But occasionally we will encounter a customer application where the analog-interfaced nanopositioning system isn't providing the resolution the customer expects. Most often, this is due to a simple issue: a mismatch between the voltage range of the customer's digital-to-analog converter (DAC) and the voltage range of the position-command input on the nanopositioner.
Consider the case where a 16-bit DAC (such as is common on multifunction cards installed in the customer's computer) offers a -10 to +10V range, but the input to the nanopositioner has a 0 to 10V range. The card's "16-bit"ness means that its 20V range is spread over 216 steps... that's 65,536 steps. So the voltage resolution is 20 ÷ 65536 = 0.3mV. If the card were set to provide a range of 0 to 10V, then its resolution would be 10 ÷ 65536 = 0.15mV ...in other words, the resolution would be improved by a factor of two. The mismatch means an entire bit of resolution is lost!
Many (but unfortunately not all) multifunction analog I/O cards offer a configuration option (accessible, for example, via National Instruments' Measurement and Automation Explorer utility, NI-MAX, or via NI-DAQmx subVIs in LabVIEW) for setting the analog range of the DAC. Certainly, any application benefits from matching the analog ranges as closely as possible. For those occasions when it is not supported by the hardware, consider HyperBit as a way of recovering that lost bit... and many more.
Read these papers for the latest on high-througput digital interfacing.
- Interfacing Fast Nanopositioners to Track-Following Servos
- High-speed, low latency communications for nanopositioning in Single-Molecule Biophysics
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