AD6650SLASHPCB datasheet(20/45 Pages) AD

20 / 45 page

AD6650

Rev. A | Page 19 of 44

RCF Filter Length

The maximum number of taps this filter can calculate, Ntaps, is

given by Equation 10. The value Ntaps − 1 is written to the

channel register within the AD6650 at Address 0x1B.

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

≤

min

SAMPIIR

RCF

CLK

taps

(10)

where:

fCLK is the external frequency oscillator.

MRCF is the RCF filter decimation rate.

fSAMPIIR is the input rate to the RCF.

The RCF coefficients are located in Address 0x40 to Address 0x6F,

and are interpreted as 20-bit twos complement numbers. When

writing the coefficient RAM, the lower addresses are multiplied

by relatively older data from the IIR, and the higher coefficient

addresses are multiplied by relatively newer data from the IIR.

The coefficients need not be symmetric, and the coefficient

length, Ntaps, can be even or odd. If the coefficients are

symmetric, both sides of the impulse response must be written

into the coefficient RAM.

The RCF stores the data from the IIR into a 46 × 48 RAM. A

RAM of 23 × 48 is assigned to I data, and a RAM of 23 × 48 is

assigned to Q data.

When the RCF is triggered to calculate a filter output, it starts

by multiplying the oldest value in the data RAM by the first

coefficient, which is pointed to by the RCF coefficient offset

products of newer data-words multiplied by the subsequent

locations in the coefficient RAM until the coefficient address

RCFOFF + Ntaps − 1 is reached.

Table 11. Three-Tap Filter

Coefficient Address

Impulse Response

Data

h(0)

N(0) oldest

h(1)

N(1)

2 = (Ntaps − 1)

h(2)

N(2) newest

The RCF coefficient offset register can be used for two purposes.

The main purpose is to allow multiple filters to be loaded into

memory and selected simply by changing the offset. The other

is to contribute to the symbol timing adjustment. If the desired

filter length is padded with 0s on the ends, the starting point

can be adjusted to form slight delays in the time the filter is

computed with reference to the high speed clock. This allows

for vernier adjustment of the symbol timing. Coarse adjustments

can be made with the RCF decimation phase.

The output rate of this filter (fSAMPR) is determined by the output

rate of the IIR stage and MRCF.

RCF

SAMPIIR

SAMPR

(11)

where:

fSAMPIIR is the input rate to the RCF.

MRCF is the RCF filter decimation rate.

RCF Output Scale Factor and Control Register

Address 0x1C is used to configure the scale factor for the RCF

filter. This 2-bit register is used to scale the output data in 6 dB

increments. The possible output scales range from 0 dB to −18 dB.

The AD6650 RCF uses a recirculating multiply accumulator

(MAC) to compute the filter. This accumulator has three bits of

growth, allowing the output of the accumulator to be up to eight

times as large as the input signal. To achieve the best filter

performance, the coefficients should be as large as possible

without overflowing the accumulator. The gain of a filter is

merely the sum of the coefficients; therefore, for normal steady

state signals, the sum of the coefficients must be less than 8. If

the sum of the coefficients is 8 or slightly less, very rare

transient events can overflow the accumulator. To prevent this,

the sum of the absolute values of the coefficients should be less

than 8. It is then impossible for the RCF filter to overflow.

The RCF filter has a 4-position mux at the output of the

accumulator. This mux chooses which 24 bits are propagated to

the output and adjusts the rounding appropriately. This can be

viewed as a gain block that can be varied in 6 dB steps and is

controlled by the 2-bit RCF scale register.

The resulting gain of the RCF (RCFgain) is then represented by

the following equation:

RCFScale

Coef

RCFgain

−

∑

(12)

where RCFScale is the value in the RCF scale register.

COMPOSITE FILTER

The total gain of the digital filters can be calculated with

Equation 13 and must be less than or equal to 1 (0 dB).

Typically, the RCF coefficient gain is scaled to compensate for

the gain of the CIC and IIR, and the RCF scale factor is set to 3.

⎟

⎠

⎞

⎜

⎝

⎛

∑

−

RCFScale

CIC

Coef

Gain

CIC

(13)

where:

Gain is the gain of the digital filters.

MCIC4 is the CIC4 decimation ratio.

SCIC4 is the CIC4 scale factor.

RCFScale is the value in the RCF scale register.

The individual responses of the CIC4 and IIR filters, along with

the composite response of all the filters, are shown in Figure 26.

AD6650/PCB 数据表(PDF) 20 Page - Analog Devices

AD6650/PCB 数据表(HTML) 20 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

数据表下载

链接网址

AD6650/PCB 数据表(PDF) 20 Page - Analog Devices

AD6650/PCB 数据表(HTML) 20 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

数据表 下载

链接网址

数据表下载