AD8022
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–PSRR
–PSRR
+PSRR
+PSRR
–100
10k
10k
100k
1M
FREQUENCY – Hz
10M
100M
100k
1M
FREQUENCY – Hz
10M
100M
Figure 34. Power Supply Rejection vs. Frequency,
Figure 33. Power Supply Rejection vs. Frequency,
VS = ±2.5 V
VS = ±12 V
THEORY OF OPERATION
APPLICATIONS
The AD8022 is a voltage-feedback op amp designed especially
for ADSL or other applications requiring very low voltage and
current noise along with low-supply current, low distortion, and
ease of use.
The low-noise AD8022 dual xDSL receiver amplifier is specifi-
cally designed for the dual differential receiver amplifier function
within xDSL transceiver hybrids as well as other low-noise
amplifier applications. The AD8022 may be used in receiving
modulated signals including Discrete Multitone (DMT) on either
end of the subscriber loop. Communication systems designers
can be challenged when designing an xDSL modem transceiver
hybrid capable of receiving the smallest signals embedded in noise
that inherently exists on twisted pair phone lines. Noise sources
include Near End Cross Talk (NEXT), Far End Cross Talk
(FEXT), background and impulse noise, all of which are fed, to
some degree, into the receiver front end. Based on a Bellcore
noise survey, the background noise level for typical twisted pair
telephone loop is said to be –140 dBm/√Hz or 31 nV/√Hz. It
is therefore important to minimize the noise added by the receiver
amplifiers in order to preserve as much Signal-to-Noise Ratio
(SNR) as is possible. With careful transceiver hybrid design
using the AD8022 dual low-noise receiver amplifier, maintaining
power density levels lower than –140 dBm/Hz in ADSL modems is
easily achieved.
The AD8022 is fabricated on Analog Devices’ proprietary eXtra-
Fast Complementary Bipolar (XFCB) process, which enables
the construction of PNP and NPN transistors with similar fTs in
the 4 GHz region. The process is dielectrically isolated to eliminate
the parasitic and latch-up problems caused by junction isolation.
These features enable the construction of high-frequency, low-
distortion amplifiers with low-supply currents.
+V
S
15⍀
15⍀
+IN
–IN
OUTPUT
7.5pF
DMT Modulation and Multitone Power Ratio (MTPR)
ADSL systems rely on Discrete Multitone (or DMT) modulation
to carry digital data over phone lines. DMT modulation appears
in the frequency domain as power contained in several individual
frequency subbands, sometimes referred to as tones or bins,
each of which is uniformly separated in frequency. (See Figures
21, 22, 24, and 25 for MTPR results while the AD8022 receives
DMT driving 800 mV rms across 500 Ω differential load). A
uniquely encoded, Quadrature Amplitude Modulation (QAM)
signal occurs at the center frequency of each subband or tone.
Difficulties will exist when decoding these subbands if a QAM
signal from one subband is corrupted by the QAM signal(s) from
other subbands, regardless of whether the corruption comes from
an adjacent subband or harmonics of other subbands. Conven-
tional methods of expressing the output signal integrity of line
receivers such as spurious free dynamic range (SFDR), single
tone harmonic distortion or THD, two-tone Intermodulation
Distortion (IMD) and third-order intercept (IP3) become
significantly less meaningful when amplifiers are required to
600A
–V
S
OFFSET NULL
Figure 35. Simplified Schematic
As shown in Figure 35, the AD8022 input stage consists of an
NPN differential pair in which each transistor operates a 300 µA
collector current. This gives the input devices a high transconduc-
tance and hence gives the AD8022 low-input noise of 2.5 nV/√Hz
@ 100 kHz. The input stage drives a folded cascode that consists
of a pair of PNP transistors. These PNP’s then drive a current
mirror that provides a differential-input to single-ended-out-
put conversion. The output stage provides a high-current gain
of 10,000, so that the AD8022 can maintain a high-dc open-
loop gain, even into low-load impedances.
REV. 0
–10–
ADI [ ADI ]
