AD8260ACPZ-R7 datasheet(21/32 Pages) AD

21 / 32 page

AD8260

Rev. 0 | Page 21 of 32

1.5kΩ

1kΩ

1kΩ 1.5kΩ

VMID

×1

AD8260

VMDO

TXEN

VMDI

VNCM

VPSB

ENBL

VGAP

VGAN

VOCM

INPP

TXOP

VPOS

VPSR

VMDO

PRAI

FDBK

50Ω

0.1µF

0V, 1.8V/3.3V

3.3V

0.1µF

1.8V OR 3.3V

ADC

FS INPUT

2V p-p

INP

INN

3.3V

RFB1

100Ω

RFB2

100Ω

0.1µF

3.3V

0.1µF

POWERLINE

CABLE, ETC.

OPTIONAL CLAMP DIODES

AND SNUBBING RESISTORS

0.1µF

50Ω

1.8V OR 3.3V

20mA DAC

1V MAX WITH

200mA pk

BIAS

ATTENUATOR

GM STAGES

LOGIC

CFB

OPTIONAL USER SELECTED

CFB REDUCES HF PEAKING

WITH CAPACITIVE LOADING

LOW-PASS

AA FILTER

+–

Figure 64. Block Diagram and Basic Application Connections

HIGH CURRENT DRIVER AMPLIFIER

The high current driver amplifier can deliver very large output

currents suitable for driving complex impedances, such as a

power line, a 50 Ω line, or a coaxial cable. The input of the

amplifier is fully differential and intended to be driven by a

differential current-output DAC, as shown in Figure 64. The

differential input signal is amplified by 1.5× and produces a

2.25 V p-p single-ended output signal from a 1.5 V p-p input

signal. A DAC with 15 mA maximum output current into a

50 Ω load provides 1.5 V p-p of input voltage and results in

2.25 V p-p at the output. A DAC whose output is 20 mA produces

an output swing of 3 V p-p (neglecting a small gain error when

driving the parallel combination of the 50 Ω load-resistor and the

internal 1 kΩ gain resistor of the AD8260).

For a 3.3 V supply rail, the maximum limit of the output voltage

is 3 V p-p and distorts severely if exceeded. The recommended

output for optimum distortion is 2 V p-p for a 3.3 V supply.

Correspondingly, larger output swings are accommodated for

higher supply voltages such as +5 V or ±5 V.

For optimum distortion, the input drive must be controlled

such that the output swing is well within saturation levels

established by the supply rail. The output swing can be reduced

by using load resistors with values less than 50 Ω or by reducing

the amplifier gain by connecting external resistors in parallel

with the internal 1 kΩ and 1.5 kΩ resistors between Pin 27,

Pin 28, and Pin 29, and between Pin 30, Pin 31, and Pin 32.

Coincidently, noise is reduced because the gain setting resistors

are the primary noise sources of the high current driver amplifier.

The output-referred noise is 14 nV/√Hz, of which 11 nV/√Hz is

due to the gain setting resistors. Matching of the gain setting

resistors is important for good common-mode rejection and the

accuracy of the differential gain. If external resistors are used,

their accuracy should be at least ±1%. How low the resistor

values can be is primarily determined by the quality of the ac

ground at Pin VOCM; as the gain setting resistors decrease in

value, the dynamic current increases, and the quality of the

decoupling capacitors needs to increase correspondingly.

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