AD6654SLASHPCB datasheet(80/88 Pages) AD

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AD6654

Rev. 0 | Page 80 of 88

Table 37. DRCF Multiply Accumulate Scale Bits

DRCF Scale <1:0>

Scale Factor

18.06 dB attenuation (left-shift 3 bits)

12.04 dB attenuation (left-shift 2 bits)

6.02 dB attenuation (left-shift 1 bit)

No scaling (0 dB)

<7:4>: DRCF Decimation Rate. This 4-bit word should be

written with one less than the decimation rate of the DRCF

filter. A value of 0 represents a decimation rate of 1 (no rate

change), and the maximum value of 15 represents a decimation

of 16. Filtering can be implemented regardless of the

decimation rate.

<3:0>: DRCF Decimation Phase Bits. This 4-bit word represents

the decimation phase used by the DRCF filter. The valid range

is 0 up to MDRCF − 1, where MDRCF is the decimation rate of the

DRCF filter. This word is primarily used for synchronization of

multiple channels of the AD6654, when more than one channel

is used for filtering one signal (one carrier).

DRCF Coefficient Offset <7:0>

This register is used to specify which section of the 64-word

coefficient memory is used for a filter. It can be used to select

between multiple filters that are loaded into memory and

referenced by this pointer. This register is shadowed, and the

filter pointer is updated every time a new filter is started. This

allows the coefficient offset to be written even while a filter is

being computed without disturbing operation. The next sample

comes out of the DRCF with the new filter.

DRCF Taps <6:0>

This register is written with one less than the number of taps

that are calculated by the DRCF filter. The filter length is given

by the decimal value of this register plus 1. A value of 0

represents a 1-tap filter, and a value of 0x28 (40 decimal)

represents a 41-tap filter.

DRCF Start Address <5:0>

This register is written with the starting address of the DRCF

coefficient memory to be updated.

DRCF Final Address <5:0>

This register is written with the ending address of the DRCF

coefficient memory to be updated.

DRCF Coefficient Memory <13:0>

DRCF Memory. This memory consists of 64 words, and each

word is 14 bits wide. The data written to this memory space is

expected to be 14-bit, twos complement format. See the

Decimating RAM Coefficient Filter (DRCF) section for the

method to program the coefficients into the coefficient

memory.

CRCF Control Register <11:0>

<11>: CRCF Bypass Bit. When this bit is set, the DRCF filter is

bypassed and, therefore, its output is the same as its input.

When this bit is cleared, the CRCF has normal operation as

programmed by its control register.

<10>: Symmetry Bit. When this bit is set, it indicates that the

CRCF is implementing a symmetrical filter and only half the

impulse response needs to be written into the CRCF coefficient

RAM. When this bit is cleared, the filter is asymmetrical and

the complete impulse response of the filter should be written

into the coefficient RAM. When this filter is symmetrical, it can

implement up to 128 filter taps.

<9:8>: CRCF Multiply Accumulate Scale Bits. The output of the

CRCF filter is scaled according to the value of these bits.

Table 38 lists the attenuation corresponding to each setting.

Table 38. CRCF Multiply Accumulate Scale Bits

CRCF Scale<1:0>

Scale Factor

18.06 dB attenuation (left-shift 3 bits)

12.04 dB attenuation (left-shift 2 bits)

6.02 dB attenuation (left-shift 1 bit)

No scaling (0 dB)

<7:4>: CRCF Decimation Rate. This 4-bit word should be writ-

ten with one less than the decimation rate of the CRCF filter. A

value of 0 represents a decimation rate of 1 (no rate change) and

the maximum value of 15 represents a decimation of 16.

Filtering operation is done regardless of the decimation rate.

<3:0>: CRCF Decimation Phase. This 4-bit word represents the

decimation phase used by the CRCF filter. The valid range is 0

to MCRCF − 1, where MCRCF is the decimation rate of the

CRCF filter. This word is primarily used for synchronization of

multiple channels of the AD6654, when more than one channel

is used for filtering one signal (one carrier).

CRCF Coefficient Offset <5:0>

This register is used to specify which section of the 64-word

coefficient memory is used for a filter. It can be used to select

between multiple filters that are loaded into memory and

referenced by this pointer. This register is shadowed, and the

filter pointer is updated every time a new filter is started. This

allows the coefficient offset to be written even while a filter is

being computed without disturbing operation. The next sample

comes out of the CRCF with the new filter.

CRCF Taps <6:0>

This register is written with one less than the number of taps

that are calculated by the CRCF filter. The filter length is given

by the decimal value of this register plus 1. A value of 0 repre-

sents a 1-tap filter, and a value of 0x28 (40 decimal) represents a

41-tap filter.

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