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AD8194 Arkusz danych(PDF) 9 Page - Analog Devices |
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AD8194 Arkusz danych(HTML) 9 Page - Analog Devices |
9 / 16 page AD8194 Rev. 0 | Page 9 of 16 THEORY OF OPERATION INTRODUCTION The primary function of the AD8194 is to switch the high speed signals from one of two (HDMI or DVI) single-link sources to one output. Each source group consists of four differential, high speed channels. The four high speed channels include a data- word clock and three Transition Minimized Differential Signaling (TMDS) data channels running at 10× the data-word clock frequency for data rates up to 2.25 Gbps. All four high speed channels of the AD8194 are identical; that is, the pixel clock can be run on any of the four TMDS channels. The AD8194 does not provide switching of the low speed DDC and CEC signals. The AD8194 is an equalized, buffered TMDS switch with low added jitter. The output pins are electrically isolated from the inputs and the input equalizer recovers and transmits an open, full-swing data eye at the output, even for heavily attenuated input signals. Because the AD8194 is a TMDS-only switch, a complete HDMI switch solution requires another component to switch the low speed DDC channels. Several low cost CMOS switches can be used along with the AD8194 to make an HDMI 1.3-compliant 2:1 link switch. The requirements for such a switch are as follows: • Low input capacitance. The HDMI 1.3 specification limits the total DDC link capacitance for an HDMI sink to less than 50 pF. This 50 pF limit includes the HDMI connector, the PCB, the capacitance of the CMOS switch, and what- ever capacitance is seen at the input of the HDMI receiver. • Low channel on resistance (RON). Switches with high on resistance degrade the quality of the DDC signals. • An appropriate form factor to switch the DDC and HPD signals as necessary. A reference design that incorporates the AD8194 and a low cost CMOS switch is described in more detail in the Evaluation Board section. In addition to the AD8194, Analog Devices, Inc., offers several HDMI switches with integrated DDC, in a variety of form factors. INPUT CHANNELS Each high speed input differential pair terminates to the 3.3 V VTTI power supply through a pair of single-ended 50 Ω on-chip resistors, as shown in Figure 19. These matched on- chip terminations absorb reflections on the input TMDS channels, properly terminating the inputs and improving overall system signal integrity. The input termination resistors all have series switches, as shown in Figure 19. The state of these switches is determined by the S_SEL signal, which also controls the input selection. The termination switches for the selected input channel are closed (terminations present), whereas the termination switches for the unselected input are open (high-Z inputs). The input equalizer of the AD8194 provides 12 dB of high frequency boost. No specific cable length is suggested for use with the AD8194 because cable performance varies widely between manufacturers; however, in general, the equalization of the AD8194 does not degrade the system signal integrity, even for short input cables. For a 24 AWG reference cable, the AD8194 can equalize more than 20 m at data rates up to 2.25 Gbps. CABLE EQ 50Ω 50Ω IP_xx IN_xx AVEE VTTI Figure 19. High Speed Input Simplified Schematic OUTPUT CHANNELS Each high speed output differential pair is terminated to the 3.3 V VTTO power supply through two single-ended 50 Ω on-chip resistors, as shown in Figure 20. These matched on- chip back terminations absorb reflections on the output TMDS channels and improve the overall system signal integrity. These termination resistors are always present in the outputs and they cannot be switched out. VTTO 50Ω 50Ω OPx ONx AVEE IOUT Figure 20. High Speed Output Simplified Schematic In a typical application, the AD8194 output is connected to the input of an HDMI/DVI receiver, which provides a second set of matched terminations in accordance with the HDMI 1.3 specification. If no receiver is connected, each of the AD8194 output pins should be tied to 3.3 V through a 50 Ω on-board termination resistor. |
Podobny numer części - AD8194_15 |
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Podobny opis - AD8194_15 |
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