AS1160 datasheet(25/29 Pages) AMSCO

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AS1160/AS1161

Datasheet - A p p l i c a t i o n I n f o r m a t i o n

Failsafe Biasing

The AS1161 has an input threshold sensitivity of ±75mV, which allows a greater differential noise margin.

However, in cases where the receiver input is not being actively driven, the increased sensitivity of the AS1161 can

pickup noise as a signal and cause unintentional locking (e.g., when the input cable is left floating).

Figure 38. Failsafe Biasing Setup

External resistors can be added to the receiver circuit to prevent noise pickup as shown in Figure 38. In such circuits,

the non-inverting receiver input is pulled up and the inverting receiver input is pulled down by high value resistors. The

pull-up and pull-down resistors (R1 and R2 in Figure 38) provide a current path through the termination resistor (RLOAD)

which biases the receiver inputs when they are not connected to an active driver.

Note: The value of the pull-up and pull-down resistors should be chosen so that sufficient current is drawn to provide

a +15mV drop across the termination resistor.

Signal Integrity

For a validation of the signal qualitiy in an application or in a simulation, the parameters tDJIT and tRNM can be used to

generate an eye pattern mask.

tDJIT measures the transmitter’s ability to place data bits in the ideal position to be sampled by the receiver. The typical

tDJIT parameter of 50ps @ 66MHz indicates that the crossing point of the Tx data is 50ps before the ideal crossing

point. The tDJITMIN and tDJITMAX parameters specify the earliest and latest time that a crossing will occur relative to the

ideal position.

Figure 39. Eye Pattern Mask Generation and Signal Quality Validation

First of all, tRNM is calculated by measuring how much of the bit the receiver needs to ensure correct sampling. This

calculated amount is subtracted from the ideal bit and what’s left of it is available for external sources of noise and is

called tRNM. It is the offset from tDJIT for the test mask within the eye opening.

The vertical limits of the mask are determined by the AS1161 receiver input threshold of ±75mV.

RLOAD

VDD

tDJIT (min)

tDJIT (typ)

tDJIT (max)

tRNM

Ideal Crossing

Vertical = 200mV/Div

Horizontal = 200ps/Div

AS1160 Datasheet(PDF) 25 Page - ams AG