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How do you ensure quality resistance measurements?

07 Oct 2015  | Martin Rowe

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Now that you see how Keithley calibrates the multi-function calibrators, we look at how they trust the values of the working resistance standards. Again, they could send the working standards to Fluke for calibration, but that also requires shipping, which carries risk of drifting caused by temperature changes, shock and vibration—no matter how well the resistance standards are packed.

Looking back on Figure 1, you can see that there are two possible methods for calibrating the working resistors, depending on the working resistor's value. Bridge configurations let Keithley compare the value of the working resistance standards against two reference resistors with values of 10kΩ and 1GΩ. Figure 5 shows four basic methods used, which cover some 15 decades of values from 0.01Ω to 10TΩ.

Calibrating working resistors

Figure 5: Keithley uses different methods for calibrating working resistors against reference resistors, depending on the resistor values.

Figure 6 shows the test setup, which uses resistance bridges to transfer values for the reference resistors to the working resistors for comparison. The instruments are used to measure voltage across the reference resistors and working resistors under test varies with setup. This forms a ratiometric bridge to enable the transfer of decade values from the NIST reference standard to higher value resistances of transfer standards.

Resistance bridges

Figure 6: This rack contains resistance bridges to transfer resistance values from reference resistors to the working resistors shown in Figure 3.

For resistances from 10MΩ to 10TΩ, Keithley uses what it calls an HRTS (High-Resistance Transfer System). This system consists of an accurate decade divider, voltage source and an electrometer that acts as a null detector. As in the case above, this forms a ratiometric bridge to enable the transfer of decade values from the NIST reference standard to higher value resistances of transfer standards.

One the next page, you'll see the resistors at the top of Keithley's resistance chain.


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