Accuracy

A measurement or output level on a power supply or SMU can differ from the actual or requested value. Accuracy represents the uncertainty of a given measurement or output level and can be defined in terms of the deviation from an ideal transfer function, as follows:

y = mx + b

where

m is the ideal gain of the system

x is the input to the system

b is the offset of the system

Applying this example to a power supply or SMU signal measurement, y is the reading obtained from the device with x as the input, and b is an offset error that you may be able to null before the measurement is performed. If m is 1 and b is 0, the output measurement is equal to the input. If m is 1.0001, then the error from the ideal is 0.01%.

Parts per million (ppm) is another common unit used to represent accuracy. The following table shows ppm to percent conversions.

Most high-resolution, high-accuracy power supplies and SMUs describe accuracy as a combination of an offset error and a gain error. These two error terms are added to determine the total accuracy specification for a given measurement. NI power supplies and SMUs typically specify offset errors with absolute units (for example, mV or μA), while gain errors are specified as a percentage of the reading or the requested value.

The following example illustrates how to calculate the accuracy of a 1 mA current measurement in the 2 mA range of an SMU with an accuracy specification of 0.03% + 0.4 μA:

Accuracy =  (0.0003 × 1 mA) + 0.4 μA = 0.7 μA

Therefore, the reading of 1 mA should be within ±0.7 μA of the actual current.

Note  Temperature can have a significant impact on the accuracy of a power supply or SMU and is a common problem for precision measurements. The temperature coefficient, or tempco, expresses the error caused by temperature. Errors are calculated as ±(% of reading + offset range)/ºC and are added to the accuracy specification when operating outside the power supply or SMU rated accuracy temperature range (usually 23±10°C or 23±5°C).