ad6437 Analog Devices, Inc., ad6437 Datasheet - Page 5

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ad6437

Manufacturer Part Number
ad6437
Description
Analog Front End For Adsl
Manufacturer
Analog Devices, Inc.
Datasheet

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This Material Copyrighted By Its Respective Manufacturer
CIRCUIT DESCRIPTION
GENERAL
The AD6437 is designed as the analog front end for the
AD20msp910 chipset or other ADSL systems. The AD6437
contains programmable filters that make it suitable for both
ATU-C and ATU-R modem applications. Many of the internal
circuit of the AD6437 can be programmed or bypassed, making
it a versatile CODEC that can be used in other ADSL applica-
tions or instrumentation and control systems.
There are four major sections to the AD6437: the transmit
channel, which contains a 12-bit DAC and a fourth order recon-
struction filter; the receive channel, which includes a PGA with
30 dB of dynamic range, a fourth order antialiasing filter and a
12-bit, 10 MSPS ADC; auxiliary and support circuitry, which
consists of a 7-bit DAC used for timing recovery and analog
switches used for bypassing internal blocks and channel loopback;
finally, there is the interface and control logic.
The AD6437 core runs from a single 5 V supply. The digital
interface circuitry can run from either a 3.3 V or 5 V supply. All
the analog input and output signals are processed in a fully
differential mode. This affords greater immunity to externally
coupled noise as well greater signal swings throughout the signal
path.
TRANSMIT CHANNEL
The data is received from the digital section (e.g., the AD6436
DME) in parallel format, and passed to the DAC. The output
from this is available externally in complementary voltage
(TX_DACOUTA, B). Typically, it will then be connected to
TXLPF for the on-chip reconstruction filter (4th order
Butterworth active filter). However, this filter can be bypassed if
a different filter is required (e.g., to use the AD6437 in a non-
standard application).
The filter can be switched between two corner frequencies, for
use in CO mode or RT mode. For downstream operation (CO
mode), the band ends at 1.1 MHz and the filter’s 3 dB corner is
set at 4 MHz. For upstream operation (RT mode), it is set to
600 kHz, with flat response ( 0.5 dB guaranteed) up to 138 kHz.
The selection is made by the txfsel bit in Control Register D
(Bit 0).
DAC Output
The DAC is a 12-bit differential output current DAC. The
digital coding is straight binary. When the digital inputs are all
1s, I
mentary (i.e.,—when I
I
REV. 0
OUTB
OUTA
is zero). The current outputs are nominally set to 20 mA
is at full scale. The value of the outputs are compli-
OUTA
is at full scale, the current from
–5–
full-scale. The full-scale output current of the DAC is set through
an external resistor. The relationship is:
e.g., R
The current can be converted to a voltage by a grounded resis-
tor. Ideally, this would be immediately converted to a large
signal to preserve SNR. However, the output compliance of the
DAC is 1.0 V. The resistors would be chosen to be <49.9
a 1.0 V output (2 V ppd) to stay within the compliance range.
The power consumption of the AD6437 can be reduced by
lowering I
increase the THD of the DAC outputs. Lowering the output
current would require the termination resistors to increase by
the same ratio to maintain a 2 V ppd output signal. This, how-
ever, will in turn raise the source impedance.
Tx LPF
The AD6437 Tx low-pass filter is designed to attenuate the
images created at the DAC output due to the sampling process.
The amount of attenuation required is specified in the ANSI
T1.413 specification in Sections 6.12 and 7.12 for the ATU-C
and ATU-R respectively. The AD6437 filter is designed to meet
or exceed these requirements.
The AD6437 Tx LPF is implemented as a fully differential 4th
order Butterworth filter. The input voltage range is 2 V ppd,
centered about 2.5 volts. The input impedance is approxi-
mately 10 k differential. The passband gain of the filter is
unity. The THD of the filter is typically less than –75 dB when
driving a 30 k|30 pF load. However, the THD rises as the load
impedance is decreased. As a result, the load impedance driven
by the filter should be kept as high as possible.
Figure 1. Typical Configuration: Transmit Path
SET
RSET
SHORTEST
CONNECTION TO GROUND
2k
OUT
is 2 k for a 20 mA full-scale output.
_full-scale. Lowering the full-scale current does
AVDD
I
OUT
CA
0.1 F
_ full-scale = 40/R
AD6437
AGND
CB
0.1 F
49.9
RB
49.9
SET
RA
Amps
0.1 F
CTA
0.1 F
CTB
AD6437
for

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