Measurement Characteristics

General Information - 1

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.

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 @30.4∝s intervals = 152 ∝s). 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