AD7714* 3 V/5 V, CMOS, 500 ÂµA Signal Conditioning ADC - dreamm

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AD7714 APPLICATIONS The on-chip PGA allows the AD7714 to handle an analog input voltage range as low as 10 mV full-scale with V REF = +1.25 V. The differential inputs of the part allow this analog input range to have an absolute value anywhere between AGND and AV DD when the part is operated in unbuffered mode. It allows the user to connect the transducer directly to the input of the AD7714. The programmable gain front end on the AD7714 allows the part to handle unipolar analog input ranges from 0 mV to +20 mV to 0 V to +2.5 V and bipolar inputs of ±20 mV to ±2.5 V. Because the part operates from a single supply these bipolar ranges are with respect to a biased-up differential input. Pressure Measurement One typical application of the AD7714 is pressure measurement. Figure 12 shows the AD7714 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/Volt of the input voltage (i.e., the voltage between its IN(+) and IN(–) terminals). Assuming a 5 V excitation voltage, the full-scale output range from the transducer is 15 mV. The excitation voltage for the bridge is also used to generate the reference voltage for the AD7714. Therefore, variations in the excitation voltage do not introduce errors in the system. Choosing resistor values of 24 kΩ and 15 kΩ as per the diagram give a 1.92 V reference voltage for the AD7714 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 AD7714 being 15 mV which corresponds with the output span from the transducer. EXCITATION VOLTAGE = +5V +5V AV DD DV DD IN+ AV DD 1µA AD7714 OUT– IN– OUT+ 24k AIN1 AIN2 AIN3 AIN4 AIN5 AIN6 REF IN (+) SWITCHING MATRIX AGND 1µA BUFFER A = 1–128 SERIAL INTERFACE CHARGING BALANCING A/D CONVERTER AUTO-ZEROED ∑∆ MODULATOR REGISTER BANK DIGITAL FILTER CLOCK GENERATION STANDBY SYNC MCLK IN MCLK OUT RESET 15k REF IN (–) DRDY POL AGND DGND BUFFER DOUT DIN CS SCLK Figure 12. Pressure Measurement Using the AD7714 Temperature Measurement Another application area for the AD7714 is in temperature measurement. Figure 13 outlines a connection from a thermocouple to the AD7714. In this application, the AD7714 is operated in its buffered mode to allow large decoupling capacitors on the front end to eliminate any noise pickup which there may have been in the thermocouple leads. When the AD7714 is operated in buffered mode, it has a reduced common mode range. In order to place the differential voltage from the thermocouple on a suitable common mode voltage, the AIN2 input of the AD7714 is biased up at the reference voltage, +2.5 V. –32– REV. B
AD7714 +5V AV DD DV DD THERMOCOUPLE JUNCTION AV DD 1µA CHARGING BALANCING A/D CONVERTER AIN2 R C C AUTO-ZEROED AIN3 ∑∆ PGA BUFFER MODULATOR AIN4 +V IN REGISTER BANK AIN5 AIN6 A = 1–128 +5V 1µA SERIAL INTERFACE AGND AD780 GND V OUT R AIN1 REF IN (+) REF IN (–) AGND SWITCHING MATRIX AD7714 DIGITAL FILTER CLOCK GENERATION DGND BUFFER DOUT DIN CS SCLK STANDBY SYNC MCLK IN MCLK OUT RESET DRDY POL 2 DV DD Figure 13. Thermocouple Measurement Using the AD7714 Figure 14 shows another temperature measurement application for the AD7714. 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 R L1 and R L4 but these simply shift the commonmode voltage. There is no voltage drop across lead resistances R L2 and R L3 as the input current to the AD7714 is very low . The lead resistances present a small source impedance so it 6.25kΩ R L1 R L2 RTD R L3 R L4 400µA REF IN (+) REF IN (–) AIN1 AGND +5V AV DD AV DD 1µA SWITCHING MATRIX AGND 1µA BUFFER DV DD PGA AIN2 A = 1–128 SERIAL INTERFACE CHARGING BALANCING A/D CONVERTER AUTO-ZEROED ∑∆ MODULATOR REGISTER BANK AD7714 DIGITAL FILTER CLOCK GENERATION STANDBY SYNC MCLK IN MCLK OUT RESET DRDY DGND BUFFER DOUT DIN CS SCLK POL Figure 14. RTD Measurement Using the AD7714 would not generally be necessary to turn on the buffer on the AD7714. 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 AGND of the AD7714. In the application shown an external 400 µA current source provides the excitation current for the PT100 and it also generates the reference voltage for the AD7714 via the 6.25 kΩ resistor. Variations in the excitation current do not affect the circuit as both the input voltage and the reference voltage vary ratiometrically 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. REV. B –33–
Page 1 and 2: a FEATURES Charge Balancing ADC 24
Page 3 and 4: AD7714-3-SPECIFICATIONS (AV DD = +3
Page 5 and 6: TIMING CHARACTERISTICS 1, 2 (AV DD
Page 7 and 8: AD7714 DIP/SOIC PIN NUMBERS PIN FUN
Page 9 and 10: AD7714 AD7714-5 OUTPUT NOISE Table
Page 11 and 12: AD7714 BUFFERED MODE NOISE Table II
Page 13 and 14: CH2-CH0 AD7714 Channel Select. Thes
Page 15 and 16: AD7714 Filter Registers. Power On/R
Page 17 and 18: CIRCUIT DESCRIPTION The AD7714 is a
Page 19 and 20: AD7714 Gain Table XIV. Input Sampli
Page 21 and 22: ANALOG FILTERING The digital filter
Page 23 and 24: System-Offset Calibration System-of
Page 25 and 26: The SYNC input can also be used as
Page 27 and 28: no longer corresponds to a full-sca
Page 29 and 30: AD7714 MICROCOMPUTER/MICROPROCESSOR
Page 31: AD7714 TABLE XV. C CODE FOR INTERFA
Page 35 and 36: PAGE INDEX Topic Page FEATURES . .

AD7714* 3 V/5 V, CMOS, 500 ÂµA Signal Conditioning ADC - dreamm

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