AD6652BC/PCB Analog Devices Inc, AD6652BC/PCB Datasheet - Page 42

• Manufacturer Part Number: AD6652BC/PCB
• Description: BOARD EVAL W/AD6652 & SOFTWARE
• Manufacturer: Analog Devices Inc

Specifications of AD6652BC/PCB

• Module/board Type: Evaluation Board
• For Use With/related Products: AD6652
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

AD6652
Because the number of average samples must be an integer
multiple of the decimation value, only the multiple number 1, 2,
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
operation, only coarse scaling is possible. Fine scaling is imple-
me ted as an offset in the request level, explained later in this
secti
14 u
reg
wher
M
N
multiple of dec
Ceil is M
num
For e
selec
s
is
implemented as a bit sh
attenuations are possible. S
to 72.24 dB. This way, S
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
appl
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) .
amples), then the actual gain due to averaging and decimation
avg
CIC
3000 or 69.54 dB ( = log
C
n
iste
a
a
ying a square root. This square root is implemented us
sing four bits of 0x10 and 0x18 of the output p
is the number of averaged s
on. The attenuation scaling, S
ber.
ted to be 3 (decimation of 1000 and av
le shift operation. The rms samples so obtained are su
e:
is the decimation ratio (1 t
xample, if a
S
rs, and is given by
CIC
athCad-
=
ceil
bit growth associated with CIC filters depends on
im
[log
speak for roundin
decim
ation ratio (1, 2, 3,
2
(
M
ation ratio M
CIC
ift operation, only multiples of 6.02 dB
CIC
2
scaling always attenuates more than
CIC
(3000)). Because attenuation is
unts for the division associated with
×
, in this case, is 12 correspondin
N
o 4096).
amples programmed as a
avg
CIC
g up to t
)]
, is programmable from 0 to
or 4).
CIC
is 1
utput from the aver-
000 an
he next whole
eragin
d N
g of 3000
ort control
avg
gisters.
is
tput
ing a
b-
t
in
Rev. 0 | Page 42 of 76
g
a-
is
Set this p
outpu
programmable from 0 to −23.99 dB in steps of 0.094 dB. The
requ
to th
requ
CI
wh re the offset is in dB.
C ntinuing with the previous example, this offset is given by
So t e request signal level is given by
wher
R is the request signal level.
DSL
desir
Therefore, in the previous example, if the desired signal level is
−13.8 dB, the request signal level, R , is programmed to be
−16.
The AGC provides a programmable second-order loop filter.
The programmable parameters, gain K and pole P , completely
defin
subt
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
gain
determines the window for calculating the peak-to-averag
ratio
The
para
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
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
o
C,
e
h
racting the request signal level is p
54 dB.
meter is as follows:
est signal level should also compensate for error, i
est signal leve
open loop transfer function for the filter, including the gain
e CIC scaling, as explained previously. Therefore, th
Offset = 72.24 − 69.54 = 2.7 dB
.
(desired signal level) is the output signal level that the user
es.
K , allow adjustment of the filter time constant, which
e:
given by
Offset
R
e the loop filter characteristics. The error term after
G
G
t signal level desired. The request signal level R is
(
closed
=
z
)
rogrammable request signal level, R , according to the
ceil
=
(
=
z
1
)
20
−
⎡
⎢
⎣
=
(
1 (
DSL
1
×
+
+
G
l is offset by the amount of error induced in
log
P
G
(
Kz
. 0
z
)
−
(
094
z
)
z
10
1 −
−
)
Offset
(
1
=
M
+
1
Pz
CIC
+
)
(
−
⎤
⎥
⎦
×
K
2
×
N
−
avg
. 0
1
−
094
Kz
)
P
−
rocessed by the loop
−
)
S
1
z
CIC
−
1
+
×
Pz
. 6
02
−
2
f any, due
e
e