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AD6622S Folha de dados(PDF) 11 Page - Analog Devices

Nome de Peças AD6622S
Descrição Electrónicos  Four-Channel, 75 MSPS Digital Transmit Signal Processor TSP
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AD6622
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The serial data frame sync output, SDFS, is pulsed high for one
SCLK cycle at the input sample rate. The input sample rate is
determined by the master clock divided by channel interpolation
factor. If the SCLK rate is not an integer multiple of the input
sample rate, the SDFS will continually adjust the period by one
SCLK cycle in order to keep the average SDFS rate equal to the
input sample rate. When the channel is in sleep mode, SDFS is
held low. The first SDFS is delayed by the channel reset latency
after the Channel Reset is removed. The channel reset latency
varies dependent on channel configuration.
The serial data input, SDIN, accepts 32-bit words as channel
input data. The 32-bit word is interpreted as two 16 bit two’s
complement quadrature words, I followed by Q, MSB first.
The first bit is shifted into the serial port starting on the second
rising edge of SCLK after SDFS goes high, as shown by the
timing diagram below.
CLK
SCLK
SDFS
SDI
CLKn
DATAn
tDSCLK
tDSDFS
tDSDFS
tSSI
tHSI
Figure 10. Serial Port Switching Characteristics
As an example of the serial port operation, consider a CLK fre-
quency of 62.208 MSPS and a channel interpolation of 2560.
In that case, the input sample rate is 24.3 kSPS (62.208 MSPS/
2560), which is also the SDFS rate. Substituting, fSCLK
≥ 32 ×
fSDFS into the equation below and solving for SCLKDIVIDER,
we find the maximum value for SCLKDIVIDER according to
Equation 2.
SCLK
f
DIVIDER
SDFS
×
f
CLK
64
1
(2)
Evaluating this equation for our example, SCLKDIVIDER must be
less than or equal to 39. Since the SCLKDIVIDER channel regis-
ter is a 5-bit unsigned number it can only range from 0 to 31.
Any value in that range will be valid for this example, but if it is
important that the SDFS period is constant, then there is another
restriction. For regular frames, the ratio fSCLK/fSDFS must be equal
to an integer of 32 or larger. For this example, constant SDFS
periods can only be achieved with an SCLK divider of 19.
In conclusion, the SDFS rate is determined by the AD6622 master
clock rate and the interpolation rate of the channel. The SDFS
rate is equal to the channel input rate. The channel interpola-
tion is equal to RCF interpolation times CIC5 interpolation,
times CIC2 interpolation
LL
L
L
RCF
CIC
CIC
×
52
(3)
The SCLK rate is determined by the AD6622 master clock
rate and SCLKDIVIDER. The SCLK is a divided version of the
AD6622 master CLK. The SCLK divide ratio is determined by
SCLKDIVIDER as shown in Equation 2. The SCLK must be fast
enough to input 32 bits of data prior to the next SDFS. Extra
SCLKs are ignored by the serial port.
PROGRAMMABLE INTERPOLATING RAM
COEFFICIENT FILTER (RCF)
Each channel has a fully independent RAM Coefficient Filter
(RCF). The RCF accepts data from the serial port, filters it, and
passes the result to the CIC filter. The RCF implements a FIR
filter with optional interpolation. The FIR filter can produce
impulse responses up to 128 output samples long. The FIR
response may be interpolated up to a factor of 128, although
the best filter performance is usually achieved if the RCF inter-
polation factor is confined to 8 or below.
FIR Filter Implementation
The RCF accepts quadrature samples from the serial port with a
fixed point resolution of 16 bits each, for I and Q.
SERIAL
PORT
DATA
MEM
RCF
RCF COARSE
SCALE
COEFFICIENT
MEM
IQ TO
CIC
FILTER
SDFS
SCLK
SDIN
16,16
ACCUMULATOR
16,16
16,16
Figure 11. RCF Block Diagram
The AD6622 RCF realizes a sum-of-products filter using a poly-
phase implementation. This mode is equivalent to an interpola-
tor followed by a FIR filter running at the interpolated rate. In
Figure 12, 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.
LRCF
fIN
a
b
c
fIN
LRCF
NRCF TAP
FIR FILTER
h[n]
fIN
LRCF
Figure 12. RCF Interpolation
The difference equation for Figure 12 is written below, where
h[n] is the RCF impulse response, b[n] is the interpolated input
sample sequence at point “b” in Figure 12, and c[n] is the out-
put sample sequence at point “c” in the Figure 12.
cn
hk
n
b n
k
NRCF
[]
[
]
[]
=−
×
=
0
1
(4)
This difference equation can be described by the transfer func-
tion from point “b” to “c” as shown Equation 5.
Hz
h n
z
bc
n
N
n
RCF
()
[ ]
= ∑
×
=
0
1
(5)
The actual implementation of this filter uses a polyphase
decomposition to skip the multiply-accumulates when b[n] 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 interpo-
lation phase. The process of selecting the coefficients and placing
them into the CMEM is broken into three steps shown below.


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