GS7025 PRO-LINX™ Serial Digital Receiver

Data Sheet

13813 - 7

October 2009

18 of 23

Figure 3-6: High Speed Digital Inputs (DDI/DDI)

DDI/DDI are high-impedance inputs which accept differential or single-ended input

drive. Two conditions must be observed when interfacing to these inputs:

1. Input signal amplitudes are between 200 and 2000mV.

2. The common mode input voltage range is as specified in Table 2-1: DC Electrical

Characteristics.

Commonly used interface examples are shown in Figure 3-7, Figure 3-8 & Figure 3-9.

Figure 3-7 illustrates the simplest interface to the GS7025 digital inputs. In this example,

the driving device generates the PECL level signals (800mV amplitudes) having a

common mode input range between 0.4 and 4.6V. This scheme is recommended when

the trace lengths are less than 1in. The value of the resistors depends on the output

driver circuitry.

Figure 3-7: Digital Inputs - simple interface

When trace lengths become greater than 1in, controlled impedance traces should be

used. The recommended interface is shown in Figure 3-8. In this case, a parallel resistor

(RLOAD) is placed near the GS7025 inputs to terminate the controlled impedance trace.

The value of RLOAD should be twice the value of the characteristic impedance of the

trace. In addition, place series resistors (RSOURCE) near the driving chip to serve as

source terminations. They should be equal to the value of the trace impedance.

Assuming 800mV output swings at the driver, RLOAD = 100Ω, RSOURCE = 50Ω and ZO

= 50Ω.

Figure 3-8: Digital Inputs - controlled impedance interface

Figure 3-9 shows the recommended interface when the GS7025 digital inputs are driven

single-endedly. In this case, the input must be AC-coupled and a matching resistor (Zo)

must be used.

SDI

GS7025

SDI

75

10nF

10nF

75

113

DDI

DDI

GS7025

DDI

DDI

GS7025