measurement application
sensitivity, and integration time.
Accuracy is defined as the closeness of agreement between the result of a measurement and its true value or accepted standard value. Imagine you are shooting arrows at a target: the accuracy of your shots would be defined by how close the arrows come to the bullseye.
Repeatability refers to the closeness of agreement between successive measurements carried out under the same conditions. Although repeatability is not typically specified on an instrument’s datasheet, it can usually be easily determined during an instrument demonstration or evaluation. Figure 4 illustrates the concepts of accuracy vs. repeatability.
Resolution is defined as the smallest portion of the signal that can be observed. The resolution of an instrument is determined by the number of digits it can display on the front panel or send to a PC over the communication bus. This can often be changed by pressing a front panel button or by sending a programming command to the instrument. In Figure 5, the user is toggling between 41
⁄2 , 51 ⁄2
on the display and has just selected the 61 display.
, and 61 ⁄2
⁄2 -digit
An SMU instrument’s usable maximum resolution depends on its overall accuracy and the resolution of its analog-to-digital converter (ADC). For example, no one would produce a 61
⁄2 -digit
instrument with an 8-bit ADC and 5% accuracy because most of the digits being displayed would be meaningless. In general, however, the higher the resolution is, the higher the bit count on the ADC and the higher the accuracy will be.
The sensitivity of a measurement is the smallest change in the measured signal that can be detected. The ultimate sensitivity of an instrument depends both on its maximum resolution and its lowest measurement range. For example, a 61
⁄2
SMU with a bottom range of 1µA would have 1pA sensitivity. However, depending on that instrument’s accuracy, that sensitivity might not be particularly useful.
Measurement instruments employ either (or both) of two basic types of analog-to-digital converters: integrating ADCs and digitizing ADCs. In general, an integrating ADC will offer higher accuracy because it cancels out the unwanted effects of AC noise from the power line.
-digit
The instrument’s integration rate, which is specified in NPLC (Number of Power Line Cycles), is adjustable. To reject AC noise, the NPLC must be equal to or greater than 1. Integrating the measurement over multiple power line cycles will reject this noise still further and thereby provide a more accurate measurement. However, this noise rejection capability comes at the expense of reading speed; one power line cycle takes 16.7ms at 60Hz or 20ms at 50Hz. Setting the NPLC to a fraction of a line cycle will provide faster measurements at the expense of more noise or lower accuracy (Figure 6).
Figure
4.In the target on the left,the shooter had high accuracy but poor repeatability
.The
target on the right shows high repeatability but poor accuracy
That means the reading rate and measurement speed of a highly accurate instrument like an SMU are determined by its NPLC setting. However, an ADC’s reading rate is only one of many factors that affects an SMU instrument’s true speed; other factors that can affect overall throughput include function and range change times, trigger in and out times, settling times, and program execution times.
Issue 2 2012
www.siliconsemiconductor.net 25
Figure
6.ADC integration time comparison (NPLC)
Figure
5.Adjusting an SMU instrument’s resolution
digits
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