LTC2424 Linear Technology, LTC2424 Datasheet - Page 24

• Manufacturer Part Number: LTC2424
• Description: 4-/8-Channel 20-Bit uPower No Latency ADCs
• Manufacturer: Linear Technology

APPLICATIONS
LTC2424/LTC2428
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 50, 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 (100 to 400 ) 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-
quently, these resistors should be low temperature coef-
ficient devices. The use of higher resistance RTDs is not
recommended in this topology, although the inclusion of
one 1000 RTD at the top on the ladder will have minimal
impact on the lower elements. The same caveat applies to
fast changing temperatures. Any fast changing sensors
should be at the top of the ladder.
The LTC2428’s Uncommitted Multiplexer Finds Use in
a Programmable Gain Scheme
If the multiplexer in the LTC2428 is not committed to
channel selection, it can be used to select various signal-
processing options such as different gains, filters or at-
tenuator characteristics. In Figure 23, the multiplexer is
shown selecting different taps on an R/2R ladder in the
feedback loop of an amplifier. This example allows selec-
tion of gain from 1 to 128 in binary steps. Other feedback
networks could be used to provide gains tailored for
specific purposes. (For example, 1x, 1.1x, 1.41x, 2x,
2.028x, 5x, 10x, 40x, etc.) Alternatively, different bandpass
characteristics or signal inversion/noninversion could be
selected. The R/2R ladder can be purchased as a network
to ensure tight temperature tracking. Alternatively, resis-
tors in a ladder or as separate dividers can be assembled
from discrete resistors. In the configuration shown, the
channel resistance of the multiplexer does not contribute
much to the error budget, as only input op amp current
24
U
INFORMATION
U
W
U
flows through the switch. The LTC1050 was chosen for
its low input current and offset voltage, as well as its
ability to drive the input of a
Insert Gain or Buffering After the Multiplexer
Separate MUXOUT and ADCIN terminals permit insertion
of a gain stage between the MUX and the ADC. If passive
filtering is used at the input to the ADC, a buffer amplifier
is strongly recommended to avoid errors resulting from
the dynamic ADC input current. If antialiasing is required,
it should be placed at the input to the MUX. If bandwidth
limiting is required to improve noise performance, a filter
with a –3dB point at 1500Hz will reduce the effective total
noise bandwidth of the system to 15Hz. A roll-off at 1500Hz
eliminates all higher order images of the base bandwidth
of 6Hz. In the example shown, the optional bandwidth-
limiting filter has a – 3dB point at 1450Hz. This filter can be
inserted after the multiplexer provided that higher source
impedance prior to the multiplexer does not reduce the
– 3dB frequency, extending settling time, and resulting in
charge sharing between samples. The settling time of this
filter to 20+ bits of accuracy is less than 2ms. In the pres-
ence of external wideband noise, this filter reduces the
apparent noise by a factor of 5. Note that the noise band-
width for noise developed in the amplifier is 150Hz. In the
example shown, the gain of the amplifier is set to 40, the
point at which amplifier noise gain dominates the LTC2428
noise. Input voltage range as shown is then 0V to 125mV
DC. The recommended capacitor at C2 for a gain of 40
would be 560pF.
An 8-Channel DC-to-Daylight Digitizer
The circuit in Figure 25 shows an example of the LTC2428’s
flexibility in digitizing a number of real-world physical
phenomena—from DC voltages to ultraviolet light. All of
the examples implement single-ended signal condition-
ing. Although differential signal conditioning is a pre-
ferred approach in applications where the sensor is a
bridge-type, is located some distance from the ADC or
operates in a high ambient noise environment, the
LTC2428’s low power dissipation allows circuit operation
in close proximity to the sensor. As a result, conditioning
the sensor output can be greatly simplified through the
ADC.