function a2d,V,Vmin,Vmax,nbit

;-------------------------------------------------------------------------------------
;
; Routine performs an analog to digital converter (ADC) simulation for a matrix
; of pixels within a focal plane array.  
;
; Input:
;   V..........voltages to digitize, scalar or array
;   Vmin.......minimun voltage of ADC
;   Vmin.......minimun voltage of ADC
;   nbit.......number of bits in ADC
;
; Output:
;   counts....voltage converted to counts
;
; Source: Mark Hervig, GATS Inc.
;
;-------------------------------------------------------------------------------------

;- setup

   c_max = 2.^float(nbit) ; max response in counts
   c_min = 0              ; min response in counts
   
   cpv  = (2.^float(nbit)) / (Vmax - Vmin)  ; counts per volt
   
;- digitize V

   counts = round( V * cpv)  ; volts to counts

;- check limits

   k = where(counts gt c_max,n) 
   if (n gt 0) then counts(k) = c_max
     
   k = where(counts lt c_min,n)
   if (n gt 0) then counts(k) = c_min

;- done

   return,counts
   end