26
LTC2404/LTC2408
APPLICATIONS INFORMATION
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ANTIALIASING
One of the advantages delta-sigma ADCs offer over con-
ventional ADCs is on-chip digital filtering. Combined with
a large oversampling ratio, the LTC2404/LTC2408 signifi-
cantly simplify antialiasing filter requirements.
The digital filter provides very high rejection except at
integer multiples of the modulator sampling frequency
(f
S
), see Figure 23. The modulator sampling frequency is
256 • F
O
, where F
O
is the notch frequency (typically 50Hz
or 60Hz). The bandwidth of signals not rejected by the
digital filter is narrow (≈0.2%) compared to the bandwidth
of the frequencies rejected.
As a result of the oversampling ratio (256) and the digital
filter, minimal (if any) antialias filtering is required in front
of the LTC2404/LTC2408. If passive RC components are
placed in front of the LTC2404/LTC2408, the input dy-
namic current should be considered (see Input Current
section). In cases where large effective RC time constants
are used, an external buffer amplifier may be required to
minimize the effects of input dynamic current.
The modulator contained within the LTC2404/LTC2408
can handle large-signal level perturbations without satu-
rating. Signal levels up to 40% of V
REF
do not saturate the
analog modulator. These signals are limited by the input
ESD protection to 300mV below ground and 300mV above
V
CC
.
The LTC2408’s Resolution and Accuracy Allows You
to Measure Points in a Ladder of Sensors
In many industrial processes, for example, cracking tow-
ers in petroleum refineries, a group of temperature mea-
surements must be related to one another. A series of
platinum RTDs that sense slow changing temperatures
can be configured into a resistive ladder, using the LTC2408
to sense each node. This approach allows a single excita-
tion current passed through the entire ladder, reducing
total supply current consumption. In addition, this ap-
proach requires only one high precision resistor, thereby
reducing cost. A group of up to seven temperatures can be
measured as a group by a single LTC2408 in a loop-pow-
ered remote acquisition unit. In the example shown in
Figure 24, the excitation current is 240µA at 0°C. The
LTC2408 requires 300µA, leaving nearly 3.5mA for the
remainder of the remote transmitter.
The resistance of any of the RTDs (PT1 to PT7) is deter-
mined from the voltage across it, as compared to the
voltage drop across the reference resistor (R1). This is a
ratiometric implementation where the voltage drop across
R1 is given by V
REF
– V
CH1
. Channel 7 is used to measure
the voltage on a representative length of wire. If the same
type and length of wire is used for all connections, then
errors associated with the voltage drops across all wiring
can be removed in software. The contribution of wiring
drop can be scaled if wire lengths are not equal.
Gain can be added to this circuit as the total voltage drop
across all the RTDs is small compared to ADC full-scale
range. The maximum recommended gain is 40, as limited
by both amplifier noise contribution, as well as the maxi-
mum voltage developed at CH0 when all sensors are at the
maximum temperature specified for platinum RTDs.
Adding gain requires that one of the resistors (PT1 to PT7)
be a precision resistor in order to eliminate the error asso-
ciated with the gain setting resistors R2 and R3. Note, that
if a precision (100Ω to 400Ω) resistor is used in place of
one of the RTDs (PT7 recommended), R1 does not need
to be a high precision resistor. Although the substitution
of a precision reference resistor for an RTD to determine
gain may suggest that R2 and R3 (and R1) need not be
precise, temperature fluctuations due to airflow may ap-
pear as noise that cannot be removed in firmware. Conse-
Figure 23. Sinc
4
Filter Rejection
INPUT FREQUENCY
0
–60
–40
0
24048 F23
–80
–100
f
S
/2 f
S
–120
–140
–20
REJECTION (dB)
