GS7025_09 Datasheet, PDF (18/23 Pages) Gennum Corporation – PRO-LINX Serial Digital Receiver

75
75 ₁₁₃
10nF
10nF
SDI
GS7025
SDI
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.
DDI
Z_O
GS7025
DDI
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.
DDI
GS7025
DDI
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.
GS7025 PRO-LINX™ Serial Digital Receiver
Data Sheet
13813 - 7
October 2009
18 of 23