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AD8390ARC-REEL7 データシート(PDF) 9 Page - Analog Devices |
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AD8390ARC-REEL7 データシート(HTML) 9 Page - Analog Devices |
9 / 16 page AD8390 Rev. C | Page 9 of 16 THEORY OF OPERATION A C B 50k Ω RG RG 50k Ω 56k Ω VCC 56k Ω VOCM BYP +OUT AD8390 –OUT +IN IADJ PWDN0 RADJ PWDN1 DGND –IN VEE VEE RF RF Figure 22. Functional Block Diagram The AD8390 is a true differential operational amplifier with common-mode feedback. The AD8390 is functionally equivalent to three op amps, as shown in Figure 22. Amplifiers A and B act like a standard dual op amp in an inverting configuration that requires four resistors to set the desired gain. The third amplifier (C) maintains the common-mode voltage (VOCM) at the output of the AD8390. VOCM is internally generated, as shown in Figure 22. The common-mode feedback amplifier (C) drives the noninverting terminals of A and B such that the difference between the output common-mode voltage and VOCM is always zero. This functionality forces the outputs to sit at midsupply, which results in differential outputs of identical amplitude and 180 degrees out of phase. The user also has the option to externally drive the VOCM pin as an input to set the dc output common-mode voltage. For details, see the Setting the Output Common-Mode Voltage section. APPLICATIONS CIRCUIT DEFINITIONS Differential voltage refers to the difference between two node voltages. For example, the output differential voltage (or output differential-mode voltage) is defined as ( ) OUT OUT DM OUT V V V − + − = , (1) V+OUT and V–OUT refer to the voltages at the +OUT and –OUT terminals with respect to a common reference. Common-mode voltage refers to the average of the two node voltages. The output common-mode voltage is defined as ( ) 2 , OUT OUT CM OUT V V V − + + = (2) ANALYZING A BASIC APPLICATION CIRCUIT The AD8390 uses high open-loop gain and negative feedback to force its differential and common-mode output voltages in such a way as to minimize the differential and common-mode error voltages. The differential error voltage is defined as the voltage between the differential inputs +IN and –IN, as shown in Figure 23. For most purposes, this voltage can be assumed to be zero. Similarly, the difference between the actual output common-mode voltage and the voltage applied to VOCM can also be assumed to be zero. Starting from these two assumptions, any application circuit can be analyzed. RF –OUT +OUT +IN –IN RG VOCM RG RL,DM VOUT,DM + – VIN,DM – + RF Figure 23. Basic Applications Circuit (IADJ Pin Not Connected, and PWDN0 and PWDN1 Held High) SETTING THE CLOSED-LOOP GAIN The differential-mode gain of the circuit in Figure 23 can be described by G F DM IN DM OUT R R V V = , , (3) CALCULATING INPUT IMPEDANCE The input impedance of the circuit in Figure 23 between the inputs (V+IN and V−IN) is simply G DM IN R R × = 2 , (4) |
同様の部品番号 - AD8390ARC-REEL7 |
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同様の説明 - AD8390ARC-REEL7 |
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