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Data Sheet

AD4684/AD4685

APPLICATIONS INFORMATION

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Rev. 0 | 16 of 32

Figure 33 shows an example of a complete signal chain connec-

tion diagram for the AD4684/AD4685. Decouple the VCC, VLOGIC,

REGCAP, and REFIO pins with suitable decoupling capacitors, as

shown in Figure 33.

The exposed pad is a ground reference point for circuitry on the

device and must be connected to the board ground.

A differential RC filter must be placed on the analog inputs to

ensure performance is achieved. For a typical application, the

recommended resistor is R = 33 Ω, and C = 330 pF.

The performance of the AD4684/AD4685 can be impacted by noise

on the digital interface. This impact is dependent on board layout

and design. Keep a minimal distance of the digital line to the digital

interface or place a 100 Ω resistor in series and close to the SDOA

pin and SDOB/ALERT pin to reduce noise from digital interface

coupling of the AD4684/AD4685.

Each of the single-ended analog inputs of the AD4684/AD4685 can

accept a voltage from 0 V to VREF and can easily be driven by an

amplifier for optimum performance. Table 8 shows the recommend-

ed components for the complete signal chain solution that can best

fit the application for the AD4684/AD4685.

The AD4684/AD4685 have a buffered internal 2.5 V reference that

can be access via the REFIO pin. The buffered internal 2.5 V

reference must use an external buffer like the ADA4807, when

connecting it to the external circuitry. Both devices have the option

to use an ultralow noise, high accuracy voltage reference ranging

from 2.5 V to 3.3 V, such as the ADR4525 or ADR4533, as an

external voltage source.

Figure 33. Typical Application Circuit (VCM Is Common-Mode Voltage, See the Power Supply Section for Additional Information on V+ and V−)

Table 8. Signal Chain Components

Companion Devices

Device Name

Description

Typical Application

ADC Driver

ADA4896-2

1 nV/√Hz, rail-to-rail output amplifier

Precision, low noise, high frequency

ADA4807-2

1 mA, rail-to-rail output amplifier

Precision, low power, high frequency

ADA4940-2

Ultra low power, fully differential, low distortion

Precision, low density, low power

LTC6227

Low distortion rail-to-rail output op amps

Precision, low noise, high frequency

External Reference

ADR4525

Ultralow noise, high accuracy, 2.5 V voltage reference

2.5 V reference voltage

ADR4533

Ultralow noise, high accuracy, 3.3 V voltage reference

3.3 V reference voltage

LDO

ADP166

Low quiescent, 150 mA, LDO regulator

3.0 V to 3.6 V supply for VCC and VLOGIC

ADP7104

Low noise, CMOS LDO regulator

5 V supply for driver amplifier

ADP7182

Low noise line regulator

−2.5 V supply for driver amplifier

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