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LTC2424/LTC2428

APPLICATIONS INFORMATION

WUU

Figure 34. Using the Multiplexer to Produce Programmable Gains of 1 to 128

24248 F34

CH0

CH1

CH2

CH3

CH4

CH5

CH6

CH7

–

8-CHANNEL

MUX

5ZS

SET

GND

1, 6, 16, 18, 22, 27, 28

CSADC

CSMUX

SCK

CLK

SDO

LTC2428

ADCINMUXOUT

128

AV = 1, 2, 4...1283

10k

10k20k

3 2, 8

1µF

SET

–

LTC1050

0.1V TO V

20-BIT

∆Σ ADC

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-

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 1000Ω 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 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

1 2 ... 26 27 28 29 30 31 32 33 34 35 36

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