9

®

AFE2124

For the transmit channel, the pulse shape and the power

spectral density scale directly with the clock rate. The power

spectral density shown in Curve 1 and the pulse template

shown in Curve 2 are measured at the output of the trans-

former. The transformer and the RC circuit on the output

provide some smoothing for the output transmission. At

lower bit rates, the amount of smoothing will be less.

rxHYB AND rxLINE INPUT

ANTI-ALIASING FILTERS

An external input antialiasing filter is needed on the hybrid

and line inputs as shown in Figure 6. The –3dB frequency of

the input anti-aliasing filter for the rxLINE and rxHYB

differential inputs should be approximately 1MHz for T1

and E1 symbol rates. Suggested values for the filter are

750

Ω for each of the two input resistors and 100pF for the

capacitor. Together, the two 750

Ω resistors and the 100pF

capacitor result in a 3dB frequency of just over 1MHz. The

750

Ω input resistors will result in minimal voltage divider

loss with the input impedance of the AFE2124.

The anti-aliasing filters will give best performance with 3dB

frequency approximately equal to the bit rate. For example,

a 3dB frequency of 320kHz may be used for a single line bit

rate of 320k bits per second.

FIGURE 6. Basic Connection Diagram for Each Channel of the AFE2124.

REF

P

V

CM

REF

N

DV

AV

DD

1/2

AFE2124

AV

DD

AV

DD

rxbaudCLK

rx48xCLK

Data Out

txbaudCLK

tx48xCLK

Data In

HDSL DSP

GNDA

GNDA

GNDA

txLINE+

txLINE–

rxHYB+

rxHYB–

rxLINE–

rxLINE+

0.01

µF

–

–

+

+

Compromise

Hybrid

Input Antialias Filter

fc

≅ 2 x Symbol Rate

0.01

µF

5V Analog

0.1

µF

0.1

µF

0.1

µF 0.1µF

0.1

µF

1 - 10

µF

100pF

750

Ω

100pF

750

Ω

750

Ω

13

Ω

13

Ω

750

Ω

5V to 3.3V Digital

1:2 Transformer

Tip

Ring

0.1

µF

0.1

µF

0.1

µF