

                      SUBPROGRAM MPP

This program helps apply Maxim's uP and pin-programmable filter
products (MAX260-MAX264). PZ can be used to first define a filter function
and to separate it into its second- and first-order components. MPP will
then implement the sections either one at a time or in pairs. For filters
requiring several pole pairs the program can be run repeatedly.

Responding to prompts, the user enters the center frequency, Q and either the
clock frequency or clock/center frequency ratio.

Available modes are 1, 2, 3, 3A and 4.

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Allowed parameter ranges are roughly as follows:

For Mode 1,3 and 4:     Q = .45 to 90

For Mode 2:             Q = .63 to 127

For Mode 1,3 and 4: clock ratio = 100-200 ( MAX260, MAX261, MAX263 )
                                = 41-140  ( MAX262, MAX264 )
For Mode 2:         clock ratio = 71-141  ( MAX260, MAX261, MAX263 )
                                = 28-100  ( MAX262, MAX264 )
***
                         ERROR CORRECTION

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

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 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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                     ERRORS RELATING TO QUANTIZATION

When clock frequency is specified and the program selects the clock ratio
there may be an error in final center frequency due to the 6-bit resolution
of the clock ratio. This error can be taken out by a small change in the clock
frequency if that is possible.

When the user defines the clock ratio, then the program will find the
closest quantized value and calculate a clock frequency, which will result
in zero error in center frequency.
***

                      POWER-DOWN FUNCTION


The MAX 260/261/262 parts are microprocessor programmable and the A and B sides
can have independent digital settings for Q and clock ratio. In the case of
the pin programmable MAX 263/264 the Q and clock ratio of the two
sections must be the same. The special code 0000000 for Q is used for a
power-down function in all of these parts, so special care must be taken
in not using it to set Q. In the case of microprocessor parts MAX 260/261/262
this restriction applies only to side A, where the power-down function takes
place. In side B the 0000000 code does result in a valid Q value.
***
                     GAIN OF NOTCH FILTERS IN MODE 3A

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

    The frequency at which the gain is to be set ( DC or high frequency )

Without further complicating the problem a simple rule to follow in answering
this question is to set gain at zero Hertz or DC. If this is done for ALL
the second order sections of a multi-section filter then the correct overall
gain will be achieved.
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                     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 in 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 would have a gain of 2, ( that is
2 times 1 ) would be incorrect since the overall gain is to be measured at
1000 Hz. There is a corrective factor 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

The output of the program is sent to the screen and an opportunity is given
to send it to a printer if one is available. The results are automatically
sent to files:

           First design: file MPP1.R
           Second design: file MPP2.R, etc. up to 9

***