11
Circuit loading error
Connecting most voltmeters to a circuit may change the operating voltage of the circuit if it loads the
circuit down. As long as the circuit resistance (source impedance) is small compared to the input
impedance of the meter, the error is not significant. For example, when measuring voltage with your
meter, as long as the source impedance is 1 k
Ω
or less, the error will be
≤
.01 %. If circuit loading does
present a problem, the percentage of error can be calculated using the appropriate formula in Figure 4.
Figure 4. Circuit
Loading Error
Combined AC and DC signal measurements
The waveform shown in Figure 5 is a simple example of an AC signal riding on a DC level. To measure
waveforms such as these, first measure the rms value of the AC component using the AC function of your
meter. Measure the DC component using the DC function of your instrument. The relationship between
the total rms value of the waveform and the AC component and the DC component is:
2
2
component)
(DC
rms)
component
(AC
Total
RMS
+
=
Figure 5.
RMS Values
Insignificance of inherent meter offset
If you short the input of your meter while the AC voltage function is selected, you should have a reading
of less than 10 digits on the display. This small offset is caused by the action of amplifier noise and offset
of the true rms converter. This offset will not significantly affect any readings until you try to measure
signals almost at the lower limit of the meter. For example:
Given: An offset of 40 digits
Given: An offset of 20 digits
0.40mV in 200 mV range
0.20mV in 200 mV range
Input signal = 10mV in 200 mV range
Input signal = 10mV in 200 mV range
2
0.4
2
10
rms
Total
+
=
2
0.2
2
10
rms
Total
+
=
0.16
100
+
=
0.04
100
+
=
1.
DC Voltage Measurements
Loading Error in % = 100 x Rs / (Rs + Rin)
Where: Rs = Source resistance in ohms of the circuit being measured.
Rin = Meter input resistance (1 x 10
7
ohms)
2.
AC Voltage Measurements
First determine input impedance as follows:
2
7
)
FC
2
(
1
10
Zin
π
+
=
Where: Zin = effective input impedance
Rin = 10
7
ohms
Cin = 100 x 10
-12
Farads
F= Frequency in Hz
Then determine source loading error as follows:
Loading Error in % = 100 x
Zin
Zs
Zs
+
Where: Zs = Source impedance
Zin = input impedance
* Vector algebra required
0 V
AC COMPONENT
DC LEVEL
