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· Typical DMM Functions · Analog to Digital · Resolution · Sensitivity
· Accuracy · Loading and Input Impedance · Speed and Settling Time · Normal and Common
· Mode Rejection Overload Protection      
TSP- and LXI-Related Products
With the wide variety of types and specifications available in today's digital multimeters, choosing one that's ideal for application can be difficult. This reference guide to understanding DMM types, specifications, and applications offers a wealth of information, including application notes, articles, data sheets, demos, technical information, webcasts, white papers, and more, to optimal precision and accuracy. And, if you have a question about any of the material, Keithley's applications engineers are available to assist you with their knowledge and expertise. Contact us today.
Typical DMM Functions
Digital multimeters convert analog signals to digital information. In general, DMMs have a minimum of five typical functions.
DC voltage AC voltage DC current AC current Resistance
While specifications vary, most DMMs can be described with block diagrams similar to Figure 1.
 
 
Analog to Digital
The A/D converts the analog input signal to a digital output and is primarily responsible for key instrument characteristics of reading speed, linearity, resolution, normal mode rejection, and precision. The digital output is shown or obtained in several ways. One way is visually, via the front panel with a display of digits and other information. Another way is electronically, with results sent via a port (GPIB, RS-232, USB, or Ethernet) to a computer for further processing.
Resolution
Resolution is defined as the smallest detectable change on any range referenced to full scale. For example, if an instrument displays a maximum of 19,999 on any range, and the smallest detect­able change in the input signal is ±1 least signifi­cant digit (LSD), then the resolution is 1/19999 or 0.005%.

Resolution is commonly expressed as a whole number plus a fraction, e.g., 5½ digits. The whole number represents the number of digits that can display the numbers from 0 to 9. The fraction indicates that the most significant digit has one or more non-zero states, that is, it can display 0, 1, or 2.
Sensitivity
Sensitivity is similar to resolution in that it deals with the smallest change of the input signal the instrument can detect. However, sensitivity is not referenced to full scale, so it is expressed in absolute terms and applies to the lowest range on any function. The sensitivity of a 7½-digit DMM is 10nV if its lowest measurement range is 200mV.
 
 
Accuracy
Accuracy is specified as a two-term specification:(% of reading + % of range) or as (ppm of reading + ppm of range). The closer to zero on the range that the percent of range term of the specification is, the greater the weight it has in the accuracy calculation. The closer to full scale on the range the percent of reading term of the specification is, the greater the weight it has in the accuracy calculation. The best accuracy is obtained near full scale.
Accuracy is also generally stated under several conditions, including ±1°C, ±5°C operating tem­perature, and 24-hour, 90-day, and one-year cali­bration intervals. The expected accuracy can be improved by controlling temperature variations in the environment and by electing more fre­quent calibration intervals. Figure 2 illustrates the effect on accuracy at various levels of input signal within the measurement range. Accuracy for both meters is specified at ±(0.1% + 1 count).
 
 
Loading and Input Impedance
Loading is the disturbance to the circuit being measured caused by the finite input impedance of the DMM. Input impedance is the equivalent resistance and capacitance of the input terminals of the DMM.
Loading error (Figure 3):
is the difference between the voltage measured by the meter (VM) and the voltage of an ideal source (VS).		   Voltage burden error (Figure 4):
is the differ­ence between the expected current through the load (RL) and the measured current (IM) caused by the finite voltage drop of the measuring instrument.
 
 
Speed and Settling Time
Every meter has a settling time associated with its input circuit. The reading rates or measure­ment speeds of instruments are independent of the settling times. For high resolution meters, it may be necessary to allow time for input settling to achieve full rated accuracy.

Several parameters affect measurement speed, including integration rate (NPLC), filter setting, ranging, AutoZero, trigger delays, and display settings. For maximum measurement speed, set these parameters:

Integration rate = 0.01
Filter = disabled Range = fixed (no auto range)
AutoZero = disabled Trigger
Delay = 0.0
Display = disabled

Note that maximum speed settings do not produce the greatest accuracy.
 
 
Normal and Common Mode Rejection
Normal mode interference is the interference mixed in with the incoming signal. Most normal mode interference is at line frequency and its harmonics. NMRR (Normal Mode Rejection Ratio) is specified in dB at line frequencies of 50Hz and 60Hz. Normal mode interference is detected as a peak noise or deviation in a DC signal.
CMRR (Common Mode Rejection Ratio) specifies the ability of a meter to reject signals common to both input HI and LO. This term is generally measured with a 1kΩ imbalance in one of the leads. A larger imbalance will cause CMRR to be worse. CMRR is specified at DC, 50Hz, or 60Hz, and (like NMRR) is expressed in dB. CMRR applies to both DC and AC measurements and appears as an offset error to the desired signal.
Mode Rejection Overload Protection
This is a measure of electrical ruggedness and should be sufficient to protect the meter from commonly encountered line voltages. Typically, the ranges most susceptible to high voltage are the lowest voltage range (e.g., 100mV) and the ohms ranges. Similar to overload protection is the maximum common mode voltage at which the meter can be used. This is the maximum voltage from earth ground that the input LO or COMMON terminal can withstand safely. The input terminal should always be at the lowest impedance.