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AD7381-4

Data Sheet

Rev. 0 | Page 16 of 31

APPLICATIONS INFORMATION

Figure 31 shows an example of a typical application circuit for

the AD7381-4. 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. In a typical

application, R = 33 Ω, C1 = 68 pF, and C2 = 68 pF are

recommended. These RC combinations must be the same for

all channels of the AD7381-4.

The four differential channels of the AD7381-4 can accept an

input voltage range from 0 V to VREF and have a wide common-

mode range to convert a variety of signals. These analog input

pins (AINx±) can easily be driven with an amplifier. Table 10

lists the recommended driver amplifiers that can best fit and

add value to the application.

The performance of the AD7381-4 device 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, SDOB, SDOC, and SDOD/ALERT pins to reduce

noise from the digital interface coupling of the AD7381-4.

The AD7381-4 can use an ultralow noise, high accuracy voltage

reference ranging from 2.5 V to 3.3 V, such as the ADR4533 or

ADR4525, as the external voltage source. A 1 µF reservoir

capacitor is recommended to be connected between the REFIO

pin and the ground. When using this external reference voltage

in another circuit within the application, for example, as a

common-mode voltage for the driver amplifier, it is

recommended to use a buffer amplifier like the ADA4807-2 for

a stable reference.

POWER SUPPLY

For a typical application, the AD7381-4 circuitry shown in

Figure 31 can be driven from a 5 V (V+) supply to power the

system. The 5 V (V+) can be supplied from the ADP7104. The

ADC driver can be supplied by a 5 V (V+) and a −2.5 V (V−)

derived from the inverting charge pump, the ADP5600, that

converts 5 V to −5 V, then to the ADP7182 for the low noise

voltage regulator to output −2.5 V. Two independent power

supply sources are derived from a low dropout (LDO) regulator

to power the VCC supply for the analog circuitry and the VLOGIC

supply for the digital interface of the AD7381-4. A very low

quiescent current LDO regulator like 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. The VCC supply and the VLOGIC

supply must be decoupled separately with a 1 µF capacitor.

Additionally, an LDO regulator supplies the AD7381-4. The on-

chip regulator provides a 1.9 V supply only for internal use on

the device. Decouple the REGCAP pin with a 1 µF capacitor to

GND.

Power-Up

The AD7381-4 is not easily damaged by power supply

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

external reference must be applied after VCC and VLOGIC are

applied. Analog and digital signals must be applied after the

external reference is applied.

The AD7381-4 requires tPOWERUP from applying VCC and VLOGIC

until the ADC conversion results are stable. Interfacing with

the AD7381-4 prior to the setup time elapsing does not have a

negative impact on ADC operation. See Figure 4 for the

recommended signal condition during power-up. It is highly

recommended to issue a software reset after power-up (see the

Software Reset section for details).Conversion results are not

guaranteed to meet data sheet specifications during this time,

however.

Table 10. 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

Ultra low power, full differential, low distortion amplifier

Precision, low density, low power

ADA4807-2

1 mA, rail-to-rail output amplifier

Precision, low power, 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

Reference Buffer

ADA4807-2

1 mA, rail-to-rail output amplifier

Precision, low power, high frequency

LDO Regulator

ADP166

Very 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 ADC driver amplifier

ADP5600

Interleaved inverting charge pump with negative LDO

Voltage inverter for negative supply

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