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ISO100BP Datasheet(PDF) 7 Page - Burr-Brown (TI) |
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ISO100BP Datasheet(HTML) 7 Page - Burr-Brown (TI) |
7 / 15 page ISO100 7 THEORY OF OPERATION The ISO100 is fundamentally a unity gain current amplifier intended to transfer small signals between electrical circuits separated by high voltages or different references. In most applications, an output voltage is obtained by passing the output current through the feedback resistor (RF). The ISO100 uses a single light emitting diode (LED) and a pair of photodiode detectors coupled together to isolate the output signal from the input. Figure 1 shows a simplified diagram of the amplifier. IREF1 and IREF2 are required only for bipolar operation to generate a midscale reference. The LED and photodiodes (D 1 and D2) are arranged such that the same amount of light falls on each photodiode. Thus, the currents generated by the diodes match very closely. As a result, the transfer function de- pends upon optical match rather than absolute performance. Laser-trimming of the components improves matching and enhances accuracy, while negative feedback improves lin- earity. Negative feedback around A1 occurs through the optical path formed by the LED and D 1. The signal is transferred across the isolation barrier by the matched light path to D 2. The overall isolation amplifier is noninverting (a positive going input produces a positive going output). INSTALLATION AND OPERATING INSTRUCTIONS UNIPOLAR OPERATION In Figure 1, assume a current, IIN, flows out of the ISO100 (IIN must be negative in unipolar operation). This causes the voltage at pin 15 to decrease. Because the amplifier is inverting, the output of A1 increases, driving current through the LED. As the LED light output increases, D1 responds by generating an increasing current. The current increases until the sum of the currents in and out of the input node (–Input to A 1) is zero. At that point, the negative feedback through D1 has stabilized the loop, and the current ID1 equals the input current plus the bias current. As a result, no bias current flows in the source. Since D1 and D2 are matched (I D1 = ID2), IIN is replicated at the output via D2. Thus, A1 functions as a unity-gain current amplifier, and A2 is a current-to-voltage converter, as described below. Current produced by D2 must either flow into A2 or RF. Since A2 is designed for low bias current (≈10nA), almost all of the current flows through RF to the output. The output voltage then becomes: V O = (ID2)RF = (ID1 ±IOS)RF ≈ –(–IIN)RF = IINRF (1) where, IOS is the difference between A1 and A2 bias currents. For input voltage operation IIN can be replaced by a voltage source (VIN) and series resistor (RIN), since the summing node of the op amp is essentially at ground. Thus, IIN = VIN/RIN. Unipolar operation does have some constraints, however. In this mode the input current must be negative so as to produce a positive output voltage from A1 to turn the LED on. A current more negative than 20nA is necessary to keep the LED turned on and the loop stabilized. When this condition is not met, the output may be indeterminant. Many sensors generate unidirectional signals, e.g., photoconductive and photodiode devices, as well as some applications of thermo- couples. However, other applications do require bipolar operation of the ISO100. BIPOLAR OPERATION To activate the bipolar mode, reference currents as shown in Figure 1 are attached to the input nodes of the op amps. The input stage stabilizes just as it did in unipolar operation. FIGURE 1. Simplified Block Diagram of the ISO100. A 1 16 15 18 D 1 D 2 A 2 8 Isolation Barrier 7 3 I REF2 R F I REF1 Input Common Output Common 17 Optical Assembly LED V OUT = IIN RF V OUT R IN –In I IN V IN – +In Input Circuit Output Circuit Connect pins 15 and 16 for bipolar, and pins 16 and 17 for unipolar. Connect pins 7 and 8 for bipolar, and pins 8 and 9 for unipolar. 9 + |
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