AD7705/AD7706
Because the AD7705 can accommodate very low input signals,
BATTERY MONITORING
R
SENSE can be kept low, reducing undesired power dissipation.
An application where the low power, single-supply operation is
required is battery monitoring in portable equipment applications.
Figure 29 shows a block diagram of a battery monitor using the
AD7705 and an external multiplexer to measure differentially
the voltage across a single cell.
Operating with a gain of 128, a 9.57 mV full-scale signal can
be measured with a resolution of 2 μV, giving 13.5 bits of flicker-
free performance in such a system.
To obtain specified performance in unbuffered mode, the
common-mode range of the input is GND to VDD, provided that
the absolute value of the analog input voltage lies between
GND − 100 mV and VDD + 30 mV. Absolute voltages of
GND − 200 mV can be accommodated on the AD7705 at 25°C
without any degradation in performance, but with significantly
increased leakage at elevated temperatures.
The second channel on the AD7705 is used to monitor current
drain from the battery. The AD7705, with its dual-input
channel, is ideally suited for measurement systems requiring
two input channels, as in this case, to monitor voltage and
current.
ON/OFF SWITCH
4-TO-1
DIFFERENTIAL
MULTIPLEXER
VCELL 1
3V
5V
VOLTAGE
REGULATORS
VDIFF1
VCELL 2
VDIFF2
V
DD
DC BATTERY
CHARGING
SOURCE
AD7705
VCELL 3
VDIFF3
VDIFF4
AIN1(+)
1.225V
REF
REF IN(+)
LOAD
AIN1(–)
AIN2(+)
VCELL 4
AIN2(–)
R
SENSE
REF IN(–)
GND
Figure 29. Battery Monitoring Using the AD7705
Rev. C | Page 41 of 44
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