Amprobe BDM40-UA Bench-Digital-Multimeter - Manual (Page 11)

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equals the slew rate of 1.75 µsec. To do this we will calculate the values for the theoretical signal with zero rise
and fall time, then calculate the values for a signal with the same period but with total slope periods equal to 1.75
µsec. A comparison of the results will show the measurement error due to the finite bandwidth. Using Figure 3 for
a reference, the total rms and DC levels are :
T
1
t
0
t
A DC V
T 3
1
t 2
0
t 3
A rms Total V
+
=
+
=
Since we can calculate two values, to find what your meter measures, use the formula:
2
DC) (V
2
rms) total (V rms VAC − =
Let's look at the waveform in Figure 3. When using your meter to measure the AC component of the signal, the
display will indicate the rms value of the AC signal riding on the DC level. (This DC level is the average value of the
waveform relation to the baseline.) The total rms value of the waveform can be calculated using the relationship:
2
DC) (V
2
rms) (VAC rms Total V + =
For our example let's use a 10kHz pulse train of 50 µsec pulses with a peak value of 1 V. Ideally, the pulses would
have a zero rise time as shown in Figure 3.
0.707 0.5
300
150
3(100)
2(0) 3(50)
rms Total V = = =
+
=
0.5
100
50
100
0 50
DC V = =
+
=
0.5 0.25 0.25 0.50
2
(0.5)
2
(0.707) rms V AC = = − = − =
When the maximum distortion in rise time of 1.75 µsec is assumed, the signal becomes the isosceles trapezoid
waveform shown in Figure 3. In this case:
0.703 0.494
300
148.25
300
3.5 144.75
3(100)
2(1.75) 3(48.25)
rms Total V = = =
+
=
+
=
0.5
100
50
100
1.75 48.25
DC V = =
+
=
0.494 0.244 0.25 0.494
2
(0.5)
2
(0.703) rms V AC = = − = − =
Note that the V DC stayed the same. So, the errors are:
V total rms: = -0.6%: V AC rms: = -1.2%
VOLTAGE MEASUREMENTS
Your DMM can make either linear voltage or AC + DC TRUE RMS voltage measurements. For both types of
measurements, plug the black test lead into the COMMON terminal and the red test lead into the V-
Ω terminal.
Linear Voltage Measurements
The controls and terminals used for making linear voltage measurements are located on the front panel. Starting
at the top left is the ACV/DCV switch. This pushbutton is interlocked with the other two white function selection
switches A and
Ω. If the DCV function switch is in the IN position (DCV selected), and any other function
selection switch is pushed, the DCV pushbutton will be released to the OUT position. Push the DCV switch to the
IN position.