Datenblatt-Suchmaschine für elektronische Bauteile |
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AD7091R Datenblatt(PDF) 3 Page - Analog Devices |
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AD7091R Datenblatt(HTML) 3 Page - Analog Devices |
3 / 8 page Circuit Note CN-0337 Rev. 0 | Page 3 of 8 U1C 1/4 AD8608 U1D 1/4 AD8608 U1A 1/4 AD8608 VR +0.1V VOUT 0.1V TO 2.4V +3.3V R10 1kΩ R4 39.2kΩ R3 1kΩ GND R1′ 95Ω R6′ 1.9kΩ R5 2kΩ RTD (Pt100) LINE 0ºC TO 300°C 100Ω TO 212.05Ω INPUT 1 2 3 1 2 3 r1 r2 r3 RX = R0 + ∆R Figure 2. RTD Signal Conditioning Circuit Using a Three-Wire Connection Circuit Design The circuit shown in Figure 2 converts the RTD resistance change from 100 Ω to 212.05 Ω to an output voltage change of 0.1 V to 2.4 V, which is compatible with the ADC input range. In addition, the circuit removes the errors associated with the wiring resistances r1 and r2. The transfer function of the circuit in Figure 2 is obtained using the superposition principle: R3 R4 R1 r R1 V R3 R4 R6 R5 R6 ) R1 r R r R1 V V 2 R 2 X 1 R OUT ) ' ( ' 1 ' ' ' ( ' (1) where: RX = R0 + ΔR R1 ′ = R1||R2 =R0, R6′ = R6||R12 r1 = r2, and neglects the voltage drop across r3. Expand Equation 1, set the term containing r1 to zero, and solve for R6 ′: R4/R3 R4/R3 R5 R6 2 ' (2) Meeting the criteria in Equation 2 removes the error due to the lead resistances, r1 = r2, (r3 is not taken into account because it is connected to the high impedance input of U1D). Substituting Equation 2 into Equation 1, obtain the transfer function: R R3 R4 R V V 0 R OUT 2 (3) Equation 3 shows that the lead wire resistance is fully compensated provided Equation 2 is met. The gain is set to the desired value by adjusting the ratio of R4/R3. Calculation of the Gain, Output Offset, and Resistor Values and Tolerances. For temperature range of 0°C to 300oC, the RTD Pt100 resistance range is 100 Ω to 212.05 Ω, and the input resistance change, ΔR, for the circuit in Figure 2 is 0 Ω to 112.05 Ω. Therefore, the gain of the circuit from Equation 3 is: mA 20.53 Ω 0 Ω 112.05 V 0.1 V 2.4 2 R V R3 R4 R V Gain OUT 0 R (4) Assuming that the current through the sensor is equal to 1 mA and R0 = 100 Ω, the required reference voltage VR is: V 1 . 0 mA 1 100 R V . Then, Equation 4 is solved for R4/R3: 06 . 41 mA 20.53 V 0.1 Ω 100 2 R3 R4 Choose R3 = 1 kΩ, then R4 = 41 kΩ. Choosing a standard value of 2 kΩ for Resistor R5, Resistor R6 ′ can be calculated from Equation 2. k Ω 907 . 1 06 . 41 2 06 . 41 k Ω 2 ' R4/R3 2 R4/R3 R5 R6 An easy way to ensure Equation 2 is met is to use the following relationships: R5 = 2R3, R6′ = R5||R4, as shown in Figure 1. If this condition is met, R1′ = R0 = 100 Ω at 0°C, and VOUT = 0 V. The output offset of the circuit must now be set to 0.1 V. An easy way to shift the output is to make the resistor R1 ′ slightly less than R0. Note that this affects the gain proportionally. The output offset of 0.1 V is approximately 4.35% of the total span of 2.3 V, therefore the ratio R1 ′/R0 must be less than 0.9565. To keep the high output level equal to 2.4 V, the ratio R4/R3 can be proportionally corrected. For example, R4 = 0.9565 × 41.06 × R3 = 39.27 kΩ. Using standard resistors values as shown in Figure 1, the circuit gives a good approximation to the required gain and the output offset. Resistor R1 ′ is formed by connecting Resistor R2 = 1.91 kΩ in parallel with resistor R1 = 100 Ω. |
Ähnliche Teilenummer - AD7091R |
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Ähnliche Beschreibung - AD7091R |
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