Looking at filtering the MPX signal

The MPX filter details

A 6th order Bessel filter is used to remove the sampling harmonics. The specification is 4bd gain, 0.1db down at 53KHz, greater tha 27db loss at 768KHz. A Bessel filter is used for flat group delay up to the 53KHz maximum for the MPX signal. Above this frequency neither the gain nor group delay are of much importance. The specififcation is a trade off between the having the minimum of gain droop and group delay variation up to 53KHz and the need for attenuation of the sampling harmonics. The need for harmonic rejection is reduced by the high reconstruction rate of the DAC.

Since it impossible to achieve a droopless filter the gain droop has to be corrected by the interpolation filter in the DSP along with correction of the droop caused by the DAC sample re-construction. (See data sheet for DAC)

The gain is determined by the pk.pk output level required. This is nominally 3.5vpk.pk. The maximum output of the DAC is 5v pk.pk, however we have decided a 6db marging is required before clipping, so the real nominal level is 2.5v pk.pk. We allow for an output of 4v pk.pk which is trimmed down to 3.5v pk.pk, so a gain of 4db is that needed.

Using rail to rail on I/O amplifiers a mere ± 5v supply would be sufficient. However to ensure minimal distorsion and maintain the op amp operation mid range a ± 15v supply is used., but this is of personal choice. If non-rail to rail amplifiers are used eg: OPA2227 types then there is a "closeness to supply" operhead to be taken into account.

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