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Data Sheet

AD9547

Table 24. Output Channel Logic Family and Pin Functionality

Mode Bits [2:0]

Logic Family and Pin Functionality

000

CMOS (both pins)

001

CMOS (positive pin), tristate (negative pin)

010

Tristate (positive pin), CMOS (negative pin)

011

Tristate (both pins)

100

LVDS

101

LVPECL

110

Unused

111

Unused

Regardless of the selected logic family, each is capable of dc

operation. However, the upper frequency is limited by the load

conditions, drive strength, and impedance matching inherent in

each logic family. Practical limitations set the maximum CMOS

frequency to approximately 250 MHz, whereas LVPECL and

LVDS are capable of 725 MHz.

In addition to the three mode bits, both distribution channel

mode registers include the following control bits:

•

Polarity invert

•

CMOS phase invert

•

Drive strength

The polarity invert bit enables the user to choose between normal

polarity and inverted polarity. Normal polarity is the default

state. Inverted polarity reverses the representation of Logic 0

and Logic 1 regardless of the logic family.

The CMOS phase invert bit applies only when the mode bits select

the CMOS logic family. In CMOS mode, both output pins of the

channel have a dedicated CMOS driver. By default, both drivers

deliver identical signals. However, setting the CMOS phase invert

bit causes the signal on an OUTxN pin to be the opposite of the

signal appearing on the OUTxP pin.

The drive strength bit allows the user to control whether the

output uses weak (0) or strong (1) drive capability (applies to

CMOS and LVDS but not LVPECL). For the CMOS family, the

strong setting implies normal CMOS drive capability, whereas

the weak setting implies low capacitive loading and allows for

reduced EMI. For the LVDS family, the weak setting provides

3.5 mA drive current for standard LVDS operation, whereas the

strong setting provides 7 mA for double terminated or double

voltage LVDS operation. Note that 3.5 mA and 7 mA are the

nominal drive current values when using the internal current

setting resistor.

Output Current Control with an External Resistor

By default, the output drivers have an internal current setting

resistor (3.12 kΩ nominal) that establishes the nominal drive

current for the LVDS and LVPECL operating modes. Instead of

using the internal resistor, the user can elect to set the external

distribution resistor bit (Register 0x0400, Bit 5) and connect an

external resistor to the OUT_RSET pin. Note that this feature

supports an external resistor value of 3.12 kΩ only, allowing for

tighter control of the output current than is possible by using

the internal current setting resistor. However, if the user elects

to use a nonstandard external resistance, the following equations

provide the output drive current as a function of the external

resistance (R):

LVDS

8325

LVDS

665

ILVPECL

The numeric subscript associated with the LVDS output cur-

rent corresponds to the logic state of the drive strength bit in the

distribution channel modes registers (Address 0x0404, Bit 3 and

Address 0x0405, Bit 3). For R = 3.12 kΩ, the equations yield

ILVDS0 = 3.5 mA, ILVDS1 = 7.0 mA, and ILVPECL = 8.0 mA. Note that

the device maintains a constant 1.238 V (nominal) across the

external resistor.

Clock Distribution Synchronization

A block diagram of the distribution synchronization functionality

appears in Figure 48. The synchronization sequence begins with

the primary synchronization signal, which ultimately results in

delivery of a synchronization strobe to the clock distribution logic.

As indicated, the primary synchronization signal originates

from the following four possible sources:

•

Direct synchronization source via the sync distribution bit

(Register 0x0A02, Bit 1)

•

Automatic synchronization source based on frequency or

phase lock detection, as controlled via the automatic synchro-

nization register (Address 0x0403)

•

Multifunction pin synchronization source via one of the

multifunction pins (M0 to M7)

•

EEPROM synchronization source via the EEPROM

All four sources of the primary synchronization signal are logic

OR’d, so any one of them can synchronize the clock distribution

output at any time. When using the multifunction pins, the syn-

chronization event is the falling edge of the selected signal. When

using the sync distribution bit, the user first sets then clears the bit.

The synchronization event is the clearing operation; that is, the

Logic 1 to Logic 0 transition of the bit.

The primary synchronization signal can synchronize

the distribution output directly, or it can enable a secondary

synchronization signal. This functionality depends on the two

sync source bits in the distribution synchronization register

(Register 0x0402, Bits[5:4]).

When sync source = 00 (direct), the falling edge of the

primary synchronization signal directly synchronizes the

distribution output.

When sync source = 01, the rising edge of the primary synchroni-

zation signal triggers the circuitry that detects a rising edge of the

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