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AD7707 Datasheet(PDF) 45 Page - Analog Devices |
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AD7707 Datasheet(HTML) 45 Page - Analog Devices |
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45 / 52 page  AD7707 Rev. B | Page 45 of 52 PRESSURE MEASUREMENT Other typical applications for the AD7707 include temperature and pressure measurement. Figure 28 shows the AD7707 used with a pressure transducer, the BP01 from Sensym. The pressure transducer is arranged in a bridge network and gives a differential output voltage between its OUT(+) and OUT(−) terminals. With rated full-scale pressure (in this case 300 mmHg) on the transducer, the differential output voltage is 3 mV/V of the input voltage (that is, the voltage between its IN(+) and IN(−) termin- als). Assuming a 5 V excitation voltage, the full-scale output range from the transducer is 15 mV. The low level input channels are ideal for this type of low signal measurement application. The excitation voltage for the bridge is also used to generate the reference voltage for the AD7707. Therefore, variations in the excitation voltage do not introduce errors in the system. Choosing resistor values of 24 kΩ and 15 kΩ, as per Figure 28, gives a 1.92 V reference voltage for the AD7707 when the excitation voltage is 5 V. Using the part with a programmed gain of 128 results in the full-scale input span of the AD7707 being 15 mV, which corresponds with the output span from the transducer. AD7707 DVDD 5V REF IN(–) AGND MCLK IN MCLK OUT DRDY REF IN(+) 24kΩ AVDD AIN1 LOCOM DGND OUT(–) SCLK DIN DOUT CS CONTROLLER 15kΩ OUT(+) EXCITATION VOLTAGE = 5V IN(–) IN(+) Figure 28. Pressure Measurement Using the AD7707 THERMOCOUPLE MEASUREMENT Another application area for the AD7707 is in temperature measurement. Figure 29 outlines a connection from a thermo- couple to the AD7707. In this application, the AD7707 is operated in its unbuffered mode to accommodate signals of ±100 mV on the front end. Cold conjunction compensation is implemented using the AD590 temperature transducer that produces an output current proportional to absolute temperature. AD7707 DVDD 5V AIN2 REF IN(–) AGND MCLK IN MCLK OUT DRDY REF IN(+) 8.2kΩ CJC AVDD AD590 5V AIN1 LOCOM DGND OUT SCLK DIN DOUT THERMOCOUPLE JUNCTION CS CONTROLLER REF192 5V GND Figure 29. Thermocouple Measurement Using the AD7707 RTD MEASUREMENT Figure 30 shows another temperature measurement application for the AD7707. In this case, the transducer is an RTD (resistive temperature device), a PT100. The arrangement is a 4-lead RTD configuration. There are voltage drops across the lead resistances, RL1 and RL4, but these simply shift the common- mode voltage. There is no voltage drop across lead resistances RL2 and RL3 because the input current to the AD7707 is very low. The lead resistances present a small source impedance so it is generally not necessary to turn on the buffer on the AD7707. If the buffer is required, the common-mode voltage should be set accordingly by inserting a small resistance between the bottom end of the RTD and GND of the AD7707. In the application shown, an external 400 μA current source provides the excitation current for the PT100 and generates the reference voltage the AD7707 via the 6.25 kΩ resistor. Variations in the excitation current do not affect the circuit because both the input voltage and the reference voltage vary radiometrically with the excitation current. However, the 6.25 kΩ resistor must have a low temperature coefficient to avoid errors in the reference voltage over temperature. AD7707 DVDD 5V REF IN(–) AGND MCLK IN MCLK OUT DRDY RTD AVDD AIN1 LOCOM DGND SCLK DIN DOUT CS CONTROLLER RL3 6.25kΩ RL2 RL4 RL1 REF IN(+) 400µA REF IN(+) REF IN(–) Figure 30. RTD Measurement Using the AD7707 |
Similar Part No. - AD7707_15 |
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Similar Description - AD7707_15 |
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