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LTC2421IMS Datasheet(PDF) 29 Page - Linear Technology |
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LTC2421IMS Datasheet(HTML) 29 Page - Linear Technology |
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29 / 32 page  29 LTC2421/LTC2422 24212f APPLICATIO S I FOR ATIO The absolute accuracy (less than 10 ppm total error) of the LTC2422 enables extremely accurate measurement of small signals sitting on large voltages. Each of the two pseudo differential measurements performed by the LTC2422 is absolutely accurate independent of the com- mon mode voltage output from the bridge. The pseudo differential result obtained from digitally subtracting the two single ended conversion results is accurate to within the noise level of the device (3 µVRMS) times the square root of 2, independent of the common mode input voltage. Typically, a bridge sensor outputs 2mV/V full scale. With a 5V excitation, this translates to a full-scale output of 10mV. Divided by the RMS noise of 8.4 µV(= 6µV • 1.414), this circuit yields 1190 counts with no averaging or ampli- fication. If more counts are required, several conversions may be averaged (the number of effective counts is in- creased by a factor of square root of 2 for each doubling of averages). An RTD Temperature Digitizer RTDs used in remote temperature measurements often have long lead lengths between the ADC and RTD sensor. These long lead lengths lead to voltage drops due to exci- tation current in the interconnect to the RTD. This voltage drop can be measured and digitally removed using the LTC2422 (see Figure 35). The excitation current (typically 200 µA) flows from the ADC through a long lead length to the remote temperature sensor (RTD). This current is applied to the RTD, whose resistance changes as a function of temperature (100 Ω to 400 Ω for 0°C to 800°C). The same excitation current flows back to the ADC ground and generates another voltage drop across the return leads. In order to get an accurate measurement of the temperature, these voltage drops must be measured and removed from the conversion result. Assuming the resistance is approximately the same for the forward and return paths (R1 = R2), the auxiliary channel on the LTC2422 can measure this drop. These errors are then removed with simple digital correction. The result of the first conversion on CH0 corresponds to an input voltage of VRTD + R1 • IEXCITATION. The result of the second conversion (CH1) is – R1 • IEXCITATION. Note, the LTC2422’s input range is not limited to the supply rails, it has underrange capabilities. The device’s input range is – 300mV to VREF + 300mV. Adding the two conversion results together, the voltage drop across the RTD’s leads are cancelled and the final result is VRTD. An Isolated, 20-Bit Data Acquisition System The LTC1535 is useful for signal isolation. Figure 36 shows a fully isolated, 20-bit differential input A/D converter imple- mented with the LTC1535 and LTC2422. Power on the isolated side is regulated by an LT1761-5.0 low noise, low dropout micropower regulator. Its output is suitable for driving bridge circuits and for ratiometric applications. During power-up, the LTC2422 becomes active at VCC = 2.3V, while the isolated side of the LTC1535 must wait for VCC2 to reach its undervoltage lockout threshold of 4.2V. Figure 35. RTD Remote Temperature Measurement VCC LTC2422 FSSET ZSSET SCK CH0 SDO FO CS CH1 GND 3-WIRE SPI INTERFACE 1 5V 9 8 7 10 6 24212 F35 2 4 3 + – VRTD Pt 100 Ω 5 IDC = 0 IEXCITATION = 200µA IEXCITATION = 200µA R2 R1 5k 25 Ω 1000pF 5k 25 Ω 0.1 µF |
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