AD8159ASVZ datasheet(20/24 Pages) AD

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AD8159

Rev. A | Page 20 of 24

DC Coupling

When ac coupling is used, the common-mode level at the input

of the device is equal to VTTI. The single-ended input signal

swings above and below VTTI equally. The user can then use

First, consider the dc-coupled case (see Figure 44). A lane on

Output Port A or Output Port B on the AD8159 is dc-coupled

to a receiving device. In this example, the output termination

voltage (VTTO) on the AD8159 is set to the same level as the

input termination voltage (VTTIR) on the receiving device, and

this level sets the high value (VHI) of the single-ended output

voltage. With pre-emphasis low (PE = 0), the maximum single-

ended current is 16 mA

Figure 22 and Figure 25 to determine the acceptable range of

common-mode levels and signal swing levels that satisfy the

input range of the AD8159.

If dc coupling is required, determining the input common-

mode level is less straightforward because the configuration of

the driver must be also be considered. In most cases, the user

would set VTTI on the AD8159 to the same level as the driver

output termination voltage, VTTOD. This prevents a continuous

dc current from flowing between the two supply nets. As a

practical matter, both devices can be terminated to the same

physical supply net.

1, which flows through the parallel

combination of the 50 Ω on-chip resistor and the 50 Ω far end

termination. Therefore, the low value (VLO) of the output

voltage is equal to

VTTO − 16 mA × (50 Ω || 50 Ω) = VTTO − 400 mV

Because the minimum allowed voltage at the output is

VCC − 1.6 V, the lowest acceptable value for VTTO is

Consider the following example: A driver is dc-coupled to the

input of the AD8159. The AD8159 input termination voltage

(VTTI) and the driver output termination voltage (VTTOD) are both

set to the same level; that is, VTTI = VTTOD = 3.3 V. If an 800 mV

differential p-p swing is desired, the total output current of the

driver is 16 mA. At balance, the output current is divided evenly

between the two sides of the differential signal path, 8 mA to each

side. This 8 mA of current flows through the parallel combina-

tion of the 54.5 Ω input termination resistor on the AD8159

and the 50 Ω output termination resistor on the driver, resulting

in a common-mode level of

VCC − 1.6 V + 0.4 V = VCC − 1.2 V

Increasing pre-emphasis to its highest level (PE = 3) results in

a maximum, single-ended current of 28 mA. In this case

VLO = VTTO − 28 mA × (50 Ω || 50 Ω) = VTTO − 700 mV

As a result, the lowest acceptable value for VTTO is

VCC − 1.6 V + 0.7 V = VCC − 0.9 V

It is expected that the minimum VTTO is 300 mV higher than the

case when PE = 0, because increasing the pre-emphasis level

results in a 300 mV lower voltage excursion at the output.

VTTI − 8 mA × (50 Ω || 54.5 Ω) = VTTI − 209 mV

The user can then use Figure 25 to determine the allowable

range of values for VTTI that meets the input compliance range

based on an 800 mV p-p differential swing.

1 The output current for Port C when PE_C = 0 is slightly higher, 20 mA. The

extra 4 mA of current (compared to Port A/Port B) is needed to support the

bidirectional feature.

OUTPUT COMPLIANCE

2 The output current for Port C when PE_C = 3 is 32 mA, for the same reason

as stated in Endnote 1.

Not surprisingly, there is also a range of voltages that satisfies

the requirements of the output devices. This range is specified

as the minimum and maximum voltage (with respect to VCC)

allowed at an output pin.

50Ω

VTTO

VTTIR

AD8159

RECEIVING DEVICE

PORT A/B: (16 + 4 × PE) mA

PORT C: (20 + 4 × PE) mA

VEE

Figure 44. DC-Coupling Output Signal from AD8159

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