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AD7707 Datasheet(PDF) 44 Page - Analog Devices |
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AD7707 Datasheet(HTML) 44 Page - Analog Devices |
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44 / 53 page  AD7707 Rev. B | Page 43 of 52 APPLICATIONS INFORMATION The AD7707 provides a low cost, high resolution analog-to- digital function with two low level input channels and one high level input channel. Because the analog-to-digital function is provided by a Σ-Δ architecture, it makes the part more immune to noisy environments, thus making the part ideal for use in industrial and process control applications. It also provides a programmable gain amplifier, a digital filter and calibration options. Thus, it provides far more system level functionality than off-the-shelf integrating ADCs without the disadvantage of having to supply a high quality integrating capacitor. In addition, using the AD7707 in a system allows the system designer to achieve a much higher level of resolution because noise performance of the AD7707 is better than that of the integrating ADCs. The on-chip PGA allows the AD7707 to handle an analog input voltage ranges as low as 10 mV full scale with VREF = 1.25 V. The pseudo differential input capability of the low level channel allows this analog input range to have an absolute value anywhere between AGND − 100 mV and AVDD + 30 mV when the part is operated in unbuffered mode. It allows the user to connect the transducer directly to the input of the AD7707. In addition, the 3-wire digital interface on the AD7707 allows data acquisition front ends to be isolated with just three wires. The AD7707 can be operated from a single 3 V or 5 V, and its low power operation ensures that very little power needs to be brought across the isolation barrier in an isolated application. DATA ACQUISITION Figure 25 shows a data acquisition system in which the low level input channel is used to digitize signals from a thermocouple and the high level input channel converts process control signals up to ±10 V in amplitude. This application shows the high level input channel being used to convert a number of input signals provided through an external mux controlled by the system microcontroller. Switching channels on the external multiplexer is equivalent to providing a step change on the AIN3 input. It takes three or four updates before the correct output code correspond- ing to the new analog input appears at the output. Therefore, when switching between channels on the external mux, the first three outputs should be ignored following the channel change, or the FSYNC bit in the setup register should be used to reset the digital filter, and ensure that the DRDY is set high until a valid result is available in the output register. SMART VALVE/ACTUATOR CONTROL Another area where the low power, single supply and high voltage input capability is of benefit is in smart valve and actuator control circuits. The AD7707 monitors the signal from the control valve. The controller and the AD7707 form a closed- loop control circuit. Figure 26 shows a block diagram of a smart actuator control circuit, which includes the AD7707. The AD7707 monitors the valve position via a high quality servo potentiometer whose output is ±10 V. Similar applications for the AD7707 include smart transmitters (see Figure 27). Here, the entire smart transmitter must operate from the 4 mA to 20 mA loop. Tolerances in the loop mean that the amount of current availa- ble to power the transmitter is as low as 3.5 mA. The AD7707 consumes only 280 μA, leaving at least 3 mA available for the rest of the transmitter. Figure 27 shows a block diagram of a smart transmitter, which includes the AD7707. AD7707 DVDD +5V AIN2 REF IN(–) AGND MCLK IN MCLK OUT CS REF IN(+) 8.2kΩ CJC AVDD AD590 +5V AIN1 LOCOM 2.5V AIN3 VBIAS HICOM DGND IN1 IN2 250Ω OUT IN3 IN4 IN5 250Ω IN6 VSS A0 A1 A2 SCLK DIN DOUT +15V –15V 0V TO 5V INPUT 4mA TO 20mA 0mA TO 20mA MICRO- CONTROLLER SCLK P0 DOUT DIN P1 P2 P3 VDD THERMOCOUPLE JUNCTION ±10V INPUT 0V TO 10V INPUT ±5V INPUT ANALOG MULTIPLEXER Figure 25. Data Acquisition System Using the AD7707 |
Similar Part No. - AD7707_17 |
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Similar Description - AD7707_17 |
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