AD8022

–10–

REV. 0

FREQUENCY – Hz

0

10k

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

100k

1M

10M

100M

–PSRR

+PSRR

Figure 33. Power Supply Rejection vs. Frequency,

FREQUENCY – Hz

0

10k

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

100k

1M

10M

100M

–PSRR

+PSRR

Figure 34. Power Supply Rejection vs. Frequency,

THEORY OF OPERATION

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 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.

7.5pF

15

15

OUTPUT

+IN

–IN

600 A

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.

APPLICATIONS

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.

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