740 Appendix-A A. Essential Electronic Measuring Devices
This shows that the root mean square value is only 70.7% of the peak value.
A.1.B AnalogMultimeter(AMM)
An analog multimeter has a needle pointer to indicate the measured parameter value
on a printed scale. The scale is generally graduated and difficult to read because of
small subdivisions. Reading is also prone to errors if the user’s line of sight is not
perpendicular to the pointer.
The input impedance of an analog multimeter is generally not very high. This
is a potential problem for testing circuits having comparable or higher impedances.
A typical analog meter has an impedance of around 200-300kΩ, which may not be
high enough for most circuits. One should therefore be very careful in interpreting
the test results. The input impedance of an analog meter is generally not stated by
the manufacturers. Instead its sensitivity is quoted. The impedance can be obtained
from the formula
R=S×Rmax,whereSis the sensitivity of the meter (usually given inkΩ/V)andRmaxis the
maximum range of the meter in volts.
Another problem with analog meters is that their pointers can get damaged if
the input DC voltage polarity is opposite to what it should be. In such a case the
pointer moves in the opposite direction and can get damaged by the force pushing it
against the opposite end. The user should remember that conventionally the positive
terminal of the meter is colored red while the negative terminal is colored black.
A.1.C DigitalMultimeter(DMM)
A digital multimeter uses a digital display to indicate the measured value of the
parameter and the units. The display is usually made of LCD but LED based
multimeters are also fairly common.
A good thing about digital multimeters is that they have a high constant input
impedance. Most DMMs come with an input impedance of about 10MΩ, which is
fairly high for typical electronic circuits. The impedance is generally written on the
back of the DMM and the user should always confirm that it is much higher than
the impedance of the circuit being tested. Care should be taken because some low
cost DMMs can have input impedance in the range of 1MΩ.
A.1.D MeasuringVoltage
Measuring voltage is analogous to measuring potential difference. That is, the volt-
age is always measured between two points. The reference point is generally chosen
to be the circuit ground. In this case, the common lead is connected toanyground
point of the circuit and the other lead is connected to the point where voltage is to
be measured.
It was mentioned earlier that the analog meters do not have very high input
impedances. If the meter’s impedance is not much greater than the circuit’s impedance,
it could lead the meter to draw some current from the circuit with the consequence
of incorrect voltage reading. Therefore one should make sure that the meter’s