AD6623SSLASHPCB datasheet(18/40 Pages) AD

18 / 40 page

REV. 0

AD6623

–18–

–2x–1 to 2x–1 – 2–y in 2–y steps. The range limits are tabulated in

Table IV for each bus. The hexadecimal values are bit-exact and

each MSB has negative weight. Note that the Product bus range is

limited by result of the multiplication and the two most significant

bits are the same except in one case.

DMEM

32 16

CMEM

256 16

INPUT

ACCUMULATOR

PRODUCT

4.18

1.15

INPUT

1.15

COEF

1.15

2.18

1.17

OUTPUT

20, 2–1, 2–2, OR 2–3

Figure 19. Interpolating FIR Filter Block Diagram

The RCF realizes a FIR filter with optional interpolation. The FIR

filter can produce impulse responses up to 256 output samples

long. The FIR response may be interpolated up to a factor of 256,

although the best filter performance is usually achieved when the

RCF interpolation factor (LRCF) is confined to eight or below. The

256

16 coefficient memory (CMEM) can be divided among an

arbitrary number of filters, one of which is selected by the Coef-

ficient Offset Pointer (channel address 0x0B). The polyphase

implementation is an efficient equivalent to an integer up-sampler

followed FIR filter running at the interpolated rate.

The AD6623 RCF realizes a sum-of-products filter using a polyphase

implementation. This mode is equivalent to an interpolator followed

by a FIR filter running at the interpolated rate. In the functional

diagram below, the interpolating block increases the rate by the RCF

interpolation factor (LRCF) by inserting LRCF–1 zero valued samples

between every input sample. The next block is a filter with a finite

impulse response length (NRCF) and an impulse response of h[n],

where n is an integer from 0 to NRCF–1.

The difference equation for Figure 20 is written below, where h[n]

is the RCF impulse response, b[n] is the interpolated input sample

sequence at point ‘b’ in the diagram above, and c[n] is the output

sample sequence at point ‘c’ in Figure 20.

NRCFTAP

FIR FILTER

h[n]

LRCF

fIN

LRCF

fIN

LRCF

Figure 20. RCF Interpolation

h n

b n

N RCF

[]=

[]

∑

[]

–

(5)

This difference equation can be described by the transfer function

from point ‘b’ to ‘c’ as:

h n

N RCF

()

–

[]

∑

= 0

(6)

The actual implementation of this filter uses a polyphase decom-

position to skip the multiply-accumulates when b[n–k] is zero.

Compared to the diagram above, this implementation has the benefits

of reducing by a factor of LRCF both the time needed to calculate

an output and the required data memory (DMEM). The price of

these benefits is that the user must place the coefficients into the coefficient

memory (CMEM) indexed by the interpolation phase. The process of

selecting the coefficients and placing them into the CMEM is broken

into three steps shown below.

The FIR accepts two’s complement I and Q samples from the serial

port with a fixed-point resolution of 16 bits each. When the serial port

provides data with less precision, the LSBs are padded with zeroes.

The Data-Mem stores the most recent 16 I and Q pairs for a total

of 32 words. The size of the Data-Mem limits the RCF impulse

response to 16

LRCF output samples. When the data words from

the Serial Port have fewer than 16 bits, the LSBs are padded with

zeroes. The Data-Mem can be accessed through the Microport

from 0x20 to 0x5F above the processing channel’s base internal

address, while the channel’s Prog bit is set (external address 4).

In order to avoid start-up transients, the Data-Mem should be

cleared before operation. The Prog bit must then be reset to

enable normal operation.

The Coef-Mem stores up to 256 16-bit filter coefficients. The Coef-

Mem can be accessed through the Microport from 0x800 to 0x8FF

above the processing channel’s base internal address, while the channel’s

Prog bit is set (external address 4). For AD6622 compatibility, the lower

128 words are also mirrored from 0x080 to 0x0FF above the processing

channel’s base internal address, while the Prog bit is set. To avoid

start-up transients, the Data-Mem should be cleared before operation.

The Prog bit must then be reset to enable channel operation.

There is a single Multiply-Accumulator (MAC) on which both the

I and Q operations must be interleaved. Two CLK cycles are required

for the MAC to multiply each coefficient by an I and Q pair. The

MAC is also used for four additional CLK cycles if the All-pass

Phase Equalizer is active.

The size of the Data-Mem and Coef-Mem combined with the

speed of the MAC determine the total number of the taps per

phase (TRCF) that may be calculated. TRCF is the number of

RCF input samples that influence each RCF output sample.

The maximum available TRCF is calculated by the equation below.

least of

floor

APE

RCF

CLK

SDO

≤

























256

–

(7)

The impulse response length at the output of the RCF is determined

by the product of the number of interfering input samples (TRCF)

and the RCF interpolation factor (LRCF), as shown by equation

(8) below. The values of NRCF and TRCF are programmed into control

registers. LRCF is not a control register, but NRCF and TRCF must

be set so that LRCF is an integer. If the integer interpolation by

the RCF results in an inconvenient sample rate at the output of

the RCF, the desired output rate can usually be achieved by

selecting non-integer interpolation in the resampling CIC

2 filter.

RCF

=×

(8)

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