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AD629BRZ-RL bảng dữ liệu(PDF) 10 Page - Analog Devices |
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10 / 16 page AD629 Rev. B | Page 10 of 16 APPLICATIONS BASIC CONNECTIONS Figure 30 shows the basic connections for operating the AD629 with a dual supply. A supply voltage of between ±3 V and ±18 V is applied between Pin 7 and Pin 4. Both supplies should be decoupled close to the pins using 0.1 μF capacitors. Electrolytic capacitors of 10 μF, also located close to the supply pins, may be required if low frequency noise is present on the power supply. While multiple amplifiers can be decoupled by a single set of 10 μF capacitors, each in amp should have its own set of 0.1 μF capacitors so that the decoupling point can be located right at the IC’s power pins. REF (–) REF (+) –VS –VS +VS +VS VOUT = ISHUNT × RSHUNT NC –IN +IN RSHUNT ISHUNT (SEE TEXT) (SEE TEXT) 0.1µF 0.1µF +3V TO +18V –3V TO –18V NC = NO CONNECT 21.1kΩ 380kΩ 380kΩ 20kΩ 380kΩ AD629 1 2 3 4 8 7 6 5 Figure 30. Basic Connections The differential input signal, which typically results from a load current flowing through a small shunt resistor, is applied to Pin 2 and Pin 3 with the polarity shown to obtain a positive gain. The common-mode range on the differential input signal can range from −270 V to +270 V, and the maximum differential range is ±13 V. When configured as shown in Figure 30, the device operates as a simple gain-of-1, differential-to-single- ended amplifier; the output voltage being the shunt resistance times the shunt current. The output is measured with respect to Pin 1 and Pin 5. Pin 1 and Pin 5 (REF(–) and REF(+)) should be grounded for a gain of unity and should be connected to the same low impedance ground plane. Failure to do this results in degraded common- mode rejection. Pin 8 is a no connect pin and should be left open. SINGLE-SUPPLY OPERATION Figure 31 shows the connections for operating the AD629 with a single supply. Because the output can swing to within only about 2 V of either rail, it is necessary to apply an offset to the output. This can be conveniently done by connecting REF(+) and REF(–) to a low impedance reference voltage (some ADCs provide this voltage as an output), which is capable of sinking current. Therefore, for a single supply of 10 V, VREF may be set to 5 V for a bipolar input signal. This allows the output to swing ±3 V around the central 5 V reference voltage. Alternatively, for unipolar input signals, VREF can be set to about 2 V, allowing the output to swing from 2 V (for a 0 V input) to within 2 V of the positive rail. REF (–) REF (+) –VS VY VX +VS +VS NC –IN +IN RSHUNT ISHUNT 0.1µF NC = NO CONNECT 21.1kΩ 380kΩ 380kΩ 20kΩ 380kΩ AD629 1 2 3 4 8 7 6 5 OUTPUT = VOUT – VREF VREF Figure 31. Operation with a Single Supply Applying a reference voltage to REF(+) and REF(–) and operating on a single supply reduces the input common-mode range of the AD629. The new input common-mode range depends upon the voltage at the inverting and noninverting inputs of the internal operational amplifier, labeled VX and VY in Figure 31. These nodes can swing to within 1 V of either rail. Therefore, for a (single) supply voltage of 10 V, VX and VY can range between 1 V and 9 V. If VREF is set to 5 V, the permissible common-mode range is +85 V to –75 V. The common-mode voltage ranges can be calculated by VCM (±) = 20 VX/VY(±) − 19 VREF SYSTEM-LEVEL DECOUPLING AND GROUNDING The use of ground planes is recommended to minimize the impedance of ground returns (and therefore the size of dc errors). Figure 32 shows how to work with grounding in a mixed-signal environment, that is, with digital and analog signals present. To isolate low level analog signals from a noisy digital environment, many data acquisition components have separate analog and digital ground returns. All ground pins from mixed-signal components, such as ADCs, should return through a low impedance analog ground plane. Digital ground lines of mixed-signal converters should also be connected to the analog ground plane. Typically, analog and digital grounds should be separated; however, it is also a requirement to minimize the voltage difference between digital and analog grounds on a converter, to keep them as small as possible (typically <0.3 V). The increased noise, caused by the converter’s digital return currents flowing through the analog ground plane, is typically negligible. Maximum isolation between analog and digital is achieved by connecting the ground planes back at the supplies. Note that Figure 32 suggests a “star” ground system for the analog circuitry, with all ground lines being connected, in this case, to the ADC’s analog ground. However, when ground planes are used, it is sufficient to connect ground pins to the nearest point on the low impedance ground plane. |
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