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AD73311L Datasheet PDF : 36 Pages
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AD73311L
FUNCTIONAL DESCRIPTION
Encoder Channel
The encoder channel consists of an input configuration block, a
switched capacitor PGA and a sigma-delta analog-to-digital
converter (ADC). An on-board digital filter, which forms part
of the sigma-delta ADC, also performs critical system-level
filtering. Due to the high level of oversampling, the input anti-
alias requirements are reduced such that a simple single pole
RC stage is sufficient to give adequate attenuation in the band
of interest.
Input Configuration Block
The input configuration block consists of a multiplexing arrange-
ment that allows selection of various input configurations. This
includes ADC input selection from either the VINP, VINN pins
or from the DAC output via the Analog Loop-Back (ALB)
arrangement. Differential inputs can be inverted and it is also
possible to use the device in single-ended mode, which allows
the option of using the VINP, VINN pins as two separate
single-ended inputs, either of which can be selected under
software control.
Programmable Gain Amplifier
The encoder section’s analog front end comprises a switched
capacitor PGA that also forms part of the sigma-delta modulator.
The SC sampling frequency is DMCLK/8. The PGA, whose
programmable gain settings are shown in Table III, may be
used to increase the signal level applied to the ADC from low
output sources such as microphones, and can be used to avoid
placing external amplifiers in the circuit. The input signal level
to the sigma-delta modulator should not exceed the maximum
input voltage permitted.
The PGA gain is set by bits IGS0, IGS1 and IGS2 (CRD:0–2)
in Control Register D.
Table III. PGA Settings for the Encoder Channel
IGS2
0
0
0
0
1
1
1
1
IGS1
0
0
1
1
0
0
1
1
IGS0
0
1
0
1
0
1
0
1
Gain (dB)
0
6
12
18
20
26
32
38
ADC
The ADC consists of an analog sigma-delta modulator and a
digital antialiasing decimation filter. The sigma-delta modu-
lator noise-shapes the signal and produces 1-bit samples at a
DMCLK/8 rate. This bitstream, representing the analog input
signal, is input to the antialiasing decimation filter. The decima-
tion filter reduces the sample rate and increases the resolution.
Analog Sigma-Delta Modulator
The AD73311L input channel employs a sigma-delta conver-
sion technique, which provides a high resolution 16-bit output
with system filtering being implemented on-chip.
Sigma-delta converters employ a technique known as over-
sampling, where the sampling rate is many times the highest
frequency of interest. In the case of the AD73311L, the initial
sampling rate of the sigma-delta modulator is DMCLK/8. The
main effect of oversampling is that the quantization noise is
spread over a very wide bandwidth, up to FS/2 = DMCLK/16
(Figure 6a). This means that the noise in the band of interest is
much reduced. Another complementary feature of sigma-delta
converters is the use of a technique called noise-shaping. This
technique has the effect of pushing the noise from the band of
interest to an out-of-band position (Figure 6b). The combi-
nation of these techniques, followed by the application of a
digital filter, reduces the noise in band sufficiently to ensure
good dynamic performance from the part (Figure 6c).
BAND
OF
INTEREST
a.
FS/2
DMCLK/16
BAND
OF
INTEREST
NOISE-SHAPING
b.
FS/2
DMCLK/16
DIGITAL FILTER
BAND
OF
INTEREST
FS/2
DMCLK/16
c.
Figure 6. Sigma-Delta Noise Reduction
Figure 7 shows the various stages of filtering that are employed
in a typical AD73311L application. In Figure 7a we see the
transfer function of the external analog antialias filter. Even
though it is a single RC pole, its cutoff frequency is sufficiently
far away from the initial sampling frequency (DMCLK/8) that
it takes care of any signals that could be aliased by the sampling
frequency. This also shows the major difference between the
initial oversampling rate and the bandwidth of interest. In Figure
7b, the signal and noise-shaping responses of the sigma-delta
modulator are shown. The signal response provides further
rejection of any high frequency signals while the noise-shaping
will push the inherent quantization noise to an out-of-band
position. The detail of Figure 7c shows the response of the
digital decimation filter (Sinc-cubed response) with nulls every
multiple of DMCLK/256, which is the decimation filter update
rate. The final detail in Figure 7d shows the application of a
final antialias filter in the DSP engine. This has the advantage
of being implemented according to the user’s requirements and
available MIPS. The filtering in Figures 7a through 7c is imple-
mented in the AD73311L.
REV. A
–9–
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