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LTC2424 Datasheet(PDF) 24 Page - Linear Technology |
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LTC2424 Datasheet(HTML) 24 Page - Linear Technology |
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24 / 28 page  24 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 Ω 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 1000 Ω 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 APPLICATIONS INFORMATION 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 ∆Σ ADC. 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 |
Similar Part No. - LTC2424 |
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Similar Description - LTC2424 |
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