AD7705 datasheet(31/32 Pages) AD

31 / 32 page

AD7705/AD7706

–31–

REV. A

Battery Monitoring

Another area where the low power, single supply operation is a

requirement is battery monitoring in portable equipment appli-

cations. Figure 26 shows a block diagram of a battery monitor

that includes the AD7705 and an external multiplexer used to

differentially measure the voltage across a single cell. The sec-

ond channel on the AD7705 is used to monitor current drain

from the battery. The AD7705 with its dual input channel is

ideally suited to measurement systems requiring two input chan-

nels, as in this case, to monitor voltage and current. Since the

AD7705 can accommodate very low input signals the RSENSE

VCELL 1

RSENSE

AIN1(+)

AIN1(–)

AIN2(+)

AIN2(–)

REF IN(–)

AD7705

VDD

REF IN(+)

1.225V

REF

VOLTAGE

REGULATORS

VCELL 2

VCELL 3

VCELL 4

VDIFF1

VDIFF2

VDIFF3

VDIFF4

DC BATTERY

CHARGING

SOURCE

LOAD

ON/OFF SWITCH

+5V

+3V

GND

4-TO-1

DIFFERENTIAL

MULTIPLEXER

Figure 26. Battery Monitoring Using the AD7705

can be kept low reducing undesired power dissipation. Operat-

ing 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. In order to obtain specified

performance in unbuffered mode, the common mode range of

the input is from GND to VDD provided that the absolute value

of the analog input voltage lies between GND – 30 mV and

VDD + 30 mV. Absolute voltages of GND – 200 mV can be

accommodated on the AD7705 at 25

°C without any degrada-

tion in performance, but leakage current increases significantly

at elevated temperatures.

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