/*

  Program:  fkcmn_to_r.c                         Revision:  21-JULY-2006
            ------------


  Differential Amplifier Low Pass Filter Calculation Program   step #2


  This program ask for you to enter five parameters:

     The cut-off frequency of the filter  "freq"  in kHz.

     The pass-band gain  "K"

     The capacitor value  "C"  in pFd.

     The value "m"  where  R2 = R  and  R3 = mR

     The value "n"  where  C1 = C  and  C2 = nC


  The program then prints out the full setup for the filter, i.e.:

     The filter cut-off frequency  "freq"  in kHz

     The filter pass-band gain  "K"

     The filter quality  "Q"

     The values of  R1, R2, R3  in Ohms

     The values of  C1 and C2  in pfd


  Recall the basic equations


     R1 = R/K     R2 = R     R3 = mR     ( recall  K = R2/R1 )

     C1 = C     C2 = nC

           sqrt(2mn)
     Q = ------------
          1 + m(K+1)

                   1                                     1
     Fcut = ------------------     thus     R = ----------------------
             2pi RC sqrt(2nm)                    2pi Fcut C sqrt(2nm)


  This program is setup to provide the values for differential Low Pass
  filters and it assumes that the gain "K" is always positive.

  Refer to  TI  sloa_054  documentation about fully differential amplifiers.

 */



#include <stdio.h>
#include <math.h>

#define     PI       3.1415926

main ()

{
        float   Fcut  ;       /*  the filter cut-off frequency in kHz     */

        float   K   ;         /*  the pass-band gain of the filter        */

        float   Q   ;         /*  the required Q of the filter pole pair  */

        float   m   ;         /*  the value where  R2 = R  and  R3 = mR   */

        float   n   ;         /*  the value where  C1 = C  and  C2 = nC   */

        float   R   ;         /*  the basic resistor value in Ohms        */

        float   R1, R2, R3  ; /* the resistor values in Ohms              */

        float   C   ;         /*  the basic capacitor value in pFd        */

        float   C1, C2   ;    /*  the capacitor values in pFd             */

        float   temp_1, temp_2, temp_3, temp_4, temp_5    ;



        printf(" \n ");
        printf("Enter the required filter cut-off frequency Fcut in kHz, \n");
        printf("       the required pass-band gain of the filter K,      \n");
        printf("       the desired basic capacitor value C in pFd        \n");
        printf("       m where m is defined as  R2 = R  and  R3 = mR     \n");
        printf("       n where n is defined as  C1 = C  and  C2 = mC     \n");
        printf(" \n ");


        printf("      Enter the filter cut-off frequency \"Fcut\" in kHz : ");
        scanf( "%f", &Fcut )  ;
        printf(" \n ")        ;

        printf("      Enter the filter gain \"K\" : ");
        scanf( "%f", &K )     ;
        printf(" \n ")        ;

        printf("      Enter the basic capacitor value \"C\" in pFd : ");
        scanf( "%f", &C )     ;
        printf(" \n ")        ;

        printf("      Enter \"m\"  where  R2 = R  and  R3 = mR : ");
        scanf( "%f", &m )     ;
        printf(" \n ")        ;

        printf("      Enter \"n\"  where  C1 = C  and  C2 = nC : ");
        scanf( "%f", &n )     ;
        printf(" \n ")        ;



        printf( "\n The filter cut-off frequency is = %7.3f kHz \n",
                Fcut )  ;

        printf( "\n The filter gain \"K\" is = %7.3f \n",
                K )  ;

        printf( "\n The basic capacitor value \"C\" is =  %7.3f pFd \n",
                C )  ;

        printf( "\n The R3 to R ratio \"m\" is =  %7.3f \n",
                m )  ;

        printf( "\n The C2 to C ratio \"n\" is =  %7.3f \n",
                n )  ;




        /*  Calculate the Q of the Filter

                     sqrt(2mn)
                Q = ------------
                     1 + m(K+1)
                                             */

        temp_1 =  sqrt( 2 * m * n )          ;

        temp_2 =  1 + ( m * ( K + 1.0 ) )    ;

             Q =  temp_1 / temp_2            ;


        printf( "\n The Q of the filter is = %7.4f   \n",
               Q ) ;




        /*  Calculate the basic Filter resistor

                             1
                R = ----------------------
                     2pi Fcut C sqrt(2nm)
                                                      */

        temp_2 =  Fcut * 1000.0    ;

        temp_3 =  C * 0.000000000001    ;

        R = 1 / ( 2 * PI * temp_2 * temp_3 * temp_1 )   ;

        printf( "\n The basic resistor value \"R\" is = %7.4f Ohm  \n",
               R ) ;



        /*  Calculate the individual resistors and capacitors

            R1 = R/K     R2 = R     R3 = mR

            C1 = C     C2 = nC
        */


        R1 = R / K  ;
        R2 = R      ;
        R3 = m * R  ;

        C1 = C      ;
        C2 = n * C  ;


        printf( "\n Resistor R1 is = %7.4f Ohm  \n",
               R1 ) ;

        printf( "\n Resistor R2 is = %7.4f Ohm  \n",
               R2 ) ;

        printf( "\n Resistor R3 is = %7.4f Ohm  \n",
               R3 ) ;


        printf( "\n Capacitor C1 is = %7.4f pFd  \n",
               C1 ) ;

        printf( "\n Capacitor C2 is = %7.4f pFd  \n",
               C2 ) ;


}