Average current measurement, Current offset correction, Current offset bias – Rainbow Electronics DS2788 User Manual
Page 9: Current measurement calibration
Average Current Measurement
The Average Current (IAVG) register reports an aver-
age current level over the preceding 28 seconds. The
register value is updated every 28s in two’s comple-
ment form, and is the average of the eight preceding
Current register updates. Figure 6 shows the format of
the Average Current register. Charge currents above
the maximum register value are reported at the maxi-
mum value (7FFFh = +51.2mV). Discharge currents
below the minimum register value are reported at the
minimum value (8000h = -51.2mV).
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 cor-
rection factor is applied to the subsequent 1023 mea-
surements. During the offset correction conversion, the
ADC does not measure the sense resistor signal. A
maximum error of 1/1024 in the Accumulated Current
(ACR) register 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 measure-
ment 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 pro-
grammable offset value to be added to raw current mea-
surements. 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 cur-
rent results and therefore the current accumulation.
COB allows read and write access. Whenever the COB
is written, the new value is applied to all subsequent
current measurements. COB can be programmed in
1.56µV steps to any value between +198.1µV and -
199.7µV. The COB value is stored as a two’s comple-
ment value in nonvolatile memory.
Current Measurement
Calibration
The DS2788’s current measurement gain can be
adjusted through the RSGAIN register, which is factory-
calibrated to meet the data sheet specified accuracy.
RSGAIN is user accessible and can be reprogrammed
after module or pack manufacture to improve the cur-
rent measurement accuracy. Adjusting RSGAIN can
correct for variation in an external sense resistor’s nom-
inal value, and allows the use of low-cost, nonprecision
D
S
2
7
8
8
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
_______________________________________________________________________________________
9
IAVG
READ ONLY
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
LSb
MSb
LSb
“S”: SIGN BIT(S)
UNITS: 1.5625µV/R
SNS
Figure 6. Average Current Register Format
ADDRESS 7Bh
S
2
6
2
5
2
4
2
3
2
2
2
1
2
0
MSb
LSb
“S”: SIGN BIT(S)
UNITS: 1.56µV/R
SNS
Figure 7. Current Offset Bias Register Format
COB
R/W AND EE