Common Terminology and Definitions for Programmable DC Power Supplies and Precision DC Sources

This tutorial is part of the National Instruments Measurement Fundamentals series. Each tutorial in this series, will teach you a specific topic of common measurement applications, by explaining the theory and giving practical examples. This tutorial covers the common terminology for programmable DC power supplies.

For additional power supply only concepts, refer to the Power Supply Fundamentals main page.
For the complete list of tutorials, return to the NI Measurement Fundamentals Main page.

Constant Current

Constant current mode is enabled on an output channel when a load attempts to draw more current than the programmed current limit. In constant current mode, the current through the load is held constant at the current limit; and the voltage level rises or falls as the load requires more or less power, respectively. Thus, the voltage level setting acts as a voltage-limiting factor. In constant current mode, the output channels act as a current source.

Note: Constant current mode is synonymous with current-controlled mode. Constant voltage mode is enabled on an output channel provided a load does not require more current than the programmed current limit. In constant voltage mode, the output voltage remains constant despite load changes. If a load attempts to draw more current than the programmed current limit, constant current mode is enabled.

Note: Constant voltage mode is synonymous with voltage-controlled mode.

Quadrants describe the sinking and sourcing capabilities of an input or output. The terms sinking and sourcing describe the direct current flow into and out of a load, respectively. Sourcing devices can drive current into the load while sinking devices absorb current and provide a path for the current to ground.

Think of the operation of a power supply output as occurring within four quadrants. Quadrants consist of the various combinations of positive and negative currents and voltage. Use the figure 3 to determine whether the output capability is sinking or sourcing for any given voltage and current, positive or negative.

Figure 3. The four possible quadrants for a programmable power supply

For example, when you have a positive voltage and a positive (sourcing) current, the output operation falls within Quadrant I, and the output is sourcing current. When you have a positive voltage and a negative (sinking) current, the output operation falls within Quadrant II, and the output is sinking current.

A single-quadrant power supply has output channels only capable of sourcing current. A four-quadrant power supply has output channels capable of both sourcing and sinking current with bipolar (positive and negative) voltage.

Ripple and Noise

Noise—unwanted signals present on the output channels—can affect devices connected to the output channels. Noise can be characterized as normal-mode or common-mode noise. Regardless of its characterization, noise is meaningful only when it is specified with an associated bandwidth.

Normal-mode Noise
Normal-mode noise is noise present between the output HI and LO, appearing either in series (constant voltage mode) or parallel (constant current mode) with the output. Normal-mode noise can be expressed as voltage noise or current noise, depending on the control mode of the output channel. In constant voltage mode, the appropriate measure of interest is a series-voltage noise source. In constant current mode, the appropriate measure of interest is a parallel-current noise source. AC to DC rectification causes ripple, a type of periodic normal-mode noise.

Common-mode Noise
Common-mode noise is noise present between the output LO and the chassis or earth ground. In this sense, the equivalent circuit is a current noise source connected across these two terminals. When you connect an impedance between the output LO and chassis or earth ground, a noise current can flow in the impedance, resulting in an unexpected offset or other undesirable error.

Considerations When Measuring Noise
Exercise care when measuring noise on an output device, such as a power supply. When verifying the specified wideband noise of a power supply, the affects of ground loops, unnecessarily long probe ground leads, and electrically noisy environments combine and can skew your measurements.

Observe the following recommendations when measuring the noise of a power supply:

Connect the probe directly to the terminals of the power supply. Do not use long leads, loose wires, or unshielded cables. Limit the probe ground lead to a few inches at most. Connect this lead directly to the output LO terminal of the appropriate channel. Limit the bandwidth of the measurement device to the bandwidth of interest. For example, making a 20 MHz noise measurement with a 200 MHz bandwidth instrument, does not yield the specified values.

Exercise caution when making measurements in a modern laboratory environment with computers, electronic ballasts, switching power supplies etc., to avoid measuring the environment noise instead of the power supply noise.

Isolation

Isolation is a means of physically and electrically separating two parts of a measurement device. Electrical isolation pertains to eliminating ground paths between two electrical systems. By providing electrical isolation, you can break ground loops, increase the common-mode range of the power supply, and level shift the signal ground reference to a single system ground.

Channel-to-Channel
The most robust isolation topology is channel-to-channel isolation. In this topology, each channel is individually isolated from one another and from other non-isolated system components. In addition, each channel has its own isolated power supply.

Rise Time and Settling time

Rise Time
Rise time specifies the time it takes for the output to transition from 10% to 90% of the programmed voltage level at the maximum current. Use the following equation to calculate the rise time for a single-pole system.

Rise Time = 2.2 × time constant

Using the rise time, you can calculate the bandwidth using the following equation.

Bandwidth = 0.35 / Rise Time

Settling Time
Settling time specifies the time required for an output channel to reach a stable mode of operation. You can calculate the settling time to any maximum level of error desired for a single pole system using the time constant and the following rule.

Settling Time = (decades × 2.3 × time constant)

Where decades is decades of settling as determined by the desired maximum error (settling to 1% error = 2 decades, 0.1% = 3 decades, etc). For example, calculating the settling to 1% error, settling time is (2 × 2.3 × time constant); calculating the settling time to 0.1%error, settling time is (3 × 2.3 × time constant), etc.

If the maximum error desired falls on an uneven number of decades, use the following equation to calculate settling time.

Settling Time = -ln(maximum error desired) × time constant

For example, calculating the settling to 0.05% error, settling time is -ln(0.0005) × time constant, or 7.6 × time constant.

Resolution

Resolution is the smallest change in output voltage or current that can be obtained by the programmable DC power supply. The formula to calculate resolution is 2^n. For example, a 12 bit ADC has a resolution of 2^12 = 4,096. Therefore, our best resolution is 1 part out of 4,096, or 0.0244% of the full scale. An ADC takes an analog signal and turns it into a binary number. Thus, each binary number from the ADC represents a certain voltage level. Resolution is the smallest input voltage change a digitizer can capture. Resolution can be expressed in bits (LSB), in proportions, or in percent of full scale.

Resolution limits the precision of a measurement. The higher the resolution (number of bits), the more precise the measurement. An 16-bit ADC divides the vertical range of the input amplifier into 65,536 discrete levels. With a vertical range of 20 V, this 16-bit ADC on a power supply can ideally resolve voltage differences down to 0.31 mV.

Relevant NI Products

Customers interested in this topic were also interested in the following NI products:

• Programmable DC Power Supplies and Precision Sources
• Modular Instruments (digital multimeters, digitizers, switching, etc...)
• LabVIEW Graphical Programming Environment
• SignalExpress Interactive Software Environment

For the complete list of tutorials, return to the NI Measurement Fundamentals Main page

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