measurement application
goes far beyond simply storing and executing a sequence of standard SCPI commands. TSP technology is based on Lua, a powerful BASIC-like scripting language. Functions like “do” loops, variables, If-Then-Else statements, and more are all supported in Lua. Therefore, TSP scripts are just as powerful as traditional test programs residing in PCs but have the advantage of actually being embedded in the instrument to optimize overall test speed. An SMU instrument’s sourcing resolution and output stability are also key to its overall performance. Let’s look at the relationship between source resolution and output stability.
When evaluating the performance of an SMU instrument’s source, it’s important to look beyond the spec sheet and the instrument’s source
readback display. The source’s actual output performance may be very different from its specified resolution or from its displayed value, which may require instrument specifiers to do their own testing to verify it.
Based solely on an SMU instrument’s spec sheet, one might conclude that the SMU instrument with the greatest programming resolution is the most accurate. The programming resolution determines the output’s “fineness” of adjustment. In Figure 8, note that the non-Keithley SMU offers 50 times greater programming resolution than the Model 2400 SourceMeter instrument.
Furthermore, based on the SMU’s “source readback” value displayed on the front panel or over the bus (Figure 9), one might conclude that the SMU showing readback values closest to the programmed values is the most stable and therefore the better choice. In this example, note that the non- Keithley SMU shows 0µV of peak-to-peak variation when sourcing a 10.001V signal, while the Model 2400 shows 30µV.
However, the picture changes dramatically when we measure the actual source output using a separate instrument. To obtain the data in the right-most column of Figure 9, we chose Keithley’s Model 2002 8?-digit digital multimeter to measure the source output of each SMU directly. The Model 2002 is one of the most accurate DMMs available on the market and is used by many calibration labs, which makes it a good choice for high accuracy applications of this type.
Figure
10a.Actual source performance: programming resolution vs. stability for non-Keithley 6?-digit SMU
To view the stability of the source outputs, we made 100 measurements using the Model 2002 at 10 NPLC to ensure maximum accuracy. We observed that the non-Keithley 61
⁄2 -digit SM (Figure 10a)
actually has almost 0.5mV peak-to-peak variation when sourcing a 10.001V signal. This is very different from the 0µV variation its source readback display indicates. In addition, this error is more than 40 times greater than the 10µV programming resolution. The Keithley Model 2400 SourceMeter instrument (Figure 10b) actually has more than 10 times better output stability than the non-Keithley 61
⁄2 -digit SMU (42.9µV vs. 438.7µV).
Figure
10b.Actual source performance: programming resolution vs. stability for Keithley Model 2400 SourceMeter instrument
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www.siliconsemiconductor.net Issue 2 2012
For the non-Keithley SMU, note that the readback voltage is exactly the same as the programmed voltage. However, the actual measured voltage is quite different from the readback voltage or the programmed voltage. The SMU readback indicates the output voltage to be exactly 10.001V; in reality, the output voltage is somewhere between 10.0014V and 10.0018V. This is a significant amount of error
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