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AD7193 데이터시트(PDF) 4 Page - Analog Devices |
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AD7193 데이터시트(HTML) 4 Page - Analog Devices |
4 / 9 page CN-0287 Circuit Note Rev. C | Page 4 of 9 Temperature measurement is a high precision and low speed application, therefore there is adequate settling time available to switch the single current source between all 4 channels, providing excellent channel-to-channel matching, low cost, and small PCB footprint. The ADG738 is an 8-to-1 multiplexer that switches the current source between channels. In order to support the 2-, 3-, and 4- wire RTD configurations, each of the four channels need two switches. In many applications, the RTD may be located remotely from the measurement circuit. The resistance from the long lead wires can generate large errors, especially for low resistance RTDs. In order to minimize the effect of the lead resistance, a 3- wire RTD configuration is supported as shown in Figure 3. Figure 3. Connector and Jumper Configuration for3-Wire RTD Sensor With S1 of the ADG738 closed and S2 opened, the voltage at the input of AD7193 is V1. With S1 opened and S2 closed, the voltage on the input of AD7193 is V2, The voltage across the RTD sensor is VRTD, the exciting current from the current source is IEXC. V1 and V2 contain the error generated by the lead resistance as shown below: EXC W3 RTD 1 I R R V × + = ) ( (1) EXC W3 RTD W2 2 I R R R V × + + = ) ( (2) EXC RTD RTD I R V × = (3) Assuming RW1 = RW2 = RW3 and combining Equations 1, 2, and 3 yields: VRTD = 2V1 – V2 (4) RRTD = VRTD/IEXC = (2V1 – V2)/IEXC (5) Equation 5 shows that the 3-wire configuration requires two separate measurements (V1 and V2) in order to calculate RRTD, thereby decreasing the output data rate. In most applications this is not a problem. The 4-wire RTD connection requires two extra sense lines, but is insensitive to wiring resistances and only requires one measurement. Figure 4 summarizes the connector configuration and jumper placements for RTD 2-wire, RTD 3-wire, RTD 4-wire, and thermocouple applications. Figure 4. Connector Configuration and Jumper Placements for EVAL-CN0287-SDPZ Board AD7193 REFIN2(-) REFIN2(+) AIN2 AIN1 S1 D ADG738 RTD IEXC IEXC S2 JP[x] +5V RRTD RREF RW1 RW2 RW3 CURRENT SOURCE +5V RTD 2-WIRE RTD 3-WIRE RTD 4-WIRE THERMOCOUPLE JPx 2 1 3 2 1 3 4 CNx RTD RTD JPx 2 1 3 2 1 3 4 CNx JPx 2 1 3 2 1 3 4 CNx JPx 2 1 3 2 1 3 4 CNx TC + – RTD |
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