Datasheet

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 (t
PSKCD
). 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
VIO
SDO
V
DD1
, V
E1
GND
1
V
DD2
, V
E2
GND
2
SCK
CNV
SDI
AD7685
1/4 AD8618
5V
1kΩ
2V REF
±10V INPUT
5V REF 5
V
5V
10µF 100nF
100nF
IN– GND
REF VDD
VIO
SDO
SCK
CNV
SDI
AD7685
1/4 AD8618
5V
1kΩ
2V REF
±10V INPUT
5V REF 5V
10µF 100nF
IN– GND
REF VDD
VIO
SDO
SCK
CNV
SDI
AD7685
1/4 AD8618
5V
1kΩ
2V REF
±10V INPUT
5V REF 5V
10µF 100nF
IN–
IN+
IN+
IN+
IN+
GND
REF VDD
VIO
SDO
SCK
CNV
SDI
AD7685
1/4 AD8618
5V
1kΩ4kΩ
4kΩ
4kΩ
4kΩ
2V REF
5V
5V
1kΩ1kΩ
1kΩ
100nF10µF
5V REF
2V REF
4kΩ
±10V INPUT
5V REF 5V
10µF 100nF
V
IA
V
IB
V
OC
V
OD
V
OA
V
OB
V
IC
V
ID
2.7V TO 5V
100nF
DATA
SCKOUT
SCKIN
CONVERT
ADuM1402C
OUT
GND
IN
ADR391
02968-046
Figure 48. A True 16-Bit Isolated Simultaneous Sampling Acquisition System