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Grasping linear power supply's data sheet (Part 1)

09 Jan 2013  | Robert Green

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For example, Keithley's Model 2200-32-3 Programmable 32V/3A DC power supply has a voltage setting accuracy specification of ±(0.03% + 3mV). Therefore, when it is set to deliver 5V, the uncertainty in the output value is (5V)(0.0003 + 3mV) or 4.5mV. Current setting accuracy is specified and calculated similarly.

Setting resolution is the smallest change in a voltage or current setting that can be selected on the power supply. This parameter is sometimes called programming resolution. The resolution specification limits the number of discrete levels that can be set. Often, this is defined by a combination of the number of user interface digits available and the number of bits available in the DAC. A DAC with more bits has finer control of its output and can deliver more distinct values for the control loop to use as a reference. However, with corrections for offset and gain errors, there will be less resolution than the number of bits in the DAC would suggest.

Changing a setting in a single step of resolution may not always cause a corresponding change in the output. However, the setting accuracy specification governs the relationship between settings and output, and a calibrated instrument should perform within this tolerance.

Figure 2: The least significant digits on the upper display correspond to the 1mV and 0.1mA readback resolution of Keithley Series 2200 instruments. The least significant digits on the lower display correspond to the setting resolution.

Setting resolution may be expressed as an absolute unit value or as a percentage of full scale. For example, the voltage setting resolution on the Keithley 2200-32-3 is 1mV and the current setting resolution is 0.1mA.

Readback accuracy is sometimes called meter ('re' for unit of measurement) accuracy. It determines how close the internally measured values are to the theoretical value of the output voltage (after setting accuracy is applied). Just as with a digital multi-meter, this is determined using a traceable reference standard. Readback accuracy is expressed as:

±(% of measured value + offset)

Readback resolution is the smallest change in internally measured output voltage or current that the power supply can discern. It is usually expressed as an absolute value but may also be given as a percentage of full scale. The voltage readback resolution on the Keithley 2200-32-3 is 1mV and the current readback resolution is 0.1mA (figure 2).

Enhancing voltage output accuracy
Voltage drop in cables that carry current between the power supply and the device under test (DUT) means that the voltage at the DUT is less than the voltage at the output terminals of the power supply. The use of heavier-gauge wire reduces the voltage drop in the test leads on any power supply. Keeping cables as short as possible also helps. If the power supply is equipped with remote sense capability, using a four-wire connection can help ensure the voltage level set on the power supply is the voltage the DUT sees.

With a four-wire connection from the power supply to the DUT, one set of leads carries the output current while the power supply uses the other set to measure voltage directly at the DUT terminals, as illustrated in figure 3. The sense leads are connected inside the power supply to a high impedance voltmeter circuit; therefore, close to zero current is flowing in the sense leads, virtually eliminating voltage drop in those leads. The power supply maintains the desired output voltage at the sense leads by increasing the voltage at the output to compensate for voltage drops in the source leads that deliver the current to the DUT.

Figure 3: Remote sense eliminates the effect of lead resistance by separating the source circuit from the sense circuit and by using the voltage detected by the sense circuit as feedback to the power supply's control circuitry. The control circuitry adjusts the output on the source leads to maintain the specified voltage across the load.

Specs related to stability
Stability specifications describe how a power supply responds to conditions caused by a changing load, AC line voltage, and temperature. Several specifications state the instrument's ability to deliver stable output over the short term.

Over the long term, the performance of a power supply inevitably changes due to ageing. Long-term stability issues are managed by requiring regular verification and calibration of the instruments.

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