Current accumulator, Figure 4. current accumulator format – Rainbow Electronics DS2740 User Manual
Page 5
DS2740
5 of 15
CURRENT RESOLUTION (1 LSB)
R
SNS
PART
CONVERSION
TIME
V
IS1
- V
IS2
20m
W
15m
W
10m
W
5m
W
DS2740
3.515s
1.5625
mV 78.13mA 104.2mA
156.3
mA
312.5
mA
DS2740B
0.878s
6.250
mV
312.5
mA 416.7mA
625
mA
1.250mA
Every 1024th conversion, the ADC measures its input offset to facilitate offset correction. Offset
correction occurs approximately once per hour in the DS2740 and four times per hour in the DS2740B.
The resulting correction factor is applied to the subsequent 1023 measurements. During the offset
correction conversion, the ADC does not measure the IS1 to IS2 signal. A maximum error of 1/1024 in
the accumulated current register (ACR) is possible, however, to reduce the error, the current measurement
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. The typical error due to offset correction is
much less than 1/1024.
CURRENT ACCUMULATOR
Current measurements are internally summed, or accumulated, at the completion of each conversion
period with the results displayed in the ACR. The accuracy of the ACR is dependent on both the current
measurement and the conversion timebase. The ACR has a range of ±204.8mVh with an LSb of
6.25µVh
. Additional registers hold fractional results of each accumulation, however, these bits are not
user accessible.
Read and write access is allowed to the ACR. Whenever the ACR is written, fractional accumulation
results are cleared. Also, a write forces the ADC to measure its offset and update the offset correction
factor. The current measurement and accumulation begin with the second conversion following a write to
the ACR. Figure 4 describes the ACR address, format, and resolution.
Figure 4. CURRENT ACCUMULATOR FORMAT
MSB—Address 10
LSB—Address 11
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
Units: 6.25
mVh/Rsns
ACR LSB
R
SNS
PART
UPDATE
INTERVAL
V
IS1
- V
IS2
20m
W
15m
W
10m
W
5m
W
DS2740
3.515s
DS2740B
0.878s
6.25
mVh
312.5
mAh
416.7
mAh
625
mAh
1.250mAh