AD8150AST datasheet(19/35 Pages) AD

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AD8150

–19–

REV. 0

Power Supplies

There are several options for the power supply voltages for the

AD8150, as there are two separate sections of the chip that require

power supplies. These are the control logic and the high-speed

data paths. Depending on the system architecture, the voltage

levels of these supplies can vary.

Logic Supplies

The control (programming) logic is CMOS and is designed to

interface with any of the various standard single-ended logic

families (CMOS or TTL). Its supply voltage pins are VDD (Pin

170, logic positive) and VSS (Pin 152, logic ground). In all cases

the logic ground should be connected to the system digital ground.

VDD should be supplied at between 3.3 V to 5 V to match the

supply voltage of the logic family that is used to drive the logic

inputs. VDD should be bypassed to ground with a 0.1

µF ceramic

capacitor. The absolute maximum voltage from VDD to VSS

is 5.5 V.

Data Path Supplies

The data path supplies have more options for their voltage lev-

els. The choices here will affect several other areas, like power

dissipation, bypassing, and common mode levels of the inputs

and outputs. The more positive voltage supply for the data paths

is VCC (Pins 41, 98, 149 and 171). The more negative supply is

VEE, which appears on many pins that will not be listed here.

The maximum allowable voltage across these supplies is 5.5 V.

The ﬁrst choice in the data path power supplies is to decide

whether to run the device as ECL (Emitter-Coupled Logic) or

PECL (Positive ECL). For ECL operation, VCC will be at ground

potential, while VEE will be at a negative supply between –3.3 V

to –5 V. This will make the common-mode voltage of the inputs

and outputs at a negative voltage, see Figure 37.

DATA

PATHS

CONTROL

LOGIC

3V TO 5V

VSS

VEE

GND

0.1 F

(ONE FOR EVERY TWO VEE PINS)

0.1 F

AD8150

VDD

VCC

Figure 37. Power Supplies and Bypassing for ECL

Operation

If the data paths are to be dc-coupled to other ECL logic devices

that run with ground as the most positive supply and a negative

voltage for VEE, then this is the proper way to run. However, if

the part is to be ac coupled, it is not necessary to have the input/

output common mode at the same level as the other system cir-

cuits, but it will probably be more convenient to use the same

supply rails for all devices.

For PECL operation, VEE will be at ground potential and VCC

will be a positive voltage from 3.3 V to 5 V. Thus, the common

mode of the inputs and outputs will be at a positive voltage.

These can then be dc coupled to other PECL operated devices.

If the data paths are ac coupled, then the common-mode levels

do not matter, see Figure 38.

DATA

PATHS

CONTROL

LOGIC

3V TO 5V

VSS

VEE

GND

0.1 F

(ONE FOR EACH VCC PIN, 4 REQ'D.)

0.1 F

AD8150

VDD

VCC

Figure 38. Power Supplies and Bypassing for PECL

Operation

POWER DISSIPATION

For analysis, the power dissipation of the AD8150 can be divided

into three separate parts. These are the control logic, the data

path circuits and the (ECL or PECL) outputs, which are part of

the data path circuits, but can be dealt with separately. The ﬁrst

of these, the control logic, is CMOS technology and does not

dissipate a signiﬁcant amount of power. This power will, of

course, be greater when the logic supply is 5 V rather than 3 V,

but overall it is not a signiﬁcant amount of power and can be

ignored for thermal analysis.

DATA

PATHS

CONTROL

LOGIC

VSS

VEE

GND

AD8150

VDD

VCC

I, DATA PATH

LOGIC

VOUT LOW – VEE

ROUT

IOUT

Figure 39. Major Power Consumption Paths

The data path circuits operate between the supplies VCC and

VEE. As described in the power supply section, this voltage can

range from 3.3 V to 5 V. The current consumed by this section

will be constant, so operating at a lower voltage can save about

40 percent in power dissipation.

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