ELECTRONIC DIGITAL INSTRUMENTS

A digital instrument is an electronic device that measures voltage, current, and/or resistance by converting the measured analog input signal into a digital represen-tation that is then displayed as a digital readout.

Advances in technology have led to digital instruments that are capable of high degrees of accuracy in the measurement of voltages, currents, and resistances over a wide range of values. Analog instruments indicate measured quantities by the deflection of a pointer on a scale, requiring the user to “eye up” the reading.

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Today’s digital instruments display measured results as discrete numbers (digits), removing much of the interpretation error from the act of reading an instrument.

Typical display technologies include liquid crystal display (LCD), light emitting diode (LED), and gas discharge.

Some instruments offer the ability to send readings to other devices such as printers or computers or to be controlled by external computers. Interfaces built into the instruments, such as RS-232-C serial communications or the IEEE-488 bus standard, provide the data transmittal and external control capabilities.

The central component of a digital instrument is the digital DC voltmeter that uses electronic circuits to sense, process, and display the measured quantities.

Input quantities other than DC voltages are converted to DC by transducers.

Examples of transducers include internal shunts used to measure current and AC-to-DC converters to measure AC quantities. The transformed analog quantity (now in the form of an equivalent DC voltage) is then converted to a digital signal.

Active electronic components, such as transistors, operational amplifiers, and integrated circuit modules perform this analog-to-digital (A/D) conversion.

Analog-to-Digital Conversion

Electronic instruments employ several different A/D conversion processes. These include dual-slope integration, ramp-and-counter, successive approximation, and voltage-to-frequency conversion. Each of these techniques produces a digital output equivalent to the measured analog input.

Figure 6-1 shows a simple version of an A/D converter. In this example, a binary counter increments one count with each clock pulse, until V_outequals V_in. This type of A/D converter is called a “digital ramp and counter” because the waveform at V_out ramps up step-by-step, like a staircase. It operates as follows:

1. A positive Start pulse is applied resetting the counter to zero. It also inhibits the AND gate so no clock pulses get through to the counter while the Start pulse is High.

2. With the counter at zero, V_out 0, so the comparator output is High.

3. When the Start pulse goes Low, the AND gate is enabled, allowing pulses to enter the counter.

Figure 6-1. Analog-to-Digital Converter.

4. As the counter advances, the digital-to-analog (D/A) output (V_out) increases one step at a time, with the size of each step equal to its resolution (see below).

5. Stepping continues until V_outreaches a step that exceeds V_in. At this point, the comparator output goes Low, stopping the pulses to the counter, and stop-ping the counter at the digital equivalent of the analog input of V_in. The A/D process is now complete.

Resolution

The following example illustrates the resolution and accuracy of the digital-ramp A/D converter.

Assume the following values for the A/D converter of Figure 6-1: D/A con-verter has a 10-bit input and a full scale analog output of 10.23 volts; the com-parator can detect a voltage difference of 1 millivolt or greater; V_inis 3.728 volts.

Since the D/A converter has a 10-bit input, the maximum number of steps possible is (2¹⁰1)  1023. With a full-scale output of 10.23 volts reached in 1023 steps, the step size is 10 millivolts. This means V_outincreases in steps of 10 mV as the counter counts up from zero. Since V_in 3.728 volts and the comparator threshold is 1 mV, then V_outhas to reach 3.729 volts or greater before the compara-tor switches Low. At 10 mV per step, this requires 373 steps.

At the end of the conversion, the counter holds the binary equivalent of 373, which is 0101110101. This is the digital equivalent of the analog input of V_in 3.728 volts. The resolution of this A/D converter is equal to the step size of the D/A converter which is 10 mV, or approximately 0.1% (.010/10.23  100  0.1%).

The resolution of an A/D converter is equal to the resolution of the D/A con-verter that it contains. The D/A output voltage V_outis a staircase waveform (digital ramp) that goes up in discrete steps until it exceeds V_in. Thus, V_outapproximates V_in. When the resolution (step size) is 10 mV, the accuracy we can expect is that V_outis within 10 mV of V_in. The resolution of the D/A converter is an inherent error, often referred to as a quantizing error. This quantizing error can be reduced by increas-ing the number of bits in the counter and in the D/A converter. It is specified as an error ± 1 least significant bit (LSB), indicating that the result can vary by that much due to the step size.

From another point of view, the input voltage V_incan take on an infinite num-ber of values, from 0 to full scale. However, the output voltage V_out has only a finite number of discrete values. This means that similar values of V_inwithin a small range could have the same digital representation. For example, if the count-er goes through 1,000 steps from zcount-ero to full scale, any value of V_in from 3.720 to 3.729 will require 373 steps, thus resulting in the same digital representation. In other words, V_in must change by 10 millivolts (the resolution) to produce a change in the digital output.

Accuracy

The D/A converter accuracy is not related to the resolution. It is related to the accuracy of the components in its circuit such as the resistors in the D/A network, comparator, level amplifiers, and the reference power supply. If a D/A has an accu-racy of 0.01% full scale, the A/D converter may be off by 0.01% full scale owing to non-perfect components. This error is in addition to the quantizing error due to resolution. These two sources of error are usually specified separately, and for a given A/D converter are usually of the same order of magnitude.

In addition to the inherent errors noted above, the accuracy of an electronic instrument depends on proper selection of the meter range. Normally, the uncer-tainty of measurements is expressed as a percent of the reading plus the number of counts of the least significant digit (LSD) displayed for that range. If the 1,000 volt DC range is selected to measure a 2 volt signal for a three-and-a-half digit digital multimeter with a nameplate accuracy of 0.5% of input voltage 1 LSD, this setup would result in a meter accuracy of 50.5%, as shown below.

Given: Meter Range Accuracy (MRA) is 0.5% of input voltage 1 LSD Meter range set to 1,000 volts DC

Input voltage is 2 volts DC

Then: Meter Accuracy   [(MRA  input V  LSD)/Input Voltage]  100

  [(0.5%  2  1)/2]  100

  50.5%

However, selecting a meter range of 2 volts DC on the same digital multimeter would result in an accuracy of 0.60%, nearly 100 times better, as shown below.

Given: Meter Range Accuracy is 0.5% of input 1 LSD Meter range set to 2 volts DC

Input voltage is 2 volts DC

Then: Meter Accuracy   [(MRA  input V  LSD)/Input Voltage]  100

  [(0.5%  2  0.002)/2]  100

  0.6%

Digital Display Resolution and Accuracy

Typical handheld digital instruments display from 3 to 5 digits. Laboratory digital instruments often offer 7 or 8 digits. The number of digits directly affects the avail-able resolution of the reading. For example, a full 4-digit display is capavail-able of pre-senting numbers from 0 to 9999 (with a decimal point somewhere in the display depending on the range setting of the instrument). This display can provide 10000 different readings for a particular range setting, so its resolution is limited to 1 part in 10000, or 0.01%. You may see this display referred to as a 10000-count display.

A 6-digit display can present numbers from 0 to 999999. This display resolution would be 1 part in 1,000,000 or 0.0001%. It may be called a 1,000,000-count display.

Examples in the previous section used LSD, Least Significant Digit, to adjust accu-racy calculations to the characteristics of the display.

The design of a digital instrument often further limits the display. A 4-digit display, by design, may display numbers from 0 to 3999, rather than to 9999. That is, the left-most digit is programmed such that it only displays the numbers 0 to 3.

This display is described as a 3¹/2-digit display or as a 4000-count display. This design does not further affect the accuracy of calculations. The value of the LSD is the same for a 3¹/2-digit display as for a 4-digit display.

Summary

Digital instruments offer a high degree of accuracy, precision, sensitivity, low cost, and designs ranging from laboratory grade to rugged field grade. The selection of a digital versus analog instrument depends on several considerations including

accuracy, resolution, speed of measurement and reading, size and portability, environmental considerations, and cost. The majority of instruments available today are digital.