AD7705/AD7706
Rev. C | Page 41 of 44
BATTERY MONITORING
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.
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.
Because the AD7705 can accommodate very low input signals,
R
SENSE
can be kept low, reducing undesired power dissipation.
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 V
DD
, provided that
the absolute value of the analog input voltage lies between
GND − 100 mV and V
DD
+ 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.
VCELL 1
R
SENSE
AIN1(+)
AIN1(–)
AIN2(+)
AIN2(–)
REF IN(–)
AD7705
V
DD
REF IN(+)
1.225V
REF
VOLTAGE
REGULATORS
VCELL 2
VCELL 3
VCELL 4
VDIFF1
VDIFF2
VDIFF3
VDIFF4
DC BATTER
CHARGING
SOURCE
LOAD
ON/OFF SWITCH
5V
3V
GND
4-TO-1
DIFFERENTIAL
MULTIPLEXER
01166-029
Figure 29. Battery Monitoring Using the AD7705
