AD8019ARU-REEL Analog Devices Inc, AD8019ARU-REEL Datasheet - Page 13

• Manufacturer Part Number: AD8019ARU-REEL
• Manufacturer: Analog Devices Inc

Specifications of AD8019ARU-REEL

• Power Supply Requirement: Single/Dual
• Package Type: TSSOP
• Slew Rate: 450V/us
• Pin Count: 14
• Lead Free Status / RoHS Status: Not Compliant

a transformer, one should do so in a way to minimize them in
order to avoid operating the line driver in a potentially unstable
environment. Limiting both distributed and interwinding capaci-
tance to less than 20 pF each should be sufficient for most
applications.
Stability Enhancements
Voltage feedback amplifiers may exhibit sensitivity to capaci-
tance present at the inverting input. Parasitic capacitance, as small
as several picofarads, in combination with the high-impedance of
the input can create a pole that can dramatically decrease the phase
margin of the amplifier. In the case of the AD8019, a compen-
sation capacitor of 10 pF–20 pF in parallel with the feedback
resistor will form a zero that can serve to cancel out the effects
of the parasitic capacitance. Placing 100
the noninverting inputs serves to isolate the inputs from each
other and from any high frequency signals that may be coupled
into the amplifier via the midsupply bias.
It may also be necessary to configure the line driver as two sepa-
rate, noninverting amplifiers rather than a single differential
driver. When doing this, the two gain resistors can share an ac
coupling capacitor of 0.1 F to minimize any dc errors.
Adhering to previously mentioned layout techniques will also be
of assistance in keeping the amplifier stable.
Receive Channel Considerations
A transformer used at the output of the differential line driver to
step up the differential output voltage to the line has the inverse
effect on signals received from the line. A voltage reduction or
attenuation equal to the inverse of the turns ratio is realized in
the receive channel of a typical bridge hybrid. The turns ratio of
the transformer may also be dictated by the ability of the receive
circuitry to resolve low-level signals in the noisy twisted pair tele-
phone plant. While higher turns ratio transformers boost transmit
signals to the appropriate level, they also effectively reduce the
received signal to noise ratio due to the reduction in the received
signal strength.
Using a transformer with as low a turns ratio as possible will limit
degradation of the received signal.
The AD8022, a dual amplifier with typical RTI voltage noise of
only 2.5 nV/ Hz and a low supply current of 4 mA/amplifier is
recommended for the receive channel.
DMT Modulation, Multi-Tone Power Ratio (MTPR) and
Out-of-Band SFDR
ADSL systems rely on Discrete Multi-Tone (or DMT) modula-
tion 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 are uniformly separated in frequency. A
uniquely encoded, Quadrature Amplitude Modulation (QAM)-
like signal occurs at the center frequency of each subband or
tone. See Figure 4 for an example of a DMT waveform in the
frequency domain, and Figure 5 for a time domain waveform.
Difficulties will exist when decoding these subbands if a QAM
signal from one subband is corrupted by the QAM signal(s)
in series with each of
from other subbands, regardless of whether the corruption
comes from an adjacent subband or harmonics of other subbands.
Conventional methods of expressing the output signal integrity
of line drivers such as 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 process DMT and other heavily
modulated waveforms. A typical ADSL upstream DMT signal
can contain as many as 27 carriers (subbands or tones) of QAM
signals. Multi-Tone Power Ratio (MTPR) is the relative differ-
ence between the measured power in a typical subband (at one
tone or carrier) versus the power at another subband specifi-
cally selected to contain no QAM data. In other words, a
selected subband (or tone) remains open or void of intentional
power (without a QAM signal) yielding an empty frequency bin.
MTPR, sometimes referred to as the ‘empty bin test,’ is typically
expressed in dBc, similar to expressing the relative difference
between single tone fundamentals and second or third harmonic
distortion components. Measurements of MTPR are typically
made on the line side or secondary side of the transformer.
MTPR versus transformer turns ratio is depicted in TPCs 30 and
31 and covers a variety of line power ranging from 10 dBm to
18 dBm. As the turns ratio increases, the driver hybrid can
deliver more undistorted power to the load due to the high
output current capability of the AD8019. Significant degrada-
tion of MTPR will occur if the output of the driver swings to
the rails, causing clipping at the DMT voltage peaks. Driving
DMT signals to such extremes not only compromises “in band”
MTPR, but will also produce spurs that exist outside of the
frequency spectrum containing the transmitted signal. “Out-
of-band” spurious free dynamic range (SFDR) can be defined
as the relative difference in amplitude between these spurs and a
tone in one of the upstream bins. Compromising out-of-band
SFDR is the equivalent of increasing near-end cross talk (NEXT).
Regardless of terminology, maintaining out-of-band SFDR
while reducing NEXT will improve the overall performance of
the modems connected at either end of the twisted pair.
–20
–40
–60
–80
20
0
0
50
FREQUENCY – kHz
100
AD8019
150