
                        SUBPROGRAM RP

This program helps to apply Maxim's resistor programmed filter
products (MAX265/266). Program input is given in response to prompts
relating to :

         Filter shape ( lowpass, highpass etc. )
         Center frequency or corner frequency
         Frequency of zero
         Q
         Mode

There are several filter operating modes: Mode 1, 2, 3, 3A etc.These determine
the way the external resistors are used to perform the filter functions.
The MAX265/266 data sheet can be consulted for help in choosing the
appropriate mode.

The program calculates a set of resistors; however the set is not
unique. Other values would work as well provided the ratios between them
are preserved. Note however that the filter output swing may be limited for
resistive loads below 10Kohm while resistor values excessively large may
result in poor noise performance.
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                   CLOCK / CENTER FREQUENCY RATIO

This ratio is set digitally in the MAX265/266. The available ratios in these
two parts are:

           MAX265       Approximately 100-200
           MAX266       Approximately 41-140

Depending on the filter operating mode, these ranges may be extended with
the external resistors and the information describing modes should be
consulted.
***
                         ERROR CORRECTION

In all switched capacitor circuits, sampling errors cause the actual
center frequencies and Q's to differ from the ideal values given by design
texts or tables. Such errors are largest with low selected Q's and low
clock/center frequency ratios. However they are a predictable function of
Q and clock/center frequency ratio.

This program is unique in that it takes these sampling errors into account.
Just as a marksman aims high when using a rifle which he knows tends to
shoot low, the program RP pre-compensates the output data so that the final
result is on target. The corrections will be evident in the printed output
when compared with "ideal" values from other design sources which do not
assume a sampled system.
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                     GAIN OF NOTCH FILTERS IN MODE 3A

When the program requires information to set the gain of notch filters it
asks questions relating to:

    The frequency at which the gain is to be set ( DC or high frequency )
    The gain which is to be given priority ( low freq, high freq, center freq )

Without further complicating the problem a simple rule to follow in answering
these questions is to set gain at zero Hertz or DC and to set the low frequency
gain as the priority. If this is done for ALL the second order sections of a
multi-section filter then the correct overall gain will be achieved. ( An
equally correct solution results from setting the gain at half the clock
frequency and giving the high frequency gain priority, provided this is done
for all stages. )
***
                     GAIN OF BANDPASS FILTERS

When a bandpass filter composed of two or more second order sections is
designed, the gain of the overall filter at the design center frequency needs
careful consideration and these programs will provide aid in arriving at the
proper gain.

Suppose a bandpass filter with a center frequency of 1000 Hz is made with two
second order sections with the following characteristics:

       Section 1: center frequeny = 771 Hz, gain at 771 Hz = 1.0 V/V
       Section 2: center frequency=1296 Hz, gain at 1296 Hz = 2.0 V/V

The assumption that the overall filter has a gain of 2, ( that is 2 times 1 )
is incorrect since the overall gain is measured at 1000 Hz. A corrective factor
is needed to calculate the proper gain at 1000 Hz and it is given in the output
of program PZ when the specifications of the individual sections are
calculated. This factor is called the GAF or "Gain adjustment factor".
Suppose that PZ gave a value for the GAF of 2.92. Then the proper value for
the gain of the above system would be:

      Gain at 1000 Hz = 1.0 * 2.0 / 2.92  =  0.685 V/V
***
                           PROGRAM OUTPUT

After all the data has been entered the program calculates the resistors
and the results appear on the screen. There is an opportunity to print the
data if a printer is attached. The same data is automatically sent to disk
files:

        First design in file RP1.R
        Second design in file RP2.R etc.

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