General Information - 1
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The heavy line illustrates the locus of possible operating points as a function of the output load, which
may be purely resistive, or possibly include external voltage or current sources. In current priority mode,
the constant current loop will regulate the output current as the load changes, until the positive or
negative voltage limit is reached. A CC (constant current) status flag indicates when the current loop is
controlling the output.
If the output voltage reaches either the positive or negative voltage limit, the unit no longer operates in
constant current mode and the output current is no longer held constant. Instead, the output current is
limited at either the positive or negative voltage limit line. When the unit is sinking power, the output
voltage will continue to increase in the positive or negative direction as more current is forced into the
unit. Note that a VL+ (positive voltage limit) or VL− (negative voltage limit) status bit will be set to
register a voltage limit at about 0.8 V before the positive or negative voltage line is reached.
The maximum current available in current priority mode is about 0.5 mA, which is ideal for testing
sensitive devices such as input diodes. In this mode, the output current must be programmed to the
desired positive or negative value. However, the positive and negative voltage limits are not
programmable, and vary with the actual output current as shown in the figure. The typical positive
voltage limit ranges from about 10.75V at no load to about 9.5V at full load. The typical negative voltage
limit ranges from about –10.75V to about –9.5V.
NOTE: Overvoltage protection is not functional in current priority mode.
Measurement Characteristics
The N3280A uses a digitizing measurement system with a single timebase for all output channels. The
number of measurement samples and the sampling interval of the timebase can be explicitly programmed.
These values will apply to measurements taken on all outputs. For example, if simultaneous
measurements are made on four output channels and one of the three channels is set to one power line
cycle (PLC), then all three channels will be set to one power line cycle per measurement.
Conversely, each output channel of the N3280A has its own measurement buffer. This means that each
output can be configured to measure a different parameter (either voltage or current), and a different
current range. However, the number of measurement samples and sampling interval for each type of
measurement is the same for all channels.
There is one voltage measurement range and three current measurement ranges. The current range must
be selected explicitly. If a measured value exceeds the presently selected range, an error message is
returned. Voltage measurements and current measurements using the 0.5A or 15mA range can be made to
full accuracy using the default measurement sample (5 data points @30.4µs intervals = 152 µs). To
achieve full accuracy on the 0.5mA current range, a longer sampling interval of one power line cycle
(PLC) is required to filter out line noise. Thus, a full accuracy measurement on the 0.5mA current range
will typically take between 18 and 21.3 ms, depending on the line frequency.
Note that faster measurements using lower PLC values (<1) are only appropriate for loads that do not
draw currents with a significant noise component. If the load current is noisy, it may be necessary to
increase the sampling interval to provide additional filtering.
All voltage and current measurements return the average value of the samples taken. Measurements can
be made using either a Rectangular or Hanning window. The default Rectangular window is used on all