
DS2782: Stand-Alone Fuel Gauge IC
9 of 25
CURRENT MEASUREMENT
In the ACTIVE mode of operation, the DS2782 continually measures the current flow into and out of the battery by
measuring the voltage drop across a low-value current-sense resistor, R
SNS
. The voltage-sense range between
SNS and VSS is ±51.2mV. The input linearly converts peak signal amplitudes up to 102.4mV as long as the
continuous signal level (average over the conversion cycle period) does not exceed ±51.2mV. The ADC samples
the input differentially at 18.6kHz and updates the Current register at the completion of each conversion cycle.
The Current register is updated every 3.515s with the current conversion result in two’s complement form. Charge
currents above the maximum register value are reported at the maximum value (7FFFh = +51.2mV). Discharge
currents below the minimum register value are reported at the minimum value (8000h = -51.2mV).
Figure 6. Current Register Format
CURRENT
Read Only
MSB—Address 0Eh
LSB—Address 0Fh
S
2
14
2
13
2
12
2
11
2
10
2
9
2
8
2
7
2
6
2
5
2
4
2
3
2
2
2
1
2
0
MSb
“S”: sign bit(s)
LSb
MSb
LSb
Units:
1.5625 V/Rsns
CURRENT RESOLUTION (1 LSB)
R
SNS
VSS
-
VSNS
20m
78.13 A
15m
104.2 A
10m
156.3 A
5m
312.5 A
1.5625 V
AVERAGE CURRENT MEASUREMENT
The Average Current register reports an average current level over the preceding 28 seconds. The register value is
updated every 28s in two’s complement form, and is the average of the 8 preceding Current register updates. The
format of the Average Current register is shown in Figure 7. Charge currents above the maximum register value
are reported at the maximum value (7FFFh = +51.2mV). Discharge currents below the minimum register value are
reported at the minimum value (8000h = -51.2mV).
Figure 7. Average Current Register Format
IAVG
R/W
MSB—Address 08h
LSB—Address 09h
S
2
14
2
13
2
12
2
11
2
10
2
9
2
8
2
7
2
6
2
5
2
4
2
3
2
2
2
1
2
0
MSb
“S”: sign bit(s)
LSb
MSb
LSb
Units:
1.5625 V/Rsns
CURRENT OFFSET CORRECTION
Every 1024th conversion, the ADC measures its input offset to facilitate offset correction. Offset correction occurs
approximately once per hour. The resulting correction factor is applied to the subsequent 1023 measurements.
During the offset correction conversion, the ADC does not measure the sense resistor signal. A maximum error of
1/1024 in the accumulated current register (ACR) is possible; however, to reduce the error, the current
measurement made just prior to the offset conversion is displayed in the current register and is substituted for the
dropped current measurement in the current accumulation process. This results in an accumulated current error
due to offset correction of less than 1/1024.