May 3, 2004

System speeds

The 6430 instruction manual gives information about system speeds on page A-5. With Auto zero off, autorange off, filter off, display off, source auto clear off, and binary reading format, at 0.01 NPLC the specs give a reading rate of 2080 rdg/second for "Measure", but only 930 for "Source/Measure Pass/Fail Test".

You can disable autozero using Keithley 6430 System Commands.vi (where you can also do a lot of not so useful things, like changing the beeper frequency). The actually remote command is :SYSTEem:AZERo OFF. It is ON at default.

Every A/D conversion (reading) is calculated from a series of zero, reference, and signal measurements. With auto zero enabled, all three of these measurements are performed for each reading to achieve rated accuracy. With auto-zero disabled, zero and reference are not measured. This increases measurement speed, but zero drift will eventually corrupt accuracy. With auto zero disabled, periodically change measurement speed.
Temperature changes across components within the instrument can cause the reference and zero values for the A/D converter to drift due to thermo-electric effects. Auto zero acts to negate the effects of drift in order to maintain measurement accuracy over time. Without auto zero enabled, measurements can drift and become erroneous.
So, use with caution. More about AZERo in 17-92. More notes, from 17-93,
Auto zero should be disabled with the :SYST:AZER OFF command for maximum source memory sweep speed; otherwise, the cache is of little use. With auto zero enabled, new A/D reference and zero values are taken for every reading and saved into the cache, slowing down sweep operation. However, with auto zero disabled, measurements may drift and become erroneous. To minimize drift when using NPLC caching with auto zero disabled, periodically send :SYST:AZER ONCE to force an immediate auto zero update.

Posted by torque at May 3, 2004 10:38 AM | TrackBack
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