Datasheet

LTC6802-2
30
68022fa
applicaTions inForMaTion
ADVANTAGES OF DELTA-SIGMA ADCS
The LTC6802-2 employs a delta-sigma analog-to-digital
converter for voltage measurement. The architecture of
delta-sigma converters can vary considerably, but the
common characteristic is that the input is sampled many
times over the course of a conversion and then filtered or
averaged to produce the digital output code. In contrast,
a SAR converter takes a single snapshot of the input
voltage and then performs the conversion on this single
sample. For measurements in a noisy environment, a
delta-sigma converter provides distinct advantages over
a SAR converter.
While SAR converters can have high sample rates, the full-
power bandwidth of a SAR converter is often greater than
1MHz, which means the converter is sensitive to noise out
to this frequency. And many SAR converters have much
higher bandwidths—up to 50MHz and beyond. It is pos-
sible to filter the input, but if the converter is multiplexed
V
+
C12
S12
C11
S11
C10
S10
C9
S9
C8
S8
C7
S7
C6
S6
C5
S5
C4
S4
C3
S3
C2
0V TO 5.5V
0V TO 5.5V
0V TO 5.5V
0V TO 5.5V
0V TO 5.5V
0V TO 5.5V
0V TO 5.5V
0V TO 5.5V
0V TO 5.5V
0V TO 5.5V
0V TO 5.5V
0V TO 5.5V
0V TO 5.5V
5.5V
3.1V
1.5V
1.5V
0V
0V
3.6V
3.6V
7.2V
CSBI
SDO
SDI
SCKI
A3
A2
A1
A0
GPIO2
GPIO1
WDTB
MMB
TOS
V
REG
V
REF
V
TEMP2
V
TEMP1
NC
V
–
S1
C1
S2
43.2V
43.2V
43.2V
39.6V
39.6V
36V
36V
32.4V
32.4V
28.8V
28.8V
25.2V
25.2V
21.6V
21.6V
18V
18V
14.4V
14.4V
10.8V
10.8V
7.2V
LTC6802-2
68022 F17
Figure 17. Typical Pin Voltages for 12 3.6V Cells
to measure several input channels a separate filter will be
required for each channel. A low frequency filter cannot
reside between a multiplexer and an ADC and achieve a
high scan rate across multiple channels. Another conse-
quence of filtering a SAR ADC is that any noise reduction
gained by filtering the input cancels the benefit of having
a high sample rate in the first place, since the filter will
take many conversion cycles to settle.
For a given sample rate, a delta-sigma converter can
achieve excellent noise rejection while settling completely
in a single conversion—something that a filtered SAR con-
verter cannot do. Noise rejection is particularly important
in high voltage switching controllers, where switching
noise will invariably be present in the measured voltage.
Other advantages of delta sigma converters are that they
are inherently monotonic, meaning they have no missing
codes, and they have excellent DC specifications.