What are the principles of operation of a digital multimeter?

Answer 1

Digital Multimeter - Principles of Operation 1. To understand a digital multimeter, it is helpful to consider its analog progenitor first. An analog multimeter has a core function of current-to-display conversion: a current through the coil of a d'Arsonval movement creates a magnetic field that deflects a rotating, pivoted permanent magnet with an attached needle and a restoring spring. Needle deflection is proportional to the current. The range of applied currents is very narrow, such as 0‑50 microamperes, and is adjusted to the signal under test by building a resistor network inside the meter box and choosing the combination to be used for each scale with a switch of some type. See Figure 1. Overall accuracy is directly proportional to the accuracy of the resistors. For resistance testing, a battery inside the box provides a current source, and the needle is zeroed by shorting the test leads together and adjusting a built-in series potentiometer until the needle reads zero ohms (actually set to full scale). Adding the resistor under test reduces the current, in a logarithmic way that can be read on a reversed (zero on the right) and very nonlinear scale. Accuracy is best at midscale so several scales are provided. For AC measurements, a diode provides rectification, the scale is adjusted to give approximate RMS readings, and capacitance may be inserted to improve accuracy over a range of frequencies. 2. A digital multimeter is essentially a voltage-to-display device, instead. The scaling resistor network and its selector switch(es) may be the same or similar. Because a voltage difference between two input terminals is measured, the meter will have a differential amplifier (diff amp) front end that operates over a range and shifts an input signal into a range and DC offset compatible with an analog-to-digital converter (ADC) that most often has a multiple-digit decimal output. Almost any ADC for DMM use has at least an overrange indication and can indicate whether the differential input signal is positive (the "positive" input terminal, as arbitrarily assigned, is more positive than the "negative" ditto for a "positive" input signal) or negative (the opposite). The number of digits reflects the internal precision of the ADC and the intended price point for the DMM to be built with it---more digits in the display means more cost to build. Conversion rates are usually slow: people can only see a few new digital values per second, and speed costs in terms of power consumption. If the range is manually set, there is a decimal point before and/or after each digit, and the switch turns on the correct one for the range in use. 3. Selecting voltage, current, resistance, power, etc. is done by the switches. For current, many meters (digital and analog) provide another set of inputs to plug the test leads into. This is meant to remind you that you're about to try to blow up the meter. Most meters have a shunt with a fraction of an ohm of resistance, measure the voltage across that, and display it as current. Some meters have a fuse in series with the ammeter shunt, to take the meter off line and remind you to be more careful. The same applies to extreme voltage measurements, which can exceed the range of the diff amp circuit and can be physically hazardous. 4. Auto-ranging adds complexity and convenience. There is already an overrange signal; this can be routed to the diff amp, if it has some processing power, or kept inside the ADC, if it has switch control line outputs. In both cases, the overrange signal causes electronic switches (relays, FETs, etc.) to be turned on and off to decrease the gain of the diff amp by a factor of ten. Since whatever goes up must come down, auto-ranging also needs an underrange signal---if the most significant digit is a zero, set underrange to TRUE and adjust the gain up by a factor of ten. Wait for stability (a few consecutive readings are the same). Adjust again if needed. 5. Note that there are 3- and 4-digit (and more) DMMs, plus 3-1/2, 3-3/4, 4-1/2, etc. The existence of the strange ones reflects historic cost considerations, but is now well established. A 3-1/2 digit meter reads 000-999 plus 1000 to 1999. Since this is twice as high as a 3-digit meter can read, it is arbitrarily called a 3-1/2 digit. A seven-segment display for this only needs the two segments that make up a "1" to perform this function. A 3-3/4 digit meter reads 000-3999, doubling the range again, but saving only one segment compared to a full 4-digit display. One could define a 3-7/8 digit meter as extending to 7999, and use a regular 4-digit display, but this is not normally done, since you are practically at the next level but probably have to charge less for it.

Answer 2

to measure frequency by digital frequency metre....the input signal of unknown frequency is first amplified through amplifier,then it is passed through the schmitt trigger,which changes the input signal into train of pulse,each pulse represent one cycle of the input signal of unknown frequency,the each pulse is passed through start/stop gate and counted by counter till the gate is open,when the gate is closed the counter sieze the flow of puls and show the no. of pulse which are in the time interval of gate i.e., the time between opening and closing of gate.To find the correct frequency we should know the correct time interval.Then the frequency can be obtained by using the formula :-

f=n/t; where

f=unknown frequency

n=no. of pulses counted by counter in the time interval

t=time interval between opening and closing of the gate