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AD28MSP02 Datasheet(PDF) 7 Page - Analog Devices

Part No. AD28MSP02
Description  Voiceband Signal Port
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD28MSP02 Datasheet(HTML) 7 Page - Analog Devices

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AD28msp02
REV. 0
–7–
Table III. Control Word Read Format
Read Request
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
1
0
0
0
0
0
00
0
0
0
0
0
000
Read Ready
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
1
1
0
0
0
0
00
0
0
0
0
0
000
Control Register Reads
To read the control register, the host processor must transfer
two control words. For each transfer, the DATA/CNTRL pin
must be low when SDIFS is asserted. If the MSB of the bit
stream is high, the SPORT recognizes the incoming serial data
as a request for control information. The protocol for reading
the control register is as follows:
1. The host processor sends a “Read Request” control word to
the AD28msp02. Since the MSB of this control word is high,
the SPORT recognized the incoming serial data as a read re-
quest and does not overwrite the control register.
2. When the AD28msp02 receives the read request, it finishes
any data transfers in progress and waits for a “Read Ready”
control word.
3. The host processor then transfers a “Read Ready” control
word to the AD28msp02. Upon receiving this control word,
the AD28msp02 transfers the control register contents to the
host processor via the SPORT.
4. When the SPORT completes the control register transfer, it
immediately resumes transmitting data at an 8 kHz rate.
This scheme allows any data transfers in progress to be com-
pleted and resolves any ambiguities between data and control
words. The format for the read control words is shown in
Table III.
DESIGN CONSIDERATIONS
Analog Input
The analog input signal to the AD28msp02 must be ac-coupled.
Figure 7 shows the recommended input circuit for the
AD28msp02’s analog input pin (either VINNORM or VINAUX).
The circuit of Figure 7 implements a first-order low-pass filter
with a 3 dB point at 20 kHz; this is the only filter that must be
implemented external to the AD28msp02 to prevent aliasing of
the sampled signal. Since the AD28msp02’s ADC uses a highly
oversampled approach that transfers the bulk of the anti-aliasing
filtering into the digital domain, the off-chip anti-aliasing filter
need only be of low order.
In the circuit shown in Figure 7, scaling of the analog input is
achieved by the resistors RIN and RFB. The input signal gain,
–RFB/RIN, can be adjusted from –12 dB to +26 dB by varying
the values of these resistors. The AD28msp02’s on-chip 20 dB
preamplifier can be enabled when there is not enough gain in
the input circuit; the preamplifier is configured by Bit 0 (IPS) of
the control register. Total gain must be configured to ensure
that a full-scale input signal (at CIN in Figure 7) produces a sig-
nal level at the input to the sigma-delta modulator of the ADC
that does not exceed VINMAX, which is specified under “Analog
Interface Electrical Characteristics.” If the total gain is increased
above unity, signal-to-noise (SNR + THD) performance will
not meet the listed specifications.
MUX
VOLTAGE
REFERENCE
VFB
VFB
VIN
AD28msp02
VIN NORM
AUX
NORM
AUX
CFB
CIN
R IN
RFB
INPUT
SIGNAL
Figure 7. Recommended Analog Input Circuit
The dc biasing of the analog input signal is accomplished with
an on-chip voltage reference which nominally equals 2.5 V. The
input signal must be ac-coupled with an external coupling ca-
pacitor (CIN). CIN and RIN should be chosen to ensure a cou-
pling corner frequency of 30 Hz. CIN should be 0.1 µF or larger.


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