LTC6957-4 datasheet(21/38 Pages) AD

21 / 38 page

LTC6957-1/LTC6957-2/

LTC6957-3/LTC6957-4

6957fb

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Figure 4

Figure 4 shows the interface between current mode logic

(CML) signals and the LTC6957 inputs. The specifics of

terminating will be dependent on the particular CML driver

used; Figure 4 shows terminations only at the load end

of the line, but the same LTC6957 interface is appropri-

ate for applications with the source end of the line also

terminated. In Figure 4a, a differential signal interface to

the LTC6957 is shown, which must be AC-coupled due to

the DC input levels required at the LTC6957.

6957 F04a

RTERM

–

LTC6957

10nF

6957 F04b

RTERM

–

LTC6957

Figure 4b. Single-Ended CML Input

Figure 4a. Differential CML Input

10nF

APPLICATIONS INFORMATION

Figure 4b shows a single-ended CML signal driving the

LTC6957. This is not commonly used because of noise

and immunity weaknesses compared to the differen-

tial CML case. Because the signal is created by a cur-

rent pulled through the termination resistor, the signal

is inherently referenced to the supply voltage to which

RTERM is tied. For that reason, the other LTC6957 should

be AC-referenced to that supply voltage as shown.

The polarity change shown here is for graphic clarity only,

and can be reversed by swapping the LTC6957 input

terminals.

LTC6957-4 Datasheet(PDF) 21 Page - Analog Devices