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AD4680/AD4681

Data Sheet

Rev. 0 | Page 16 of 29

APPLICATIONS INFORMATION

Figure 34 shows an example of a typical application circuit for

the AD4680/AD4681. Decouple the VCC, VLOGIC, REGCAP, and

REFIO pins with suitable decoupling capacitors as shown.

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 optimal performance is achieved. On a typical application,

it is recommended that resistance (R) = 33 Ω, C1 = 68 pF, and

C2 = 330 pF.

The performance of the AD4680/AD4681devices may be

impacted by noise on the digital interface. This impact depends

on the on-board layout and design. Keep a minimal distance

between the digital line and 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 the digital interface coupling of the

AD4680/AD4681.

The two differential channels of the AD4680/AD4681 can

accept an input voltage range from 0 V to VREF, and have a wide

common-mode range that allows the conversion of a variety of

signals. These analog input pins can easily be driven with an

amplifier. Table 9 lists the recommended driver amplifiers that

best fit and add value to the application.

The AD4680 and the AD4681 have a buffered internal 2.5 V

reference that can be accessed via the REFIO pin. The buffered

internal 2.5V reference must use an external buffer like the

ADA4807-2, when connecting it to the external circuitry. Both

devices have an option to use an ultralow noise, high accuracy

voltage reference as an external voltage source, ranging from

2.5 V to 3.3 V, such as the ADR4533 and ADR4525.

POWER SUPPLY

The typical application circuit in Figure 34 can be powered by

a single 5 V (V+) voltage source that supplies the entire signal

chain. The 5 V supply can come from a low noise, CMOS, low

dropout (LDO) regulator (ADP7105). The driver amplifier

supply is supplied by the 5 V (V+) and −2.5 V (V−) derived

from the inverter (ADM660), which then converts the +5 V to

−5 V, then to the ADP7182 low noise voltage regulator to output

the −2.5 V. The two independent supplies of the AD4680/

AD4681, VCC and VLOGIC, that supply the analog circuitry and

digital interface, respectively, can be supplied by a low

quiescent current LDO regulator such as the ADP166. The

ADP166 is a suitable supply with a fixed output voltage range

from 1.2 V to 3.3 V for typical VCC and VLOGIC levels. Decouple

both the VCC supply and the VLOGIC supply separately with a 1 μF

capacitor. Additionally, an internal LDO regulator supplies the

AD4680/AD4681. The on-chip regulator provides a 1.9 V supply

for internal use on the device only. Decouple the REGCAP pin

with a 1 μF capacitor connected to GND.

Power-Up

The AD4680/AD4681 are not easily damaged by power supply

sequencing. VCC and VLOGIC can be applied in any sequence. An

external reference must be applied after VCC and VLOGIC are

applied.

The AD4680/AD4681 require a tPOWERUP time from applying

VCC and VLOGIC until the ADC conversion results are stable.

Applying CS pulses or interfacing with the AD4680/AD4681

prior to the setup time elapsing does not have a negative impact

on ADC operation. Conversion results are not guaranteed to

meet data sheet specifications during this time, however, and

must be ignored.

Table 9. Signal Chain Components

Companion Devices

Part Name

Description

Typical Application

ADC Driver

ADA4896-2

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

Precision, low noise, high frequency

ADA4940-2

Ultralow power, full differential, low distortion

Precision, low density, low power

ADA4807-2

1 mA, rail-to-rail output amplifier

Precision, low power, high frequency

LTC6227

Low distortion rail-to-rail output op amp

Precision, low noise, high frequency

External Reference

ADR4525

Ultralow noise, high accuracy voltage reference

2.5 V reference voltage

ADR4533

Ultralow noise, high accuracy voltage reference

3.3 V reference voltage

LDO Regulator

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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