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High Accuracy 24 Bit Low Current Measurement Module



Gen 3 RI 7100A ATE Systems can accept an improved low-current measurement module. These improved modules began being offered in January 2003.

The new module uses 24 bit measurement circuitry and a unique "switched sense real-time calibration" to achieve sub-nanoamp accuracy.

The following graph shows -1V to +1V into a 100 Mohm resistor. Note from the measured current that the resistor (+/-10% tolerance) is off slightly in value. At 1 Volt Stimulus we would expect 10nA and are measureing slightly above 9nA.




This low current measurement is in addition to the parametric measure's previous capabilities and does not compromise any existing features. Test plans written for the existing parametric measure will work fine with the new module.



In the Static Digital's 'Measure' panel, the mode button offers a new selection: Low Current. The Low Current mode is a 'force voltage/measure current' mode. It operates the same as the iMeas mode, except that the 24 bit circuitry and the switched sense real-time calibration are used for the actual measurement. Note that the current gain ranging is still incorporated (controlled by the 'Current Measure Max' and 'Current Limit' buttons). Although the low-current mode will work for any range, the minimum uncertainties are at Current Measure Max and Current Limit settings of 2 ua.

Note that two additional new modes are added: 'vCal' and 'iCal'. These are to assist in calibration/service and should not be used for normal operation.

Although slower than the other modes, the low current mode is still relatively fast. As the following table shows, a single measurement takes about 13 ms with 170 picoamp standard deviation.


Averages
Time*
Std. Dev.
1
13.2 ms
167 pA
10
81 ms
52.3 pA
100
726 ms
16.3 pA
*Time is the total time for a test plan that takes 1 measurement.

The following graph shows peak-to-peak deviation, at 100 pA/div. for 1 average, 10 averages and 100 averages.




In addition to the new hardware, a software patch and new calibration plans are required to implement the low current measurement capability.

A common source of uncertainty is common mode error. Common mode error causes the same current to read differently at different voltages. Following is a graph of current measurement into an open vs. voltage. Note that the measurement error does not change with voltage, indicating the common mode error is very small.



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