AD9481 datasheet(18/29 Pages) AD

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AD9481

Rev. 0 | Page 17 of 28

APPLICATIONS

The AD9481 uses a 1.5 bit per stage architecture. The analog

inputs drive an integrated high bandwidth track-and-hold

circuit that samples the signal prior to quantization by the 8-bit

core. For ease of use, the part includes an on-board reference

and input logic that accepts TTL, CMOS, or LVPECL levels. The

digital output logic levels are CMOS-compatible.

ANALOG INPUTS

The analog input to the AD9481 is a differential buffer. For best

dynamic performance, impedances at VIN+ and VIN− should

match. Optimal performance is obtained when the analog

inputs are driven differentially. SNR and SINAD performance

can degrade if the analog input is driven with a single-ended

signal. The analog inputs self-bias to approximately 1.9 V; this

common-mode voltage can be externally overdriven by

approximately ±300 mV if required.

A wideband transformer, such as the Mini-Circuits ADT1-1WT,

can provide the differential analog inputs for applications that

require a single-ended-to-differential conversion. Note that the

filter and center-tap capacitor on the secondary side is optional

and dependent on application requirements. An RC filter at the

secondary side helps reduce any wideband noise getting aliased

by the ADC.

AD9481

VIN+

AVDD

(R, C OPTIONAL)

AGND

0.1

µF

10pF

Ω

49.9

Ω

VIN–

Figure 28. Driving the ADC with an RF Transformer

For dc-coupled applications, the AD8138/AD8139 or AD8351

can serve as a convenient ADC driver, depending on

requirements. Figure 29 shows an example with the AD8138.

The AD9481 PCB has an optional AD8351 on board, as shown

in Figure 39 and Figure 40. The AD8351 typically yields better

performance for frequencies greater than 30 MHz to 40 MHz.

The AD9481’s linearity and SFDR start to degrade at higher

analog frequencies (see the Typical Performance Characteristics

section). For higher frequency applications, the AD9480 with

LVDS outputs and superior AC performance should be

considered.

AD9481

AD8138

VIN+

AVDD

AGND

0.1

µF

20pF

Ω

VIN–

49.9

Ω

1.3k

Ω

499

Ω

499

Ω

523

Ω

499

Ω

Figure 29. Driving the ADC with the AD8138

The AD9481 can be easily configured for different full-scale

ranges. See the Voltage Reference section for more information.

Optimal performance is achieved with a 1 V p-p analog input.

SENSE = GND

VIN+

2.0V

500mV

2.0V

VIN–

DIGITALOUT = ALL 1s

DIGITALOUT = ALL 0s

Figure 30. Analog Input Full Scale

VOLTAGE REFERENCE

A stable and accurate 1.0 V reference is built into the AD9481.

Users can choose this internal reference or provide an external

reference for greater accuracy and flexibility. Figure 32 shows

the typical reference variation with temperature. Table 8

summarizes the available reference configurations.

SELECT

LOGIC

ADC

CORE

0.5V

Ω

VREF

0.1

µF

SENSE

VIN+

VIN–

Ω

Figure 31. Internal Reference Equivalent Circuit

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