AD6652BBC datasheet(42/76 Pages) AD

42 / 76 page

AD6652

Rev. 0 | Page 42 of 76

tput

gisters.

bit growth associated with CIC filters depends on

unts for the division associated with

operation, only coarse scaling is possible. Fine scaling is imple-

secti

14 u

ort control

iste

wher

o 4096).

amples programmed as a

or 4).

athCad-

g up to t

ber.

xample, if a

ation ratio

MCIC is 1

Navg is

ted to be 3 (decimation of 1000 and av

g of 3000

amples), then the actual gain due to averaging and decimation

3000 or 69.54 dB ( = log2(3000)). Because attenuation is

ift operation, only multiples of 6.02 dB

utput from the aver-

appl

ing a

simp

tracted from the request signal level, R, specified in Registers

(0x0B, 0x14), leaving an error term to be processed by the loop

filter, G(z).

rogrammable request signal level, R, according to the

t signal level desired. The request signal level

R is

requ

f any, due

to th

requ

l is offset by the amount of error induced in

Because the number of average samples must be an integer

multiple of the decimation value, only the multiple number 1, 2,

Set this p

outpu

3, or 4 is programmed. This number is programmed in Ou

Port Control Registers 0x10:1–0 and 0x18:1–0. These averaged

samples are then decimated with decimation ratios programm

ble from 1 to 4096. This 12-bit decimation ratio is defined in

Registers 0x11 and 0x19.

The average and decimate operations are linked together and

implemented using a first-order CIC filter and FIFO re

The gain and

the decimation ratio. To compensate for the gain associated

with these operations, attenuation scaling is provided before the

CIC filter.

This scaling operation acco

the veraging operation as well as the traditional bit growth

CI filters. Because this scaling is implemented as a bit shif

me ted as an offset in the request level, explained later in this

on. The attenuation scaling, SCIC, is programmable from 0 to

sing four bits of 0x10 and 0x18 of the output p

reg

rs, and is given by

)]

(

[log

avg

CIC

ceil

MCIC is the decimation ratio (1 t

Navg is the number of averaged s

multiple of dec

ation ratio (1, 2, 3,

Ceil is M

speak for roundin

he next whole

num

For e

decim

000 an

selec

eragin

implemented as a bit sh

attenuations are possible. SCIC, in this case, is 12 correspondin

to 72.24 dB. This way,

SCIC scaling always attenuates more than

sufficient to compensate for the gain changes in average and

decimate sections and, therefore, prevents overflows in the AGC

loop. But it is also evident that the CIC scaling is inducing a

gain error (difference between gain due to CIC and attenuation

provided) of up to 6.02 dB. This error should be compensated

for in the request signal level, as explained below.

Logarithm to the Base 2 is applied to the o

age and decimate section. These decimated power samples (in

log rithmic domain) are converted to rms signal samples by

ying a square root. This square root is implemented us

le shift operation. The rms samples so obtained are su

programmable from 0 to −23.99 dB in steps of 0.094 dB. The

est signal level should also compensate for error, i

e CIC scaling, as explained previously. Therefore, th

est signal leve

given by

)

(

log

−

CIC

avg

CIC

Offset

wh re the offset is in dB.

C ntinuing with the previous example, this offset is given by

Offset = 72.24 − 69.54 = 2.7 dB

So t e request signal level is given by

094

)

(

⎥

⎤

⎢

⎡

−

Offset

DSL

ceil

094

⎦

⎣

wher

DSL

desir

Therefore, in the previous example, if the desired signal level is

−16.

The AGC provides a programmable second-order loop filter.

defin

subt

rocessed by the loop

gain

ratio

The

para

R is the request signal level.

(desired signal level) is the output signal level that the user

es.

−13.8 dB, the request signal level, R, is programmed to be

54 dB.

The programmable parameters, gain K and pole P, completely

e the loop filter characteristics. The error term after

racting the request signal level is p

filter, G(z). The open loop poles of the second-order loop filter

are 1 and

P, respectively. The loop filter parameters, pole P and

K, allow adjustment of the filter time constant, which

determines the window for calculating the peak-to-averag

open loop transfer function for the filter, including the gain

meter is as follows:

)

(

−

− +

−

If the AGC is properly configured (in terms of offset in request

level), then there are no gains except the filter gain K. Under

−

)

(

these circumstances, a closed loop expression for the AGC loop

is possible and is given by

)

(

)

(

)

(

)

(

−

closed

The gain parameter

K and pole P are programmable through

registers (0x0E and 0x0F, respectively, for AGC Channel A and

Channel B) from 0 to 0.996 in steps of 0.0039 using 8-bit

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