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Publication Date: 2008-Jul-23
Document File: 3 page(s) / 161K

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Thermal sensors are included in many integrated circuits to provide circuit temperature information: for example, for thermal shutdown protection system, or temperature monitoring during different operating states or manufacturing steps. In many on chip analog and digital systems, this function is a secondary feature of the circuit and the information is digitized by a generic on chip ADC (analog to Digital Converter) that is not dedicated to it. The proposed solution adapts the temperature dependent voltage generated by a current biased semiconductor diode junction to fit a given input dynamic range of a generic ADC, over a specified temperature range. It optimizes temperature measurement condition by optimizing the ADC quantization step at low cost: very few components are necessary, and it can be easily implemented in standard silicon technology.

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Thermal  sensors  are  included  in  many  integrated  circuits  to  provide  circuit  temperature  information:  for  example,  for  thermal  shutdown  protection  system,  or  temperature  monitoring  during  different  operating  states  or  manufacturing  steps.  In  many  on  chip  analog  and  digital  systems,  this  function  is  a  secondary  feature  of  the  circuit  and  the  information  is  digitized  by  a  generic  on  chip  ADC  (analog  to  Digital  Converter)  that  is  not  dedicated  to  it.  The  proposed  solution  adapts  the  temperature  dependent  voltage  generated  by  a  current  biased  semiconductor  diode  junction  to  fit  a  given  input  dynamic  range  of  a  generic  ADC,  over  a  specified  temperature  range.  It  optimizes  temperature  measurement  condition  by  optimizing  the  ADC  quantization  step  at  low  cost:  very  few  components  are  necessary,  and  it  can  be  easily  implemented  in  standard  silicon  technology.


To  realise  on  chip  temperature  sensor  function,  diode  junction  are  widely  used.  Figure1  describes  some  common  implementation  of  such  device  for  temperature  sensing.  Silicon  diode  junction  based  thermal  sensor  usually  have  a  temperature  to  voltage  conversion  gain  around  -2mV/C  with  a  voltage  offset  around  0.8V  at  25C.  (Or  n*-2mv/C  and  n*0.8V  offset  if  n  diodes  are  stacked).  In  the  following,  we  will  name  offset  (Vbe)  this  non  temperature  dependent  voltage  part.  Thus,  for  a  given  temperature  range,  the  output  voltage  range  is  fixed  and  an  important  amount  of  the  voltage,  the  offset,  does  not  provide  temperature  information.  Moreover,  in  many  systems,  the  voltage  temperature  information  issue  from  the  thermal  sensor  is  digitized  by  an  ADC.  For  best  ADC  performance,  meaning  its  best  ENOB  (effective  number  of  bits)  operation  condition,  an  optimum  input  voltage  dynamic  range  exists  that  is  the  trade  off  between  SNR  (signal  to  noise  ratio)  and  THD  (total  harmonic  distortion).  Thus  to  fit  this  best  ADC  measurement  condition  over  a  given  temperature  range,  the  conversion  gain  of  the  thermal  sensor  needs  to  be  adjusted.  Moreover,  the  best  temperature  measurement  resolution  will  be  obtain  when  the  ADC  input  range  fits  the  only  temperature  dependent  part  of  t...