202 used to conver t the small currents  from the photodet ectors to voltages  have  programmable gain and other   paramet ers that allow  them  to be configured ideally for  nearly any geometry or technology detector . 

The analog- to-digital (A/D) converters  in the ASIC  are a proprietary  design,  and provide both fast  response and improved DC fidelity  for the signals  as compared to conventional  discrete A/D converters.  Since the amplifiers are used primarily  for sine-cosine signals that  are intended to be ratiometrically   compared,  common voltage  references  are used  for all of the A/D converters.  This assures  that the signals   all continue to track over time, tempe rature, and common mode disturbances.  

The ASIC also  includes  the LED pulsing circuitry  which is fully selectable for current  level,  pulse width,   repetition rate, and other  param eters.  Circuitry  is also included  for monitoring and controlling the LED current.   In normal  operation,  an external  transistor  is used to switch  the LED current,  and a small  external  current   sense resist or is also provided  to allow  for constant  current  drive.  The current  drive level is controlled using   internal  D/A devices  to keep  the light levels  at the detectors at  a constant  level to end-of-life. Thus  at  beginning- of-life, the system  is set up for robust  operation under  nominal  conditions,  and the current through   the LED is then varied as required to keep the system  margins  in place  in spite  of variations  in temperature,   radiation,  and ageing of components. This  sort of scheme nearly  doubles  the useable  life of the LED as it  degrades.  Moreover,  since  the LED  is pulsed,  there is a linear  reduction of current -induced degradation of  the LED proportional to  the duty cycle,  allowing  very long on-orbit lifetime.  The ASIC is  procured to a  government - controlled SMD at  V-quality level  which eliminates a lot of  conc erns about quality and reliability.  

Figure  2. ARA Space  nanoSeries Encoder  Readhead  

The new readhead mechanical  design  is shown  in Figure 2. The small readhead  (25 x 50 x 46 mm) contains   the ASIC,  the LED and driver  transistor,  the photod iode detector  with a precision  grating, and  the optic  to  collimate  and fold the light.  The readhead and the code  disk hub can be fabricated from  titanium,  aluminum   or stainless  steel,  depending on the application.  The precision code disk (chrome pattern printed on glass)  is pre -mounted at  the factory  to the hub.    The readhead is powered from the remote elec tronics  through a shielded cable with a MicroD connector.   All of the signaling wires  are using LVDS drivers  and receivers  and are twisted  pairs  for low EMI  susceptibility  and emission.  

This design also requires  a remote electronics  assembly  that can process data from 1 to 4 readheads.  The  small  125 x 125 x 40 mm remote box (not shown)  or 3U size remote card (Figure  3) contains  a powerful  data  processor  implement ed in a rad hard Field  Programmable Gate Array with critical  portions  of the design   implemented as a pipel ined processor.  This processor applies  the mathematical  algorithm s that correct  the  encoder  signals  for optimal  performance.  The details  of the algorithms  are proprietary  to BEI, but the end  result  is easily  demonstrated using  BEI test and diagnostic  equipment.  Research continues   to improve  the