6

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AFE1104

THEORY OF OPERATION

The transmit channel consists of a switched-capacitor pulse

forming network followed by a differential line driver. The

pulse forming network receives symbol data from

SPAROW’S DAC+ line and generates a 2B1Q output wave-

form. The output meets the pulse mask and power spectral

density requirements defined in European Telecommunica-

tions Standards Institute document RTR/TM-03036 for E1

mode and in sections 6.2.1 and 6.2.2.1 of Bellcore technical

advisory TA-NWT-001210 for T1 mode. The differential

line driver uses a composite output stage combining class B

operation (for high efficiency driving large signals) with

class AB operation (to minimize crossover distortion).

The receive channel is designed around a fourth-order delta

sigma A/D converter. It includes a difference amplifier that

can be used with an external compromise hybrid for first

order analog crosstalk reduction. A programmable gain

amplifier with gains of 0dB to +9dB is also included. The

delta sigma modulator operating at a 24X oversampling ratio

produces 14 bits of resolution at output rates up to 584kHz.

The basic functionality of the AFE1104 is illustrated in

Figure 1 shown below.

The receive channel operates by summing the two differen-

tial inputs, one from the line (rxLINE) and the other from the

compromise hybrid (rxHYB). The connection of these two

inputs so that the hybrid signal is subtracted from the line

signal is described in the paragraph titled “Echo Cancella-

tion in the AFE”. The equivalent gain for each input in the

difference amp is 1. The resulting signal then passes to a

programmable gain amplifier which can be set for gains of

0dB through 9dB. The ADC converts the signal to a

14-bit digital word, rxD13 - rxD0.

rxLOOP INPUT

rxLOOP is the loopback control signal. When enabled, the

Loopback is enabled by applying a positive signal (Logic 1)

to rxLOOP.

ECHO CANCELLATION IN THE AFE

The rxHYB input is designed to be subtracted from the

rxLINE input for first order echo cancellation. To accom-

plish this, note that the rxLINE input is connected to the

same polarity signal at the transformer (positive to positive

and negative to negative) while the rxHYB input is con-

nected to opposite polarity through the compromise hybrid

(negative to positive and positive to negative) as shown in

Figure 2.

RECEIVE DATA CODING

The data from the receive channel A/D converter is in two’s

complement code.

FIGURE 1. Functional Block Diagram of AFE1104.

ADC

Pulse Former

Programmable

Gain Amp

Difference

Amplifier

Differential

Line Driver

txLINE

P

txLINE

N

rxHYB

P

rxHYB

N

rxLINE

P

rxLINE

N

txDAT

rxD13 - rxD0

14

ANALOG INPUT

OUTPUT CODE (rxD13 - rxD0)

Positive Full Scale

01111111111111

Mid Scale

00000000000000

Negative Full Scale

10000000000000

RECEIVE CHANNEL PROGRAMMABLE

GAIN AMPLIFIER

The gain of the amplifier at the input of the Receive Channel

is set by two gain control pins, rxGAIN1 and rxGAIN0. The

resulting gain between 0dB and +9dB is shown below.

rxGAIN1

rxGAIN0

GAIN

0

0

0dB

0

1

3.25dB

1

0

6dB

1

1

9dB