AD6620SSLASHPCB datasheet(26/44 Pages) AD

26 / 44 page

AD6620

–26–

REV. A

RCF Output Scale Factor

The scale factor associated with the RCF, SOUT, behaves differ-

ently than the scale factors in the CIC stages. This scalar, at the

RCF output, controls the weight of the 16-bit output data going

to the parallel port or to the serial port when using 16-bit words.

SOUT determines which of the 23 RCF output bits are used

based on the equation below. OLRCF is the 23-bit RCF output

data; POL represents the output port data. POL is rounded to

the 16 bits desired. The weight of the rounding is adjusted by

SOUT. When the serial port is used with 24-bit or 32-bit words,

SOUT is ignored.

POL

round

bits

RCF

OUT

=×

()

( –

)

Another way to consider the effects of the RCF Output Scale

factor is discussed here. If both CIC scalars follow the previous

recommendations, the following chart can be used to determine

what value to use for the RCF scale factor. In order to determine

this, the “gain” of the impulse response must first be determined.

This can be done by integrating the coefficients used for the

RCF filter remembering to normalize the values against the full-

scale input range of 2

19.

There are several possibilities when setting the “gain” of the

RCF coefficients. Following these guidelines will preserve at

least three bits in the sum of products registers.

( ) =

∑

1; 0 dB dc gain in RCF filter. Numeric wraparound

very unlikely. The RCF Scale factor should be nominally

set to 4.

( ) ≤

∑

1; slight loss in RCF filter. Numeric wraparound

is impossible. The RCF Scale factor should be nominally

set to 4.

( ) =

∑

; where the absolute value of m is a number less

than 1 and is scaled to account for losses elsewhere in the

system, such as conversion gain errors, attenuator losses or

CIC scaling errors. The gain should be scaled down to avoid

wraparound in the RCF process, however, then the RCF

Scale Factor can be adjusted up to increase the signal level.

The value of m can also be negative to account for an inver-

sion through an amplifier. The RCF Scale factor should be

set where needed to produce the desired full-scale results

with a fully loaded receiver input signal.

The RCF Scale factor has the effect as shown in the following

table. Each successive gain step doubles or halves the overall

gain of the stage. Overall gain through the RCF stage is the

cascaded gain of the RCF Scale factor shown below and the

RCF coefficient gain discussed previously.

Table VI.

RCF Gain

RCF Scale Factor (Address 309h)

1/8

1/4

1/2

Gain through the RCF of the AD6620 is thus:

Gain

coefficients

RCF

Unique B Operation

Unique B works in conjunction with dual channel mode. In this

mode, both the A and B channels can have different FIR coeffi-

cients. This can prove useful in many applications where each

signal path has known differences. Another option is that FIR

gain for one path could be different than the other. During

diversity selection, one path could be tailored for weak signals

and the other for strong signals, providing extra dynamic range.

To use the Unique B mode, set Bit 3 high in register 309h. This

will cause the internal state machine to use a different set of

coefficients for the B channel than the A. With Bit 3 set low for

normal operation, the FIR coefficient index is incremented only

after both the A and B channels are computed. However when

this bit is set high, the index is incremented after each A channel

and B channel computation. Therefore, filters are computed nor-

mally. When downloaded to the AD6620, they should be inter-

leaved with the A channel terms occupying the even RCF

Coefficient locations and the B channel terms occupying the

odd locations. Both filters must be the same length and fit in the

allocated memory space.

With Unique B set to ‘0,’ the following table illustrates how the

coefficients are distributed.

Table VII.

Coefficient

Address

W(0)

W(1)

W(2)

W(3)

……

With Unique B set to ‘1,’ the following table illustrates how the

coefficients are distributed.

Table VIII.

Coefficient

Address

Wa(0)

Wb(0)

Wa(1)

Wb(1)

……

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