how to make Analog mater,Analog mater

Build an Analog Bar Graph Expanded Scale Voltmeter

An expanded scale voltmeter (ESV) can save your plane. That may be a strong statement, but it's true. The crucial radio link that lets you control your plane relies on nickel-cadmium (NiCd) batteries in the transmitter and receiver. If either of these batteries goes dead, you'll lose control and your plane will likely crash or fly away.

The completed ESV, with test lead plugged in. My test lead has two cables, one for testing receiver packs, and one for transmitter packs.

Unlike carbon-zinc or alkaline cells, NiCd cells have a very flat discharge curve. This means that their output voltage remains relatively constant (ranging from about 1.28V down to 1.17V) until they are almost dead, and then drops off suddenly. Testing the voltage with an ordinary analog voltmeter is therefore nearly useless, because it will always read close to 1.2V no matter how much charge is left.

A digital volt meter would be a slight improvement, because it has enough resolution to show the changes in this narrow range. However, another characteristic of NiCd cells is that they tend to show full voltage when they aren't doing anything, even if they are almost completely dead. If you use a resistor to discharge a NiCd cell to 0V (this is not recommended), and then remove the resistor and wait a few minutes, measuring the voltage will still indicate about 1.2V.

What is an ESV?

An ESV does two things that an ordinary voltmeter does not.

First, it acts like a magnifying glass on part of the scale of an ordinary meter, namely the part of the scale we are interested in. For example, an ESV for testing a single NiCd cell would range from 1.17V at one end of the scale to 1.28V at the other end of the scale. With the full scale covering this narrow range, it's easy to see voltage changes within the range.

	The ESV being used to test a fully charged 4.8V receiver pack. All the LEDs are lit.

Secondly, and ESV applies a known load to the cells while it's testing them. Only by testing them while they are doing a known amount of work can one accurately get an estimate of their state of charge (just like you can't assess your cardiovascular fitness by measuring your pulse and breathing while you are resting).

Instead of an analog meter movement or a digital display, the ESV described in this article uses a bar graph display consisting of eight LEDs. If no LEDs are lit, the NiCd battery being tested is dead. If all the LEDs are lit, it is close to fully charged.

A switch selects the number of cells being tested, either four for receiver batteries, or eight for transmitter batteries. The vast majority of receiver and transmitter batteries consist of AA cells, and this ESV is calibrated for those types of cells (larger, lower-resistance cells will show a higher voltage for a given load than will the smaller cells).

How it Works

The battery being tested is connected to the "+" and "-" inputs. Depending on the setting of switch S1, the battery voltage is applied across just R1a through R1d (to test a 4-cell battery), or also R2a through R2d (to test an 8-cell battery). This will put a load of approximately 200mA on the battery.

Schematic diagram. Click to enlarge.

The voltage across the R1 resistors is what will be tested. When testing four cells, this is the full battery voltage. When testing eight cells, it is half the battery voltage, because the R2 and R1 resistors together form a voltage divider. The test voltage is applied to one input of each of the eight voltage comparators Z1a through Z2d.

The other input of each comparator is connected to a point on an 8-way voltage divider, made up of R4 through R10. R3 and Zener diode D1 form a stable voltage reference to supply this voltage divider. R20 sets the voltage at the top of the divider, and R11 and R21 set the voltage at the bottom. R20 and R21 should be adjusted to give 5.13 and 4.66V at the top and bottom respectively.

Whenever the test voltage exceeds the divider voltage at a given comparator, the comparator's output goes low, lighting the corresponding LED. So, if the test voltage exceeds 5.13V, all the LEDs will light. If it is less than 4.66V, none of the LEDs will light.

Resistors R4 through R10 were chosen to produce voltages that are evenly separated in time along the battery's discharge curve. In other words, if four LEDs light, it means half the available operating time has been used up, which is not necessarily the same as the voltage being half way between 4.66V and 5.13V.

Power for the entire circuit is provided by a 9V alkaline battery. Since S1 is a center-off momentary toggle switch, power is only used when actually testing a battery, so the 9V battery will last a long time.

Resistors R12 through R19 were chosen to limit LED current to about 10mA. Most LEDs can handle 30mA, but the comparators can only sink at most 16mA, and the lower current levels lengthen the life of the 9V battery.

Construction

	Printed circuit pattern. Actual size is 2.6" x 2.1".

I've provided a printed circuit board layout for those who wish to build that way. My article, Making Excellent Printed Circuit Boards, gives tips on etching your own boards. Alternatively, you could build the ESV on a general purpose circuit board, or even a perforated board using point-to-point wiring.

Start by installing wire jumpers J1 and J2.

Select resistors for R1a through R2d. Either use 1% tolerance resistors, or hand pick from a larger selection of 5% resistors to find those closest to 100Ω. What is most important is that the combined parallel resistance of R1a through R1d be equal to that of R2a through R2b.

Next install R1a through R2d. After doing this, measure the resistance across R1a with a digital Ohm-meter. Repeat for R2a. If they aren't within 1% of each other, you'll need to install a compensating resistor in the extra set of holes of the group with the higher resistance. The value of this adjusting resistance should be RH x RL / (RH - RL), where RH is the higher of the two resistances, and RL is the lower of the two. For example, if you measured R2 as 26Ω and R1 as 25Ω, then RH is 26, RL is 25, and the adjusting resistor to be installed next to R2d would be 26 x 25 / (26 - 25), which is 650Ω. The closest commonly available value is 680Ω, which will do fine.

Component placement diagram.

Install R3 through R21, C1, and C2. Next install D1, making sure the banded end is adjacent to R20. Install sockets for the two integrated circuits.

Next install the eight LEDs. They will have a flat spot on one side, and this flat spot should be adjacent to the corresponding resistor (to the right on the component layout diagram). Depending on the enclosure you intend to use, you might want the LEDs to stand up fairly high to reach to (or through) the front panel.

Connect a 9V battery clip to the 9V+ and 9V- inputs, with the red lead at 9V+. Connect an appropriate test lead to the "+" and "-" inputs. In my prototype, I made a pair of banana jacks out of pieces of brass tubing, but you can also solder the test leads directly to the circuit board. My test lead has two plugs on the end, one for the transmitter charging jack, and one for the receiver charging jack. Be sure to check your transmitter's documentation as to the polarity of the charging jack; it varies from brand to brand.

Finally, install S1. This is most easily done by first soldering short lengths of solid hook-up wire to the switch terminals, and then inserting these into the holes in the circuit board.

	The completed circuit board. The prototype used vertically mounted trimmer potentiometers rather than the more convenient horizontally mounted ones in the parts list. Notice the extra adjusting resistor installed in the R1 group.

Calibration and Testing

Do not install Z1 and Z2 in their sockets yet. Connect a 9V battery to the battery clip. With S1 in its center-off position, nothing should happen.

Turn both R20 and R21 to approximately their midpoints. Clip the black lead from a digital voltmeter to the 9V- lead (the easiest way to do this is to clip the voltmeter lead to J2). Connect the voltmeter's red lead to the center terminal of R20 or the top end of R4 (the end furthest from R20). While holding S1 in either of its two on positions, adjust R20 until the voltmeter reads 5.13V.

Next, move the voltmeter's red lead to the center terminal of R21 or the rightmost end of R10 (the end closest to R21). Adjust R21 until the voltmeter reads 4.66V. Now move the red lead back to R20, and readjust R20 to give 5.13V. Repeat this, going back and forth between R20 and R21, adjusting for 5.13V and 4.66V respectively. After going back and forth a few times, you should have both voltages correct. Disconnect the digital voltmeter.

Front panel label. Actual size is 2.5" x 1".

Install Z1 and Z2, being careful to put them in the right way around. Connect a fully charged 4-cell receiver battery to the ESV's test leads. With S1 switched to the receiver position (to the left), all eight LEDs should light. With S1 switched to the transmitter position, none of the LEDs should light.

Next, connect a fully charged 8-cell transmitter battery. With S1 switched to the transmitter position, all eight LEDs should light. Avoid switching S1 to the receiver position when testing a transmitter battery or R1a through R1d will overheat.

Try testing the transmitter battery when it's installed in the transmitter, by plugging the test lead into the charging jack. If you can't get the LEDs to light, it could be that your transmitter has a reverse polarity protection diode in it. If so, you'll need to bypass this diode. Most modelers bypass it with a small fuse, so the reverse polarity protection is still there, but the battery voltage can be tested without removing the battery from the transmitter.

	I attached the board to a smoked transparent front panel (using the switch to hold it in place). One of the brass tubes used as a banana jack is visible near the bottom. The enclosure is home-made, complete with a compartment for the 9V battery.

Enclosure

The ESV can be installed in a variety of enclosures. I constructed one from gray plastic, with a smoked transparent plastic cover. That way, the only holes I needed to drill were for the switch, test leads, and screws. The LEDs are visible through the cover, even in daylight. If a professional appearance is not important, you could even just cover the whole circuit in clear heatshrink tubing and hold the 9V battery to the backside with Velcro® or servo tape. Regardless of the enclosure you use, I've provided a suitable front panel label that you can photocopy and glue on.

Using the ESV

I use my ESV every time I go flying, and part way through the day when spending a long time at the field. I've found its estimates of remaining charge to be fairly accurate, as confirmed by measuring the remaining capacity at the end of the day by discharging at a known current.

If four or less LEDs light, I start testing more often (before each flight). If two or less LEDs light, I won't fly without recharging the offending battery first.

In addition to indicating remaining charge, an ESV can also provide an early indication of some kinds of problems. If a fully charged battery lights less than eight LEDs, figure out why. Possibilities include a poor connection in a switch, a broken power lead or cell-to-cell connection (with only pressure still holding things together), corrosion, a bad cell, or voltage depression (caused by prolonged overcharging). In all cases, there is a disaster waiting to happen, and the problem should be rectified.

Please remember however that an ESV is not a substitute for regular battery and wiring inspections. If a connection is intact but is about to break, the ESV will give no indication that anything is wrong.

Parts List

The following table lists all the parts along with DigiKey part numbers. Radio Shack part numbers are also shown for those parts available at your local Radio Shack store.

Part	Description	DigiKey Part	Radio Shack Part
R1a…R1d, R2a…R2d	100Ω ¼W resistor	100QBK-ND	271-1311
R3	2.2kΩ ¼W resistor	2.2KQBK-ND	271-1325
R4	1.2kΩ ¼W resistor	1.2KQBK-ND
R5	330Ω ¼W resistor	330QBK-ND	271-1315
R6	390Ω ¼W resistor	390QBK-ND
R7	470Ω ¼W resistor	470QBK-ND	271-1317
R8, R9, R12…R19	680Ω ¼W resistor	680QBK-ND
R10	1kΩ ¼W resistor	1KQBK-ND	271-1321
R11	39kΩ ¼W resistor	39KQBK-ND
R20, R21	10kΩ trimmer	3316F-103-ND	271-282
C1, C2	0.1uF capacitor	P4923-ND	272-109
D1	1N5232 5.6V 500mW Zener or 1N4734 5.6V 1W Zener	1N5232ADICT-ND 1N4734BDICT-ND
LED1…LED8	Red LED	P300-ND	276-041
S1	DPDT center-off momentary toggle switch	CKN1129-ND	275-620 (not momentary)
Z1, Z2	LM339 quad comparator	LM339N-ND	276-1712
Miscellaneous	9V battery clip	BS6I-ND	270-325

digital Voltmeter diagram

front side
Copyright of this circuit belongs to smart kit electronics. In this page we will use this circuit to discuss for improvements and we will introduce some changes based on original schematic.

General Description
This is an easy to build, but nevertheless very accurate and useful digital voltmeter. It has been designed as a panel meter and can be used in DC power supplies or anywhere else it is necessary to have an accurate indication of the voltage present. The circuit employs the ADC (Analogue to Digital Converter) I.C. CL7107 made by INTERSIL. This IC incorporates in a 40 pin case all the circuitry necessary to convert an analogue signal to digital and can drive a series of four seven segment LED displays directly. The circuits built into the IC are an analogue to digital converter, a comparator, a clock, a decoder and a seven segment LED display driver. The circuit as it is described here can display any DC voltage in the range of 0-1999 Volts.

Technical Specifications - Characteristics
Supply Voltage: ............. +/- 5 V (Symmetrical)
Power requirements: ..... 200 mA (maximum)
Measuring range: .......... +/- 0-1,999 VDC in four ranges
Accuracy: ....................... 0.1 %
FEATURES
- Small size
- Easy construction
- Low cost.
- Simple adjustment.
- Easy to read from a distance.
- Few external components.

How it Works
In order to understand the principle of operation of the circuit it is necessary to explain how the ADC IC works. This IC has the following very important features:
- Great accuracy.
- It is not affected by noise.
- No need for a sample and hold circuit.
- It has a built-in clock.
- It has no need for high accuracy external components.

Schematic (fixed 22-2-04)

7-segment display pinout MAN6960

An Analogue to Digital Converter, (ADC from now on) is better known as a dual slope converter or integrating converter. This type of converter is generally preferred over other types as it offers accuracy, simplicity in design and a relative indifference to noise which makes it very reliable. The operation of the circuit is better understood if it is described in two stages. During the first stage and for a given period the input voltage is integrated, and in the output of the integrator at the end of this period, there is a voltage which is directly proportional to the input voltage. At the end of the preset period the integrator is fed with an internal reference voltage and the output of the circuit is gradually reduced until it reaches the level of the zero reference voltage. This second phase is known as the negative slope period and its duration depends on the output of the integrator in the first period. As the duration of the first operation is fixed and the length of the second is variable it is possible to compare the two and this way the input voltage is in fact compared to the internal reference voltage and the result is coded and is send to the display.

back side

All this sounds quite easy but it is in fact a series of very complex operations which are all made by the ADC IC with the help of a few external components which are used to configure the circuit for the job. In detail the circuit works as follows. The voltage to be measured is applied across points 1 and 2 of the circuit and through the circuit R3, R4 and C4 is finally applied to pins 30 and 31 of the IC. These are the input of the IC as you can see from its diagram. (IN HIGH & IN LOW respectively). The resistor R1 together with C1 are used to set the frequency of the internal oscillator (clock) which is set at about 48 Hz. At this clock rate there are about three different readings per second. The capacitor C2 which is connected between pins 33 and 34 of the IC has been selected to compensate for the error caused by the internal reference voltage and also keeps the display steady. The capacitor C3 and the resistor R5 are together the circuit that does the integration of the input voltage and at the same time prevent any division of the input voltage making the circuit faster and more reliable as the possibility of error is greatly reduced. The capacitor C5 forces the instrument to display zero when there is no voltage at its input. The resistor R2 together with P1 are used to adjust the instrument during set-up so that it displays zero when the input is zero. The resistor R6 controls the current that is allowed to flow through the displays so that there is sufficient brightness with out damaging them. The IC as we have already mentioned above is capable to drive four common anode LED displays. The three rightmost displays are connected so that they can display all the numbers from 0 to 9 while the first from the left can only display the number 1 and when the voltage is negative the «-« sign. The whole circuit operates from a symmetrical ρ 5 VDC supply which is applied at pins 1 (+5 V), 21 (0 V) and 26 (-5 V) of the IC.

Construction
First of all let us consider a few basics in building electronic circuits on a printed circuit board. The board is made of a thin insulating material clad with a thin layer of conductive copper that is shaped in such a way as to form the necessary conductors between the various components of the circuit. The use of a properly designed printed circuit board is very desirable as it speeds construction up considerably and reduces the possibility of making errors. To protect the board during storage from oxidation and assure it gets to you in perfect condition the copper is tinned during manufacturing and covered with a special varnish that protects it from getting oxidised and also makes soldering easier.
Soldering the components to the board is the only way to build your circuit and from the way you do it depends greatly your success or failure. This work is not very difficult and if you stick to a few rules you should have no problems. The soldering iron that you use must be light and its power should not exceed the 25 Watts. The tip should be fine and must be kept clean at all times. For this purpose come very handy specially made sponges that are kept wet and from time to time you can wipe the hot tip on them to remove all the residues that tend to accumulate on it.
DO NOT file or sandpaper a dirty or worn out tip. If the tip cannot be cleaned, replace it. There are many different types of solder in the market and you should choose a good quality one that contains the necessary flux in its core, to assure a perfect joint every time.
DO NOT use soldering flux apart from that which is already included in your solder. Too much flux can cause many problems and is one of the main causes of circuit malfunction. If nevertheless you have to use extra flux, as it is the case when you have to tin copper wires, clean it very thoroughly after you finish your work.
In order to solder a component correctly you should do the following:
- Clean the component leads with a small piece of emery paper.
- Bend them at the correct distance from the component’s body and insert the component in its place on the board.
- You may find sometimes a component with heavier gauge leads than usual, that are too thick to enter in the holes of the p.c. board. In this case use a mini drill to enlarge the holes slightly. Do not make the holes too large as this is going to make soldering difficult afterwards.

Parts placement

PCB dimensions: 77,6mm x 44,18mm or scale it at 35%

- Take the hot iron and place its tip on the component lead while holding the end of the solder wire at the point where the lead emerges from the board. The iron tip must touch the lead slightly above the p.c. board.
- When the solder starts to melt and flow wait till it covers evenly the area around the hole and the flux boils and gets out from underneath the solder. The whole operation should not take more than 5 seconds. Remove the iron and allow the solder to cool naturally without blowing on it or moving the component. If everything was done properly the surface of the joint must have a bright metallic finish and its edges should be smoothly ended on the component lead and the board track. If the solder looks dull, cracked, or has the shape of a blob then you have made a dry joint and you should remove the solder (with a pump, or a solder wick) and redo it.
- Take care not to overheat the tracks as it is very easy to lift them from the board and break them.
- When you are soldering a sensitive component it is good practice to hold the lead from the component side of the board with a pair of long-nose pliers to divert any heat that could possibly damage the component.
- Make sure that you do not use more solder than it is necessary as you are running the risk of short-circuiting adjacent tracks on the board, especially if they are very close together.
- When you finish your work, cut off the excess of the component leads and clean the board thoroughly with a suitable solvent to remove all flux residues that may still remain on it.

As it is recommended start working by identifying the components and separating them in groups. There are two points in the construction of this project that you should observe:
First of all the display IC’s are placed from the copper side of the board and second the jumper connection which is marked by a dashed line on the component side at the same place where the displays are located is not a single jumper but it should be changed according to the use of the instrument. This jumper is used to control the decimal point of the display.
If you are going to use the instrument for only one range you can make the jumper connection between the rightmost hole on the board and the one corresponding to the desired position for the decimal point for your particular application. If you are planning to use the voltmeter in different ranges you should use a single pole three position switch to shift the decimal point to the correct place for the range of measurement selected. (This switch could preferably be combined with the switch that is used to actually change the sensitivity of the instrument).
Apart from this consideration, and the fact that the small size of the board and the great number of joints on it which calls for a very fine tipped soldering iron, the construction of the project is very straightforward.
Insert the IC socket and solder it in place, solder the pins, continue with the resistors the capacitors and the multi-turn trimmer P1. Turn the board over and very carefully solder the display IC’s from the copper side of the board. Remember to inspect the joints of the base of the IC as one row will be covered by the displays and will be impossible to see any mistake that you may have made after you have soldered the displays into place.
The value of R3 controls in fact the range of measurement of the voltmeter and if you provide for some means to switch different resistors in its place you can use the instrument over a range of voltages.
For the replacement resistors follow the table below:
0 - 2 V ............ R3 = 0 ohm 1%
0 - 20 V ........... R3 = 1.2 Kohm 1%
0 - 200 V .......... R3 = 12 Kohm 1%
0 - 2000 V ......... R3 = 120 Kohm 1%
When you have finished all the soldering on the board and you are sure that everything is OK you can insert the IC in its place. The IC is CMOS and is very sensitive to static electricity. It comes wrapped in aluminium foil to protect it from static discharges and it should be handled with great care to avoid damaging it. Try to avoid touching its pins with your hands and keep the circuit and your body at ground potential when you insert it in its place.
Connect the circuit to a suitable power supply ρ 5 VDC and turn the supply on. The displays should light immediately and should form a number. Short circuit the input (0 V) and adjust the trimmer P1 until the display indicates exactly «0».

Parts
R1 180k
R2 22k
R3 12k
R4 1M
R5 470k
R6 560 Ohm
C1 100pF
C2, C6, C7 100nF
C3 47nF
C4 10nF
C5 220nF
P1 20k trimmer multi turn
U1 ICL 7107
LD1,2,3,4 MAN 6960 common anode led displays

If it does not work
Check your work for possible dry joints, bridges across adjacent tracks or soldering flux residues that usually cause problems.
Check again all the external connections to and from the circuit to see if there is a mistake there.
- See that there are no components missing or inserted in the wrong places.
- Make sure that all the polarised components have been soldered the right way round. - Make sure the supply has the correct voltage and is connected the right way round to your circuit.
- Check your project for faulty or damaged components.

Sample Power supply 1

Sample Power Supply 2

voltmeter articl

Demonstration voltmeter from a physics class

A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit. Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit; digital voltmeters give a numerical display of voltage by use of ananalog to digital converter.

Voltmeters are made in a wide range of styles. Instruments permanently mounted in a panel are used to monitor generators or other fixed apparatus. Portable instruments, usually equipped to also measure current and resistance in the form of a multimeter, are standard test instruments used in electrical and electronics work. Any measurement that can be converted to a voltage can be displayed on a meter that is suitably calibrated; for example, pressure, temperature, flow or level in a chemical process plant.

General purpose analog voltmeters may have an accuracy of a few percent of full scale, and are used with voltages from a fraction of a volt to several thousand volts. Digital meters can be made with high accuracy, typically better than 1%. Specially calibrated test instruments have higher accuracies, with laboratory instruments capable of measuring to accuracies of a few parts per million. Meters using amplifiers can measure tiny voltages of microvolts or less.

Part of the problem of making an accurate voltmeter is that of calibration to check its accuracy. In laboratories, the Weston Cell is used as a standard voltage for precision work. Precision voltage references are available based on electronic circuits.

Contents   [show]

[edit]Analog voltmeter

A moving coil galvanometer of thed'Arsonval type.

• The red wire carries the current to be measured.
• The restoring spring is shown in green.
• N and S are the north and south poles of the magnet.

A moving coil galvanometer can be used as a voltmeter by inserting a resistor in series with the instrument. It employs a small coil of fine wire suspended in a strong magnetic field. When an electric current is applied, the galvanometer's indicator rotates and compresses a small spring. The angular rotation is proportional to the current through the coil. For use as a voltmeter, a series resistance is added so that the angular rotation becomes proportional to the applied voltage.

One of the design objectives of the instrument is to disturb the circuit as little as possible and so the instrument should draw a minimum of current to operate. This is achieved by using a sensitive ammeter or microammeter in series with a high resistance.

The sensitivity of such a meter can be expressed as "ohms per volt", the number of ohms resistance in the meter circuit divided by the full scale measured value. For example a meter with a sensitivity of 1000 ohms per volt would draw 1 milliampere at full scale voltage; if the full scale was 200 volts, the resistance at the instrument's terminals would be 200,000 ohms and at full scale the meter would draw 1 milliampere from the circuit under test. For multi-range instruments, the input resistance varies as the instrument is switched to different ranges.

Moving-coil instruments with a permanent-magnet field respond only to direct current. Measurement of AC voltage requires a rectifier in the circuit so that the coil deflects in only one direction. Moving-coil instruments are also made with the zero position in the middle of the scale instead of at one end; these are useful if the voltage reverses its polarity.

Voltmeters operating on the electrostatic principle use the mutual repulsion between two charged plates to deflect a pointer attached to a spring. Meters of this type draw negligible current but are sensitive to voltages over about 100 volts and work with either alternating or direct current.

[edit]VTVMs and FET-VMs

The sensitivity and input resistance of a voltmeter can be increased if the current required to deflect the meter pointer is supplied by an amplifier and power supply instead of by the circuit under test. The electronic amplifier between input and meter gives two benefits; a rugged moving coil instrument can be used, since its sensitivity need not be high, and the input resistance can be made high, reducing the current drawn from the circuit under test. Amplified voltmeters often have an input resistance of 1, 10, or 20 megohms which is independent of the range selected. A once-popular form of this instrument used a vacuum tube in the amplifer circuit and so was called the vacuum tube voltmeter, or VTVM. These were almost always powered by the local AC line current and so were not particularly portable. Today these circuits use a solid-state amplifier using field-effect transistors, hence FET-VM, and appear in handheld digital multimeters as well as in bench and laboratory instruments. These are now so ubiquitous that they have largely replaced non-amplified multimeters except in the least expensive price ranges.

Most VTVMs and FET-VMs handle DC voltage, AC voltage, and resistance measurements; modern FET-VMs add current measurements and often other functions as well. A specialized form of the VTVM or FET-VM is the AC voltmeter. These instruments are optimized for measuring AC voltage. They have much wider bandwidth and better sensitivity than a typical multifunction device.

[edit]Digital voltmeter

Two digital voltmeters. Note the 40 microvolt difference between the twomeasurements, an offset of 34 parts per million.

The first digital voltmeter was invented and produced by Andrew Kay of Non-Linear Systems (and later founder of Kaypro) in 1954.

Digital voltmeters (DVMs) are usually designed around a special type of analog-to-digital converter called an integrating converter. Voltmeter accuracy is affected by many factors, including temperature and supply voltage variations. To ensure that a digital voltmeter's reading is within the manufacturer's specified tolerances, they should be periodically calibrated against avoltage standard such as the Weston cell.

Digital voltmeters necessarily have input amplifiers, and, like vacuum tube voltmeters, generally have a constant input resistance of 10 megohms regardless of set measurement range.