REV. 0

AD8019

–13–

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

Ω in series with each of

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

µF 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)

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.

FREQUENCY – kHz

–80

50

–60

–40

–20

0

20

0

100

150

Figure 4. DMT Waveform in the Frequency Domain

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.