Operator`s manual

7
Your instrument has a crest factor range of 1.0 to 3.0 at full scale. Going down from full-scale, the crest
factor capability increases from 3.0 to: Full-Scale x 3
(i.e. 6 at half-scale) RMS Value , If an input signal
has a crest factor of 3.0 or less, voltage measurements will not be in error due to dynamic range
limitations at full-scale. If the crest factor of a waveform is not known, and you wish to know if it falls within
the crest factor of your meter, measure the signal with both your meter and an ac coupled oscilloscope. If
the rms reading on your meter is 1/3 of the peak voltage on the waveform or less, then the crest is 3.0.
For readings at less than full-scale, use the preceding formula to determine the maximum crest factor. At
half-scale the maximum crest factor is: 2 x 3
= 1
The waveforms in Figure 2 show signals with increasing values of crest factor. As you can see from the
series of waveforms, a signal with a crest factor above 3.0 is unusual.
For an ac coupled pulse train: Crest Factor =
1)D/1( −
Where D = duty cycle or the ratio of pulse width to cycle length. Reversing this formula, we find that your
meter can accurately measure pulse trains at full-scale with a duty cycle above 10% without being limited
by crest factor .
1(1/D)3.0Factor Crest −== : 1-(1/D) 9.0
=
:
1/D 10.0 =
Bandwidth
Bandwidth defines the range of frequencies where the response of the voltmeter's amplifiers is no more
than 3 dB down (half-power levels). Your instrument has a bandwidth of greater than 200kHz.
Slew rate
Slew rate is also called the rate limit or the voltage velocity limit. It defines the maximum rate of change of
the output of the amplifiers for a large input signal. Slew rate limitations are not a factor in measuring
voltages within specified frequencies and amplitude limits of this DMM.
Rise and fall time effect on accuracy
The rise and fall time of a waveform are the length of time it takes a waveform to change between the
points that are 10% and 90% of the peak value. When discussing these periods, we'll only mention rise
time. Errors due to rise to fall time can be caused either by bandwidth or slew rate limitations. Slew rate
should not affect your measurement with this DMM.
Figure 3 Waveform Distortion
An approximate way of converting bandwidth to rise time limit is to divide 0.35 by the 3 dB down
frequency. For your instrument this will be 0.35/200kHz = 1.75 µsec. The following example will help you
calculate errors due to this limitation when measuring rectangular pulses. These calculations will be
rough because ideal waveforms are used in the analysis.
Ideally, the rectangular pulses would have zero rise and fall time and would be the right angled waveform
shown in Figure 3. In practice, every waveform has a rise and fall time and looks more like the waveform
in Figure 3. When calculating the error caused by the bandwidth of your Instrument, we will assume that
T
t
0
t
1
DC Level
A
t
1
t
1 +
t
0
100usec
48.25
usec
1.75usec
DC Level
A
1.75usec
50usec
DC Level DC Level
Ideal
Distortion
Components Example