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LTC2404C 数据表(PDF) 26 Page - Linear Technology |
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LTC2404C 数据表(HTML) 26 Page - Linear Technology |
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26 / 36 page  26 LTC2404/LTC2408 APPLICATIONS INFORMATION 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 (fS), see Figure 23. The modulator sampling frequency is 256 • FO, where FO is the notch frequency (typically 50Hz or 60Hz). The bandwidth of signals not rejected by the digital filter is narrow ( ≈0.2%)comparedtothebandwidth 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 VREF do not saturate the analog modulator. These signals are limited by the input ESD protection to 300mV below ground and 300mV above VCC. 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 240 µA at 0°C. The LTC2408 requires 300 µA, 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 VREF – VCH1. 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 (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- Figure 23. Sync4 Filter Rejection INPUT FREQUENCY 0 –60 –40 0 24048 F23 –80 –100 fS/2 fS –120 –140 –20 |
类似零件编号 - LTC2404C |
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类似说明 - LTC2404C |
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