AD73311
–14–
REV. B
Analog Smoothing Filter & PGA
The output of the single-bit DAC is sampled at DMCLK/8,
therefore it is necessary to filter the output to reconstruct the
low frequency signal. The decoder’s analog smoothing filter
consists of a continuous-time filter preceded by a third-order
switched-capacitor filter. The continuous-time filter forms part
of the output programmable gain amplifier (PGA). The PGA
can be used to adjust the output signal level from –15 dB to
+6 dB in 3 dB steps, as shown in Table V. The PGA gain is
set by bits OGS0, OGS1 and OGS2 (CRD:4-6) in Control
Register D.
Table V. PGA Settings for the Decoder Channel
OG2 OG1 OG0 Gain (dB)
00 0+6
00 1+3
01 00
01 1–3
10 0–6
10 1–9
11 0–12
11 1–15
Differential Output Amplifiers
The decoder has a differential analog output pair (VOUTP and
VOUTN). The output channel can be muted by setting the
MUTE bit (CRD:7) in Control Register D. The output signal is
dc-biased to the codec’s on-chip voltage reference.
Voltage Reference
The AD73311 reference, REFCAP, is a bandgap reference that
provides a low noise, temperature-compensated reference to the
DAC and ADC. A buffered version of the reference is also made
available on the REFOUT pin and can be used to bias other
external analog circuitry. The reference has a default nominal
value of 1.2 V but can be set to a nominal value of 2.4 V by
setting the 5VEN bit (CRC:7) of CRC. The 5 V mode is gener-
ally only usable when V
DD
= 5 V.
The reference output (REFOUT) can be enabled for biasing
external circuitry by setting the RU bit (CRC:6) of CRC.
Serial Port (SPORT)
The codec communicates with a host processor via the bidirec-
tional synchronous serial port (SPORT) which is compatible
with most modern DSPs. The SPORT is used to transmit and
receive digital data and control information.
In both transmit and receive modes, data is transferred at the
serial clock (SCLK) rate with the MSB being transferred first.
Due to the fact that the SPORT uses a common serial register for
serial input and output, communications between an AD73311
codec and a host processor (DSP engine) must always be
initiated by the codec itself. This ensures that there is no danger
of the information being sent to the codec being corrupted by
ADC samples being output by the codec.
SPORT Overview
The AD73311 SPORT is a flexible, full-duplex, synchronous
serial port whose protocol has been designed to allow up to
eight AD73311 devices to be connected, in cascade, to a single
DSP via a six-wire interface. It has a very flexible architecture
that can be configured by programming two of the internal
control registers. The AD73311 SPORT has three distinct
modes of operation: Control Mode, Data Mode and Mixed
Control/Data Mode.
In Control Mode (CRA:0 = 0), the device’s internal configura-
tion can be programmed by writing to the five internal control
registers. In this mode, control information can be written to or
read from the codec. In Data Mode (CRA:0 = 1), information
that is sent to the device is used to update the decoder section
(DAC), while the encoder section (ADC) data is read from the
device. In this mode, only DAC and ADC data is written to or
read from the device. Mixed mode (CRA:0 = 1 and CRA:1 = 1)
allows the user to choose whether the information being sent to
the device contains either control information or DAC data.
This is achieved by using the MSB of the 16-bit frame as a flag
bit. Mixed mode reduces the resolution to 15 bits with the MSB
being used to indicate whether the information in the 16-bit
frame is control information or DAC/ADC data.
The SPORT features a single 16-bit serial register that is used
for both input and output data transfers. As the input and out-
put data must share the same register there are some precautions
that must be observed. The primary precaution is that no infor-
mation must be written to the SPORT without reference to an
output sample event, which is when the serial register will be
overwritten with the latest ADC sample word. Once the SPORT
starts to output the latest ADC word then it is safe for the DSP
to write new control or data words to the codec. In certain con-
figurations, data can be written to the device to coincide with
the output sample being shifted out of the serial register—see
section on interfacing devices. The serial clock rate (CRB:2–3)
defines how many 16-bit words can be written to a device before
the next output sample event will happen.
The SPORT block diagram, shown in Figure 9, details the five
control registers (A–E), external MCLK to internal DMCLK
divider and serial clock divider. The divider rates are controlled
by the setting of Control Register B. The AD73311 features a
master clock divider that allows users the flexibility of dividing
externally available high frequency DSP or CPU clocks to gen-
erate a lower frequency master clock internally in the codec
which may be more suitable for either serial transfer or sampling
rate requirements. The master clock divider has five divider
options (÷ 1 default condition, ÷ 2, ÷ 3, ÷ 4, ÷ 5) that are set by
loading the master clock divider field in Register B with the
appropriate code. Once the internal device master clock (DMCLK)
has been set using the master clock divider, the sample rate and
serial clock settings are derived from DMCLK.
The SPORT can work at four different serial clock (SCLK)
rates: chosen from DMCLK, DMCLK/2, DMCLK/4 or
DMCLK/8, where DMCLK is the internal or device master
clock resulting from the external or pin master clock being
divided by the master clock divider. When working at the lower
SCLK rate of DMCLK/8, which is intended for interfacing with
slower DSPs, the SPORT will support a maximum of two de-
vices in cascade with the sample rate of DMCLK/256.
