LTC2424CG#PBF Linear Technology, LTC2424CG#PBF Datasheet - Page 31

• Manufacturer Part Number: LTC2424CG#PBF
• Description: IC ADC 20BIT 4CH MICROPWR 28SSOP
• Manufacturer: Linear Technology

Specifications of LTC2424CG#PBF

• Number Of Bits: 20
• Sampling Rate (per Second): 7.5
• Data Interface: MICROWIRE™, Serial, SPI™
• Number Of Converters: 1
• Power Dissipation (max): 1mW
• Voltage Supply Source: Single Supply
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP (0.200", 5.30mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

APPLICATIONS
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.
U
20k
20k
20k
20k
20k
20k
20k
INFORMATION
Figure 34. Using the Multiplexer to Produce Programmable Gains of 1 to 128
U
10k
10k
10k
10k
10k
10k
10k
10k
2
4
8
16
32
64
128
W
V
IN
10
11
12
13
14
15
17
9
5 ZS
3
2
U
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
–
+
LTC1050
SET
5V
8-CHANNEL
AV = 1, 2, 4...128
MUX
6
MUXOUT
1, 6, 16, 18, 22, 27, 28
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 34, 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
7
4
ADCIN
+
0.1V TO V
–
20-BIT
GND
LTC2428
ADC
3
FS
SET
CC
V
CC
2, 8
CSMUX
CSADC
LTC2424/LTC2428
SDO
SCK
CLK
24248 F34
D
F
IN
O
26
23
20
25
19
21
24
5V
V
1 F
CC
31