EVAL-AD73422EB AD [Analog Devices], EVAL-AD73422EB Datasheet - Page 13

• Manufacturer Part Number: EVAL-AD73422EB
• Description: Dual Low Power CMOS Analog Front End with DSP Microcomputer
• Manufacturer: AD [Analog Devices]

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 AD73422 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 nominal value of
1.2 V.
The reference output (REFOUT) can be enabled for biasing
external circuitry by setting the RU bit (CRC:6) of CRC.
Analog and Digital Gain Taps
The AD73422 features analog and digital feedback paths be-
tween input and output. The amount of feedback is determined
by the gain setting that is programmed in the control registers.
This feature can typically be used for balancing the effective
impedance between input and output when used in Subscriber
Line Interface Circuit (SLIC) interfacing.
Analog Gain Tap
The analog gain tap is configured as a programmable differential
amplifier whose input is taken from the ADC’s input signal
path. The output of the analog gain tap is summed with the
output of the DAC. The gain is programmable using Control
Register F (CRF:0-4) to achieve a gain of –1 to +1 in 32 steps,
with muting being achieved through a separate control setting
(Control Register F Bit _). The gain increment per step is 0.0625.
The AGT is enabled by powering up the AGT control bit in the
power control register (CRC:1). When this bit is set (=1) CRF
becomes an AGT control register with CRF:0-4 holding the
AGT coefficient, CRF:5 becomes an AGT enable and CRF:7
becomes an AGT mute control bit. Control bit CRF:5 connects/
disconnects the AGT output to the summer block at the output
of the DAC section while control bit CRF:7 overrides the gain
tap setting with a mute, or zero gain, setting (which is omitted
from the gain settings). Table III shows the gain versus digital
setting for the AGT.
REV. 0
REFOUT
REFCAP
VOUTP1
VOUTN1
VFBN1
VFBP1
VINN1
VINP1
INVERTING
OP AMPS
V
REF
Figure 7. Analog Input/Output Section
LOOP-BACK
ANALOG
SELECT
+6/–15dB
REFERENCE
PGA
GAIN
1
CONTINUOUS
LOW-PASS
INVERT
FILTER
TIME
AFE SECTION
AD73422
SINGLE-ENDED
ENABLE
V
REF
GAIN TAP
ANALOG
0/38dB
PGA
–13–
AGTC4
0
0
0
0
0
—
0
1
—
1
1
1
*AGE and DGT weights are given for the case of VFBNx (connected to the
Digital Gain Tap
The digital gain tap features a programmable gain block whose
input is taken from the bitstream from the ADC’s sigma-delta
modulator. This single bit input (1 or 0) is used to add or sub-
tract a programmable value, which is the digital gain tap setting,
to the output of the DAC section’s interpolator. The program-
mable setting has 16-bit resolution and is programmed using the
settings in Control Registers G and H.
*AGE and DGT weights are given for the case of VFBNx (connected to the
AFE Serial Port (SPORT2)
The AFE section communicates with the DSP section via its
bidirectional synchronous serial port (SPORT2), which interfaces
to either SPORT0 or SPORT1 of the DSP section. SPORT2 is
used to transmit and receive digital data and control informa-
tion. The dual AFE is implemented using two separate AFE
blocks that are internally cascaded with serial port access to the
input of AFE Channel 1 and the output of AFE Channel 2.
This allows other single or dual codec devices to be cascaded
together (up to a limit of eight codec units).
In both transmit and receive modes, data is transferred at the
serial clock (SCLK2) rate with the MSB being transferred first.
Communications between the AFE section and the DSP section
must always be initiated by the AFE section (AFE is in master
mode—DSP SPORT is in slave mode). This ensures that there
is no collision between input data and output samples.
sigma-delta modulator’s positive input) being at a higher potential than VFBPx
(connected to the sigma-delta modulator’s negative input).
sigma-delta modulator’s positive input) being at a higher potential than VFBPx
(connected to the sigma-delta modulator’s negative input).
AGTC3
0
0
0
0
0
—
1
0
—
1
1
1
Table III. Analog Gain Tap Settings*
Table IV. Digital Gain Tap Settings*
DGT15-0 (Hex)
0x8000
0x9000
0xA000
0xC000
0xE000
0x0000
0x2000
0x4000
0x6000
0x7FFF
AGTC2
0
0
0
0
1
—
1
0
—
1
1
1
AGTC1 AGTC0
0
0
1
1
0
—
1
0
—
0
1
1
Gain
–1.00
–0.875
–0.75
–0.5
–0.25
–0.00
+0.25
+0.5
+0.75
+0.99999
0
1
0
1
0
—
1
0
—
1
0
1
AD73422
Gain
+1.00
+0.9375
+0.875
+0.8125
+0.075
—
+0.0625
–0.0625
—
–0.875
–0.9375
–1.00