AD8159 datasheet(21/24 Pages) AD

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AD8159

Rev. A | Page 21 of 24

AC Coupling

In general, more VTTO supply headroom is required with ac-

coupled outputs. When the outputs are ac-coupled, the average

single-ended current does not see the far end 50 Ω termination

because the capacitor acts as a dc block. For example, with

PE = 0, the single-ended output current alternates from 0 mA

to 16 mA, or 8 mA on average. This 8 mA current flows entirely

through the on-chip 50 Ω termination resistor due to the dc

block.

The single-ended output voltage has an average value of

VTTO − 8 mA × 50 Ω = VTTO − 400 mV

For appropriate data patterns,

1 the capacitor acts as a short and

the voltage swing is 400 mV p-p, identical to the dc-coupled case.

The low output voltage is, therefore,

VTTO − 400 mV − 200 mV = VTTO − 600 mV

1AC coupling requires that the signal pattern have no long term dc

component. Codes such as 8b/10b, for example, ensure that the data

pattern is benign in an ac-coupled link.

The lowest acceptable value for VTTO is

VCC − 1.6 V + 0.6 V = VCC − 1.0 V

The same exercise can be repeated for other pre-emphasis

settings.

Output Compliance Table

To simplify the task of interfacing to the AD8159 output,

Table 13 is useful as a quick-reference. It provides the minimum

and maximum values for output termination voltage for both

ac and dc coupling. The values in the table are valid for any pre-

emphasis setting.

Table 13. Output Compliance

AC-Coupled

DC-Coupled

Minimum

(V)

Maximum

(V)

Minimum

(V)

Maximum

(V)

VTTO

VCC – 0.5

VCC + 0.6

VCC – 0.9

VCC + 0.6

VTTOI

VCC – 0.4

VCC + 0.6

VCC – 0.8

VCC + 0.6

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