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AD623ARM-REEL7 데이터시트(PDF) 11 Page - Analog Devices |
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AD623ARM-REEL7 데이터시트(HTML) 11 Page - Analog Devices |
11 / 16 page AD623 –11– REV. C Figure 38. Gain Nonlinearity (G = –100, 15 ppm/Div) V– 0 0.5 1 (V–) +0.5 (V+) –1.5 V+ (V+) –1.5 (V+) –0.5 1.5 OUTPUT CURRENT – mA 2 Figure 39. Output Voltage Swing vs. Output Current THEORY OF OPERATION The AD623 is an instrumentation amplifier based on a modified classic three op amp approach, to assure single or dual supply operation even at common-mode voltages at the negative supply rail. Low voltage offsets, input and output, as well as absolute gain accuracy, and one external resistor to set the gain, make the AD623 one of the most versatile instrumentation amplifiers in its class. The input signal is applied to PNP transistors acting as voltage buffers and providing a common-mode signal to the input amplifiers (Figure 40). An absolute value 50 k Ω resistor in each of the amplifiers’ feedback assures gain programmability. The differential output is VO = 1+ 100 kΩ RG V C The differential voltage is then converted to a single-ended voltage using the output amplifier, which also rejects any common- mode signal at the output of the input amplifiers. Since all the amplifiers can swing to either supply rails, as well as have their common-mode range extended to below the nega- tive supply rail, the range over which the AD623 can operate is further enhanced (Figures 19 and 20). The output voltage at Pin 6 is measured with respect to the potential at Pin 5. The impedance of the reference pin is 100 k Ω, so in applications requiring V/I conversion, a small resistor between Pins 5 and 6 is all that is needed. 50k 50k 50k POS SUPPLY 7 INVERTING 2 1 4 50k 50k 50k 8 4 NEG SUPPLY NON- INVERTING 3 7 GAIN OUT 6 REF 5 + – + – Figure 40. Simplified Schematic The bandwidth of the AD623 is reduced as the gain is increased, since all the amplifiers are of voltage feedback type. At unity gain, it is the output amplifier that limits the bandwidth. There- fore even at higher gains the AD623 bandwidth does not roll off as quickly. APPLICATIONS Basic Connection Figure 41 shows the basic connection circuit for the AD623. The +VS and –VS terminals are connected to the power supply. The supply can be either bipolar (VS = ± 2.5 V to ± 6 V) or single supply (–VS = 0 V, +VS = 3.0 V to 12 V). Power supplies should be capacitively decoupled close to the devices power pins. For best results, use surface mount 0.1 µF ceramic chip capacitors and 10 µF electrolytic tantalum capacitors. The input voltage, which can be either single-ended (tie either –IN or +IN to ground) or differential is amplified by the pro- grammed gain. The output signal appears as the voltage difference between the Output pin and the externally applied voltage on the REF input. For a ground referenced output, REF should be grounded. GAIN SELECTION The AD623’s gain is resistor programmed by RG, or more pre- cisely, by whatever impedance appears between Pins 1 and 8. The AD623 is designed to offer accurate gains using 0.1%–1% tolerance resistors. Table I shows required values of RG for various gains. Note that for G = 1, the RG terminals are uncon- nected (RG = ). For any arbitrary gain, RG can be calculated by using the formula RG = 100 kΩ/(G – 1) REFERENCE TERMINAL The reference terminal potential defines the zero output voltage and is especially useful when the load does not share a precise ground with the rest of the system. It provides a direct means of injecting a precise offset to the output. The reference terminal is also useful when bipolar signals are being amplified as it can be used to provide a virtual ground voltage. The voltage on the reference terminal can be varied from –VS to +VS. |
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