There are typically 5 specifications found on a power supply's data sheet that relate to its voltage level accuracy, they are:
- Load Effect (Load Regulation): Voltage variations on the supply’s output level caused by changes to the load that the supply is connected to. The error here is caused by the supplies regulation circuitry not able to maintain the exact voltage level after a load change or change in current draw from the supply.
- Source Effect (Line Regulation): Error on the supply’s set voltage level caused by non-ideal input AC line power.
- Programming resolution: When setting a supply’s level, that level is turned into a digital value that is used by a digital to analog converter (DAC). The output of the DAC is used by the supply internally as a reference to set the correct output voltage level. In the digital to analog conversion process there is always quantization error and that is what the programming resolution represents. The higher resolution the DAC (more bits) the less error.
- Programming Accuracy: This is the key specification that encompasses includes the three just mentioned (load effect, source effect, and prog resolution) as well as parts tolerances such as amplifier drift.
- Programming Temperature Coefficient: If the power supply is not being operated in its ideal temperature range a temperature coefficient error value is added for every degree out of that range.
The specs will typically be in the form of a static value (such as 4 mV), a percentage of the voltage range, a percentage of the programmed voltage level, or a combination of two of them.
- Programming accuracy is 0.016% of programmed value plus 1.5 mV, which is 5*.00016 + .0015 = 2.3 mV
- Temperature coefficient is 1 degree so 1*(5*30e-6 + 40 uV) = 190 uV
When working with a supply that has built in measurement capability (like most supplies on the market today) remember that the measurement accuracy is separate from the programming accuracy. Also the measurement accuracy error spec may be worse or larger than the programming accuracy spec. As an example, below is a figure from Agilent's N6741B power supply data sheet. The voltage programming accuracy spec is circled in green and the voltage measurement accuracy is circled in red.
As you can see the voltage measurement accuracy is worse than the voltage programming accuracy. In cases like this it is more accurate to use the programmed voltage level and not the measured voltage value.
In the accuracy calculation we did it was assumed that the supply was being used in constant voltage mode (CV), where the supply regulates the output voltage at a set level and allows the current to fluctuate as the load changes (most common way to use a power supply). What if you are operating the supply in constant current mode (CC), where the supply regulates at a certain current level and the voltage is allowed to fluctuate with the load? The calculation is the same except you use the current specs versus the voltage specs.