AD8145_15 datasheet(19/24 Pages) AD

Part #	AD8145

Description	High Speed, Triple Differential Receiver with Comparators

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AD8145

Rev. A | Page 19 of 24

DRIVING A CAPACITIVE LOAD

The AD8145 typically drives either high impedance loads over

short PCB traces, such as crosspoint switch inputs, or doubly

terminated coaxial cables. A gain of 1 is commonly used in

the high impedance case because a 6 dB transmission line

termination loss is not incurred. A gain of 2 is required when

driving cables to compensate for the 6 dB termination loss.

In all cases, the output must drive the parasitic capacitance of

the feedback loop, conservatively estimated to be 1 pF, in addition

to the capacitance presented by the actual load. When driving a

high impedance input, it is recommended that a small series

resistor be used to buffer the input capacitance of the device

being driven. Clearly, the resistor value must be small enough to

preserve the required bandwidth. In the ideal doubly terminated

cable case, the AD8145 output sees a purely resistive load. In

reality, there is some residual capacitance, which is buffered by

the series termination resistor. Figure 42 illustrates the high

impedance case, and Figure 43 illustrates the cable driving case.

RS

CIN

GAIN

REF

R

0.01µF

+5V

–5V

C

OUT

VIN

VREF

Figure 42. Buffering the Input Capacitance of a High-Z Load with G = 1

GAIN

REF

R

0.01µF

+5V

–5V

C

VREF

VIN

RS

CS

RL

OUT

Figure 43. Driving a Doubly Terminated Cable with G = 2

POWER-DOWN

The power-down feature can be used to reduce power consumption

when a particular device is not in use and does not place the output

in a high-Z state when asserted. The power-down feature is

asserted when the voltage applied to the power-down pin drops

to approximately 2 V below the positive supply. The AD8145 is

enabled by pulling the power-down pin to the positive supply.

AUTOMOTIVE PRODUCTS

The AD8145W product is qualified per the AEC-Q100 for use

in automotive applications. This model meets stringent automotive

performance and quality requirements and offers the same

functionalities as the commercial and industrial grade devices but

operates over a temperature range of −40°C to +105°C. For more

information, contact your local Analog Devices, Inc., sales

representative.

COMPARATORS

In addition to general-purpose applications, the two on-chip

comparators can be used to decode video sync pulses from the

received common-mode voltages or to receive differential digital

information. Built-in hysteresis helps to eliminate false triggers

from noise.

The comparator outputs are designed to drive source-terminated

transmission lines. The source termination technique uses a resistor

in-series with each comparator output such that the sum of the

comparator source resistance (≈20 Ω) and the series resistor

equals the transmission line characteristic impedance. The load

end of the transmission line is high impedance. When the signal

is launched into the source termination, its initial value is one-

half of its source value because its amplitude is divided-by-2 by

the voltage divider formed by the source termination and the

transmission line. At the load, the signal experiences nearly

100% positive reflection due to the high impedance load and

is restored to nearly its full value. This technique is commonly

used in PCB layouts that involve high speed digital logic.

An internal linear voltage regulator derives power for the

comparators from the positive supply; therefore, the AD8145

must always have a minimum positive supply voltage of 4.5 V.

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