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AD623AN 数据表(PDF) 14 Page - Analog Devices |
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AD623AN 数据表(HTML) 14 Page - Analog Devices |
14 / 16 page AD623 –14– REV. C plane. Maximum isolation between analog and digital is achieved by connecting the ground planes back at the supplies. The digi- tal return currents from the ADC, which flow in the analog ground plane will, in general, have a negligible effect on noise performance. If there is only a single power supply available, it must be shared by both digital and analog circuitry. Figure 46 shows how to minimize interference between the digital and analog circuitry. As in the previous case, separate analog and digital ground planes should be used (reasonably thick traces can be used as an alternative to a digital ground plane). These ground planes should be connected at the power supply’s ground pin. Separate traces should be run from the power supply to the supply pins of the digital and analog circuits. Ideally, each device should have its own power supply trace, but these can be shared by a num- ber of devices as long as a single trace is not used to route cur- rent to both digital and analog circuitry. Ground Returns for Input Bias Currents Input bias currents are those dc currents that must flow in order to bias the input transistors of an amplifier. These are usually transistor base currents. When amplifying “floating” input sources such as transformers or ac-coupled sources, there must be a direct dc path into each input in order that the bias current can flow. Figure 47 shows how a bias current path can be provided for the cases of transformer coupling, capacitive ac-coupling and for a thermocouple application. In dc-coupled resistive bridge LOAD TO POWER SUPPLY GROUND RG –INPUT +INPUT AD623 VOUT +VS REFERENCE –VS Figure 47a. Ground Returns for Bias Currents with Transformer Coupled Inputs LOAD TO POWER SUPPLY GROUND RG –INPUT +INPUT AD623 VOUT +VS REFERENCE –VS Figure 47b. Ground Returns for Bias Currents with Thermocouple Inputs LOAD TO POWER SUPPLY GROUND RG –INPUT +INPUT AD623 VOUT +VS REFERENCE –VS 100k 100k Figure 47c. Ground Returns for Bias Currents with AC Coupled Inputs applications, providing this path is generally not necessary as the bias current simply flows from the bridge supply through the bridge and into the amplifier. However, if the impedances that the two inputs see are large and differ by a large amount (>10 k Ω), the offset current of the input stage will cause dc errors propor- tional with the input offset voltage of the amplifier. Output Buffering The AD623 is designed to drive loads of 10 k Ω or greater. If the load is less that this value, the AD623’s output should be buff- ered with a precision single supply op amp such as the OP113. This op amp can swing from 0 V to 4 V on its output while driving a load as small as 600 Ω. Table III summarizes the per- formance of some other buffer op amps. +5V RG VIN VOUT 0.1 F AD623 REF OP113 +5V 0.1 F Figure 48. Output Buffering Table III. Buffering Options Op Amp Comments OP113 Single Supply, High Output Current OP191 Rail-to-Rail Input and Output, Low Supply Current OP150 Rail-to-Rail Input and Output, High Output Current A Single Supply Data Acquisition System Interfacing bipolar signals to single supply analog to digital converters (ADCs) presents a challenge. The bipolar signal must be “mapped” into the input range of the ADC. Figure 49 shows how this translation can be achieved. 10mV +5V 0.1 F AD623 REF RG 1.02k +5V REFOUT REFIN AIN AD7776 +5V 0.1 F Figure 49. A Single Supply Data Acquisition System The bridge circuit is excited by a +5 V supply. The full-scale output voltage from the bridge ( ± 10 mV) therefore has a common-mode level of 2.5 V. The AD623 removes the common- mode component and amplifies the input signal by a factor of 100 (RGAIN = 1.02 kΩ). This results in an output signal of ± 1V. In order to prevent this signal from running into the AD623’s ground rail, the voltage on the REF pin has to be raised to at least 1 V. In this example, the 2 V reference voltage from the AD7776 ADC is used to bias the AD623’s output voltage to 2 V ±1 V. This corresponds to the input range of the ADC. |
类似零件编号 - AD623AN |
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类似说明 - AD623AN |
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