators”.

If the input signal to be measured is low, the operational amplifier brings it to the level of analog-to- digital converter.

Selector switches are provided in attenuator and operational amplifier circuits for range selection, but in modern meters the provision auto-ranging is present.

Analog to Digital Conversion The most naturally occurring phenomena arc analog in nature. Analog quantities are continuous functions with time and most transducers give an analog output. With the introductions of digital computer, the data taken in physical system has to be converted into a digital form. When the system engineers made their first attempts at applying computational power,, speed and data handing capability of digital computer to automatic control systems, they were faced with a tremendous task of making the digital computer to work with analog devices. The data to be fed to digital devices, normally appears in analog form. For example a temperature difference would be represented by the voltage output of the thermocouple, the strain of a mechanical member would be represented by the voltage resulting from, a unbalanced strain gauge resistance bridge. Therefore the need arises for a device that converts analog information into digital form. Therefore, analog to digital (AID) conversion devices are used in measurement and instrumentation systems. Analog to Digital (A/D) Conversion Techniques Analog to digital (A/D) conversion is slightly more complex than digital-to- analog (D/A) conversion. A number of methods may be used for this purpose. Following four methods are most commonly used.

a) Successive approximation method (potentiometric type)
b) Voltage to time conversion method, (ramp type)
c) Voltage to frequency conversion method, (integration type)
d) Dual slope integration method. Dual slope integration A/D converter The block diagram of a dual slope integration AID converter is shown in the fig.

The reference voltage and the input analog voltage are sequentially connected to the integrator with help of a switch. The reference voltage and the input analog voltage must be of opposite polarity. The input voltage is integrated for a fixed input sample time. The integrated value is then discharged at affixed rate and the time to do this is measured by a counter.

The two types of command area given by the logical control to the counter and they are they are (i) Reset and (ii) convert.

1. When the convert command is received by the counter, it automatically resets to all zeros and the switch connects the input voltage to the integrator. The output from the comparator is designed such that at this time it will permit the counter to count up for a Output from the integrator will be steadily increasing in value.

On the next count after the converter has counted all the way up to all I (i.e. the next count will cause it to go all zeros and start over again), the switch changes position. The input voltage to is disconnected and the switch connects the reference voltage to the integrator. Therefore, the integrator now integrates the opposite polarity voltage, which causes the output to decrease towards zero voltage. Mean while, the counter is counting up from zero again.

When the output of the integrator goes to zero, if causes the comparator to switch its output, there by stopping the counter Via the control logic. The binary number in the counter at this time is proportional to the amount of time that it took the integrator to integrate down from its starting point to zero. Therefore, the binary count is proportional to input voltage. Display The output of AID converter goes to display, which show in the shape of digits. Electronic displays serve as output units of instrumentation systems. The digital data resulting from a measurement application is binary coded decimal (BCD) form. Thus in display applications, where machine communicates with a human being some sort of “decoding” is required. If the language of the machine is BCD, we have to decode (translate) it into a decimal code, then it can be easily understood by the human operator. In many cases, decoding binary into octal might facilitate man machine communications. The decoder converts the binary code, such as BCD, into a non-binary code that can be understood by man, such as decimal or 7- segment code. The decimal code has 10m different symbols while the 7-segment code has 7 separate line used in activating 7 segment. Decimal displays.

Three types of display formats commonly used like.

i) Segmental (7-segment)
ii) Dot matrix (5 x 7 matrix)
iii) Decimal indicator (Nixie tube).

Thus each of above formats require the use of an appropriate code converter between the BCD data input and the output display. In case of a Nixie tube there is one anode and 10 cathodes, each shaped like the digit to be displayed

Grounding a particular cathode to ionise and the digit is displayed on a account of the resulting glow. Figure (a) (b) shows a typical 7 segment display.

Grounding a particular cathode to ionise and the digit is displayed on a account of the resulting glow. Figure (a) (b) shows a typical 7 segment display. Suppose digit “0” is to be displayed by a 7 segment display, this requires that segment a, b, c, d, e, f should be lit up. Similarly for digit “2” segments a, b, d. e and g should be lit up.

Thus by properly choosing the segments any number from “0” to “9” can he displayed.

A common supply voltage drives the anodes of the LEDs and when a switch closes, the corresponding LED is forward biased and emits light. LCDs are also used instead of LEDs.