REV. A
AD73311L
–10–
F
B
= 4kHz
FS
INIT
= DMCLK/8
a. Analog Antialias Filter Transfer Function
F
B
= 4kHz
FS
INIT
= DMCLK/8
SIGNAL TRANSFER FUNCTION
NOISE TRANSFER FUNCTION
b. Analog Sigma-Delta Modulator Transfer Function
F
B
= 4kHz FS
INTER
= DMCLK/256
c. Digital Decimator Transfer Function
F
B
= 4kHz FS
FINAL
= 8kHz
FS
INTER
= DMCLK/256
d. Final Filter LPF (HPF) Transfer Function
Figure 7. AD73311L ADC Frequency Responses
Decimation Filter
The digital
fi
lter used in the AD73311L carries out two impor-
tant functions. Firstly, it removes the out-of-band quantization
noise, which is shaped by the analog modulator and secondly, it
decimates the high frequency bitstream to a lower rate 15-bit word.
The antialiasing decimation
fi
lter is a sinc-cubed digital
fi
lter
that reduces the sampling rate from DMCLK/8 at the modula-
tor to an output rate at the SPORT of DMCLK/M (where M
depends on the sample rate setting
—
M = 256 @ 64 kHz; M =
512 @ 32 kHz, M = 1024 @ 16 kHz, M = 2048 @ 8 kHz), and
increases the resolution from a single bit to 15 bits. Its Z trans-
form is given as: [(1
–
Z
–
N
)/(1
–
Z
–
1
)]
3
where N is determined by
the sampling rate (N = 32 @ 64 kHz, N = 64 @ 32 kHz, N =
128 @ 16 kHz, N = 256 @ 8 kHz). This ensures a minimal
group delay of 25
μ
s at the 64 kHz sampling rate.
ADC Coding
The ADC coding scheme is in twos complement format (see
Figure 8). The output words are formed by the decimation
fi
lter, which grows the word length from the single-bit output of
the sigma-delta modulator to a 15-bit word, which is the 16-bit
transfer being used as a flag bit to indicate either control or data
in the frame.
V
INN
V
INP
V
REF
+ (V
REF
0.32875)
V
REF
V
REF
–
(V
REF
0.32875)
10...00
00...00
01...11
ADC CODE DIFFERENTIAL
V
INN
V
INP
V
REF
+ (V
REF
0.6575)
V
REF
V
REF
–
(V
REF
0.6575)
10...00
00...00
01...11
ADC CODE SINGLE-ENDED
ANALOG
INPUT
ANALOG
INPUT
Figure 8. ADC Transfer Function
Decoder Channel
The decoder channel consists of a digital interpolator, digital
sigma-delta modulator, a single bit digital-to-analog converter
(DAC), an analog smoothing
fi
lter and a programmable gain
ampli
fi
er with differential output.
DAC Coding
The DAC coding scheme is in twos complement format with
0x7FFF being full-scale positive and 0x8000 being full-scale
negative.
Interpolation Filter
The anti-imaging interpolation
fi
lter is a sinc-cubed digital
fi
lter
which up-samples the 16-bit input words from the SPORT
input rate of DMCLK/M (where M depends on the sample rate
setting
—
M = 256 @ 64 kHz; M = 512 @ 32 kHz, M = 1024 @
16 kHz, M = 2048 @ 8 kHz), to a rate of DMCLK/8 while
fi
ltering to attenuate images produced by the interpolation pro-
cess. Its Z transform is given as: [(1
–
Z
–
N
)/(1
–
Z
–
1
)]
3
where N is
determined by the sampling rate (N = 32 @ 64 kHz, N = 64 @
32 kHz, N = 128 @ 16 kHz, N = 256 @ 8 kHz). The DAC
receives 16-bit samples from the host DSP processor at a rate of
DMCLK/M. If the host processor fails to write a new value to
the serial port, the existing (previous) data is read again. The
data stream is
fi
ltered by the anti-imaging interpolation
fi
lter,
but there is an option to bypass the interpolator for the mini-
mum group delay con
fi
guration by setting the IBYP bit (CRE:5) of
Control Register E. The interpolation
fi
lter has the same charac-
teristics as the ADC
’
s antialiasing decimation
fi
lter.