Specifications

Precision DCV measurements
combined with peak measurements
Power supplies often have
ripple voltages that are riding
on top of the desired DC output.
These ripple voltages are speci-
fied and tested to be a certain
level or less. Frequencies of the
AC signal are often power-line
related, but they can be associ-
ated with higher-frequency
byproducts of switching power
supplies. For example, Figure 1
shows a DC signal with an
AC component.
A common approach to this
measurement problem is to
make both a DCV and ACV
RMS measurement. However,
there are limitations to this
approach:
• Making two measurements
takes more time – especially
changing function and range.
• A typical ACV RMS measure-
ment lacks valuable peak
information.
• Having to digitize to get peak
information takes time.
The 34410A and 34411A
DMMs provide a secondary
measurement function called
peak measurements that you
can activate when you make
precision DCV (or ACV)
measurements. Here is an
improvement on the above
approach:
• Enable the peak measure-
ment function.
• Make a precision DCV
measurement using 1 or
more periods of power-line
cycle integration time to
reject power line frequencies
and random noise.
• The DCV and peak measure-
ment data is displayed.
Peak measurements occur at
20-µs intervals during the
aperture of the DCV measure-
ment, so any peak that is at
least 20 µs wide can be detected.
Several scenarios that can be
determined from the two mea-
surements:
1. The DCV and peak-to-peak
data are within tolerance –
passed
2. The DCV is correct, but the
peak-to-peak value exceeds
a limit – failed
3. The DCV is slightly off,
but the peak-to-peak value
is OK – failed
In cases 2 and 3, the peak-to-peak
ripple voltage is in question.
Case 2 may be excessive noise
spikes due to failed output
filtering. Case 3 could be
distortion that is creating an
asymmetrical AC component
that adds a DC component
to the DCV measurement. In
that case, the ripple may retain
the same peak-to-peak voltage.
When the primary measurement
fails, more information about
the signal is required. The
34411A DMM provides wave-
form capture at 50 k readings/s
that can sample the signal to
provide additional diagnostic
information.
6
Figure 1. A 5 V DC signal with an AC signal
5 V
DC