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Quiet Digital To Analog Converter System

IP.com Disclosure Number: IPCOM000048612D
Original Publication Date: 1982-Feb-01
Included in the Prior Art Database: 2005-Feb-08
Document File: 3 page(s) / 31K

Publishing Venue

IBM

Related People

Davis, DE: AUTHOR [+2]

Abstract

The circuit shown in the figure provides for the transfer of asynchronous data (bit 0, bit 1...bit N) to a digital to analog converter (DAC) in a way so that the DAC is protected from spurious data that occurs during the switching of the input data. Because the DAC is protected, it sees only monotonic data and. therefore, its output does not have to recover from spurious data.

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Quiet Digital To Analog Converter System

The circuit shown in the figure provides for the transfer of asynchronous data (bit 0, bit 1...bit N) to a digital to analog converter (DAC) in a way so that the DAC is protected from spurious data that occurs during the switching of the input data. Because the DAC is protected, it sees only monotonic data and. therefore, its output does not have to recover from spurious data.

The circuit of the figure is typically employed in electron-beam lithography tools. Bits 0, 1,...N are derived from, for example, a laser interferometer and represent positional information from the X-Y stage. The DAC output drives a magnetic deflection circuit that deflects the electron beam in such a manner as to follow the positional information and cause the electron beam to remain stationary with respect to the X-Y stage. The deflection typically occurs while the electron beam is writing patterns, and it is necessary that the DAC output be free of spurious pulses that would deflect the beam and cause positional errors in the patterns being written.

In operation, the digital position data (bits 0, 1,...N) changes at a variable rate (depending on the speed of the X-Y stage) and is asynchronous. The positional data is monotonic, and the least significant bit (LSB) changes each time the data changes. The skew in the data between bits may be as much as 400-500 ns.

When the data changes, the LSB changes, and this transition produces a pulse from the output of pulse generator 1, made up of delay device 3 and exclusive-OR gate 5. Either a positive-going or negative-going transition produces a positive-going pulse at the output of the exclusive-OR gate. Single- shot multivibrators 7 and 9 are such as to be triggered by the positive edge of the output pulses from 0R gate 5 and are retriggerable. Single-shot reduces a negative pulse of, for example, 500 ns width at its output from a single positive- going input pulse. If another positive going pulse occurs before the, for example, 500 ns time has elapsed, single-shot 7 will retrigger and extend its output pulse by another 500 ns. Single-shot 9 operates in the same manner except that it is...