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MAX3983

Applications Information

Signal-Detect Output Leakage Current

Considerations

If all four RX or TX signal-detect outputs are to be con-

nected together to form one signal detect, the leakage

current of the output stage needs to be considered.

Each SD output sinks a maximum of 25µA when assert-

ed, so when four are connected together, a maximum of

100µA is possible. The value of the pullup resistor con-

nected to pullup voltage V

PULLUP

should be selected so

the leakage current does not cause the output voltage to

fall below the threshold of the next stage. For example, if

the signal-detect outputs are connected together and to

a stage with a logic-high threshold of 1.5V, the pullup

resistor needs to be chosen so V

PULLUP

- I

LEAKAGE

PULLUP

> 1.5V. In this case, if V

PULLUP

= 3.0V,

PULLUP

should be less than 15kΩ.

Autodetect

The MAX3983 can automatically detect an incoming sig-

nal and enable the appropriate outputs. Autodetect of

the RX side is done by connecting RX_SD[1:4] together

with a pullup resistor (value 4.7kΩ to 10kΩ to V

) to

RX_ENABLE. For the TX side, this is done by connecting

TX_SD[1:4] together with a pullup resistor (value 4.7kΩ

to 10kΩ to V

) to TX_ENABLE (Figure 4). If signal is

detected on all channels, SD is high and forces the cor-

responding ENABLE high. Leaving the inputs to the

MAX3983 open (i.e., floating) is not recommended, as

noise amplification can occur and create undesirable

output signals. Autodetect is recommended to eliminate

noise amplification or possible oscillation. When using

autodetect, the link length is determined by the received

signal strength. It is possible to reach longer distances if

the autodetect configuration is not used.

Using Loopback with Autodetect

If the MAX3983 is configured for autodetection,

RX_ENABLE is controlled by the RX_SD[1:4] outputs.

Since loopback requires RX_ENABLE to be high, a sim-

ple OR gate can be used to enable the RX outputs

when either RX_SD[1:4] is high or when LOOPBACK is

high (Figure 5).

InfiniBand and 10Gbase-CX4 Transition

Time Specification

InfiniBand specifies a minimum transition time (20% to

80%) of 100ps and CX4 specifies a minimum of 60ps.

Both are specified at the connector interface to the

cable. The output transition times of the MAX3983 are

45ps (typ) and therefore require some care to increase

this time. Approximately 3in of FR4 with 4-mil-wide lines

is sufficient to lengthen the transition time to 60ps. For

100ps transition times, additional length can be used or

an additional 1.5pF capacitor can be placed across the

outputs of the MAX3983. Do not use high-speed dielec-

tric material for the circuit board if the application

requires the use of the InfiniBand or CX4 type connector

system. With such materials, the fast edges of the

Quad Copper-Cable Signal Conditioner

12 ______________________________________________________________________________________

MAX3983

RX OR TX_SD1

RX OR TX_SD2

RX OR TX_SD3

RX OR TX_SD4

RX OR TX_ENABLE

3.0V ≤ V

PULLUP

≤ 5.5V

4.7kΩ ≤ R ≤ 10kΩ

Figure 4. Autodetection Using Corresponding Signal-Detect

Outputs and Enable Input

MAX3983

RX_SD1

RX_SD2

RX_SD3

RX_SD4

RX_ENABLE

LOOPBACK

3.0V ≤ V

PULLUP

≤ 5.5V

4.7kΩ ≤ R ≤ 10kΩ

TO HOST

Figure 5. Loopback in Autodetect Mode

1 2 ... 7 8 9 10 11 12 13 14 15 16

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