User`s guide

Operation and Connections Tutorial 5
Model N6705A User’s Guide 107
Measurement Considerations
Dynamic Current Correction
The DC Power Analyzer measures the output current across an
internal current monitor. This current monitor is located on the
positive output rail on the inboard side of the output capacitor (see
figure). This current measurement scheme is used by the majority of
power supplies on the market today.
For the majority of power supply applications this method of
measuring output current yields accurate measurements. However,
with an output capacitor, when there is a significant change in
voltage over time, the additional output current does not all flow into
the user's load; some of it flows through the output capacitor. Thus,
in this momentary situation, the instrument’s measurement circuit is
not only measuring the output current going to the user’s load, but
also the output current flowing through the output capacitor.
Because the load never sees this additional current, this results in an
inaccurate output current measurement.
Normally when the output current is measured and averaged over a
number of samples, this inaccuracy is insignificant. However,
because the DC Power Analyzer has built-in scope and data logging
functions, which can sample the output current at up to about 50
kHz, this inaccuracy becomes evident.
Dynamic Current Correction compensates for the current flowing
into the output capacitor. This feature is turned on by default. The
DC Power Analyzer automatically calculates what the additional
current is, and subtracts it from the current measurement. Thus, with
compensation mode on, the DC Power Analyzer measures the output
current flowing through the user’s load accurately.
Note that turning the current compensation circuit on increases the
peak to peak noise in current measurements on some power
modules. It may also limit the measurement bandwidth as explained
in the next section. If either condition is a significant factor in your
application, you should turn Dynamic Current Correction off.
To turn Dynamic Current Correction off for each output, press the
Meter View key, then Properties . Uncheck the box labeled
“Compensate current measurements during voltage transients”.
current
monitor
+ output
- output
output
capacitor
current
load
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