Rainbow Electronics DS2781 User Manual
Page 8
DS2781: Two-Cell Standalone Fuel Gauge IC
8 of 30
Figure 5. 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
LSb
MSb LSb
“S”: sign bit(s)
Units: 1.5625
mV/Rsns
CURRENT RESOLUTION (1 LSB)
R
SNS
VSS
-
VSNS
20m
W 15mW 10mW 5mW
1.5625
mV 78.13mA 104.2mA 156.3mA 312.5mA
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.
CURRENT OFFSET BIAS
The Current Offset Bias (COB) register allows a programmable offset value to be added to raw current
measurements. The result of the raw current measurement plus COB is displayed as the current measurement
result in the CURRENT register, and is used for current accumulation. COB can be used to correct for a static
offset error, or can be used to intentionally skew the current results and therefore the current accumulation.
Read and write access is allowed to COB. Whenever the COB is written, the new value is applied to all subsequent
current measurements. COB can be programmed in 1.56
mV steps to any value between +198.1mV and -199.7mV.
The COB value is stored as a two’s complement value in volatile memory, and must be initialized through the
interface on power-up.
Figure 6. Current Offset Bias Register Format
Address
7B
S 2
6
2
5
2
4
2
3
2
2
2
1
2
0
MSb
LSb
“S”:
sign
bit(s)
Units:
1.56
mV/Rsns