AD8021 datasheet(25/28 Pages) AD

25 / 28 page

AD7685

Rev. C | Page 25 of 28

TRUE 16-BIT ISOLATED APPLICATION EXAMPLE

In applications where high accuracy and isolation are required,

for example, power monitoring, motor control, and some

medical equipment, the circuit given in Figure 48, using the

AD7685 and the ADuM1402C digital isolator, provides a

compact and high performance solution.

Multiple AD7685s are daisy-chained to reduce the number of

signals to isolate. Note that the SCKOUT, which is a readback of

the AD7685’s clock, has a very short skew with the DATA

signal. This skew is the channel-to-channel matching

propagation delay of the digital isolator (tPSKCD). This allows

running the serial interface at the maximum speed of the digital

isolator (45 Mbps for the ADuM1402C), which would have

been otherwise limited by the cascade of the propagation delays

of the digital isolator.

The complete analog chain runs on a 5 V single supply using

the ADR391 low dropout reference voltage and the rail-to-rail

CMOS AD8618 amplifier while offering true bipolar input range.

IN– GND

REF VDD VIOSDO

VDD1, VE1

GND1

VDD2, VE2

GND2

SCK

CNV

SDI

AD7685

1/4 AD8618

1kΩ

2V REF

±10V INPUT

5V REF

10µF

100nF

IN– GND

REF VDD VIOSDO

SCK

CNV

SDI

AD7685

1/4 AD8618

1kΩ

2V REF

±10V INPUT

5V REF

10µF

100nF

IN– GND

REF VDD VIOSDO

SCK

CNV

SDI

AD7685

1/4 AD8618

1kΩ

2V REF

±10V INPUT

5V REF

10µF

100nF

IN–

IN+

GND

REF VDD VIOSDO

SCK

CNV

SDI

AD7685

1/4 AD8618

1kΩ

4kΩ

2V REF

1kΩ

100nF

10µF

5V REF

2V REF

4kΩ

±10V INPUT

5V REF

10µF

100nF

VIA

VIB

VOC

VOD

VOA

VOB

VIC

VID

2.7V TO 5V

100nF

DATA

SCKOUT

SCKIN

CONVERT

ADuM1402C

OUT

GND

ADR391

Figure 48. A True 16-Bit Isolated Simultaneous Sampling Acquisition System

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