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Unwanted signals and understanding power supply specifications

Ideally, when you set your DC power supply to a certain voltage, it will maintain that voltage and have little or no noise. However, many external conditions can affect the power supply's output. Since, a power supply is used to power a device under test (DUT), any variations or noise in the output is directly coupled to the DUT. Following are some of the commons sources of unwanted power supply signals, and how they are specified in a data sheet.

Source effects and line regulation

Changes in the AC line voltage can affect the output of a DC power supply. In some regions, AC line voltages can vary greatly. A motor or piece of large equipment can cause voltage to drop in power lines when it pulls excessive current. The ability to maintain a set DC voltage during a change in line voltage is specified as line regulation. Line regulation is typically specified in two ways. First the power supply will only function properly if the AC line voltage is close to the proper voltage, typically ± 10%. Line regulation also affects the output accuracy and is specified as a percent of output with an offset.

Load effects, load regulation and transient response

Loads often require varying amounts of current. Load regulation is a power supply's ability to maintain a constant voltage regardless of the demands of the load for more or less current. A power supply's load regulation specification tells how accurately the voltage will be maintained for example the E3632A load regulation is specified as 0.01% of output plus 2mV. A power supply's transient response specification tells how fast the power supply can return to the desired voltage for example, 50mseconds for a large change in the load.

Illustration of a load that alternates between two resistances.

Figure 1. The top graph illustrates a load that alternates between two resistances. When the resistance drops the power supply will source more current an increase in current will cause a drop in the output voltage.

As the load resistance increases, the output current will cause a small drop in the output voltage of the power supply due to the output impedance. Any resistance in the connecting wire will add to this resistance and increase the voltage drop. You can minimize the voltage drop by using the largest possible hook-up wire. Using a power supply with remote sense will compensate for the voltage drop due to lead resistance. An external capacitor can help minimize the voltage drop and speed the transient response. Transient response will be specified as a length of time, the change in load should also be provided.

Ripple and noise

Ripple and noise are sometimes referred to as periodic and random deviations (PaRD). Quality bench power supplies are designed to have minimum noise on the outputs. Sensitive applications involving low-voltage measurements on the DUT can be particularly susceptible to power supply noise, as it can be coupled into measurement circuits. The peak-to-peak measurement is important for applications where noise can be detrimental to a DUT, such as an RF mixer. To function correctly, many devices require a steady supply voltage with minimum noise. Along with a RMS and peak-to-peak value, the frequency range for the ripple and noise specification should also be provided.

Conducted and radiated noise, normal and common mode noise

In order to gain efficiency, switching power supplies create a higher-frequency (20 kHz-100kHz) AC line signal. The high-frequency signal can be coupled into a measurement circuit through the output terminals (conducted) or radiated as electromagnetic interference. Radiated noise can be coupled into the wires connecting the power supply and DUT. Noise traveling in the same direction as the power from the supply is considered normal mode. A second source of noise is common mode which is noise coupled through ground loops.

Normal mode noise is ripple related to the line frequency, which may be switched at a higher frequency, plus some random noise. Both of these are very low 0.35mV rms for the Keysight E3632A power supplies. Using shielded twisted-pair wire and avoiding large sources of EMI will minimize the normal mode noise supplied to the DUT.

Common mode noise can be a problem for very sensitive circuitry that is referenced to earth ground. When a circuit is referenced to earth ground, a small AC current flows from the output terminals to earth ground. Any impedance to earth ground will create a voltage drop equal to the current flow multiplied by the impedance. To minimize this effect, the output terminal can be grounded at the output terminal. Alternately, you can offset any impedance from your DUT to earth ground with complementary impedance from the supply to earth ground, canceling any generated voltages. If the circuit is not referenced to earth ground, common mode power line noise is typically not a problem.

Keysight's E3600Series power supplies are linear supplies. The figure below shows a simplified diagram of the normal mode and common mode noise sources.

Simplified diagram of an E3632A bench power supply

Figure 2. A simplified diagram of an E3632A bench power supply, illustrating normal and common mode noise sources.

Power supplies rely on an internal voltage reference to determine the voltage output. The accuracy of the voltage reference changes with time and temperature, so when power-supply manufacturers provide a specification for accuracy, they must also provide a time and temperature– For example, a 1-year accuracy between 20 and 30 degrees C would cover the temperature range in most development areas. Additional adders may be provided for larger temperature ranges and stability over time.

Resolution

Typically, a modern power supply will use a D/A to set the output voltage via a control circuit, and it may use an A/D to read back the voltage or current output. The resolution of the D/A and the control circuit will be provided as the program resolution while the read back resolution will provide the resolution of the measurement circuit and A/D. Finally, the front panel will have some limit on how many digits it can display which is specified as display resolution. For current measurements, the E3632A has more – programming and read-back resolution than the front panel.

E3632A resolution:

 VoltageCurrent
Program1mV0.5mA
Readback0.5mV0.1mA
Meter1mV1mA

Getting the most from your power supply

Often a lower-wattage power supply will have less noise than a higher power model some are designed to have excellent stability more program and read back resolution. Look for a power supply that specifies both RMS and peak-to-peak noise. When you connect your DUT to the power supply, use shielded twisted-pair cable with large conductors. Use a separate set of wires for each DUT to maintain low source impedance and minimize voltage drops in the leads. Keep wiring runs as short as possible, and if you use a shield, connect only one end of the shield to the low power-supply terminal. Some power supplies offer a voltage sense feature that can compensate for long lead runs.

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