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MCP602-ISN Datasheet(PDF) 15 Page - Microchip Technology |
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MCP602-ISN Datasheet(HTML) 15 Page - Microchip Technology |
15 / 34 page © 2007 Microchip Technology Inc. DS21314G-page 15 MCP601/1R/2/3/4 4.8.2 INSTRUMENTATION AMPLIFIER CIRCUITS Instrumentation amplifiers have a differential input that subtracts one input voltage from another and rejects common mode signals. These amplifiers also provide a single-ended output voltage. The three-op amp instrumentation amplifier is illustrated in Figure 4-10. One advantage of this approach is unity- gain operation, while one disadvantage is that the common mode input range is reduced as R2/RG gets larger. FIGURE 4-10: Three-Op Amp Instrumentation Amplifier. The two-op amp instrumentation amplifier is shown in Figure 4-11. While its power consumption is lower than the three-op amp version, its main drawbacks are that the common mode range is reduced with higher gains and it must be configured in gains of two or higher. FIGURE 4-11: Two-Op Amp Instrumentation Amplifier. Both instrumentation amplifiers should use a bulk bypass capacitor of at least 1 µF. The CMRR of these amplifiers will be set by both the op amp CMRR and resistor matching. 4.8.3 PHOTO DETECTION The MCP601/1R/2/3/4 op amps can be used to easily convert the signal from a sensor that produces an output current (such as a photo diode) into a voltage (a transimpedance amplifier). This is implemented with a single resistor (R2) in the feedback loop of the amplifiers shown in Figure 4-12 and Figure 4-13. The optional capacitor (C2) sometimes provides stability for these circuits. A photodiode configured in the Photovoltaic mode has zero voltage potential placed across it (Figure 4-12). In this mode, the light sensitivity and linearity is maximized, making it best suited for precision applications. The key amplifier specifications for this application are: low input bias current, low noise, common mode input voltage range (including ground), and rail-to-rail output. FIGURE 4-12: Photovoltaic Mode Detector. In contrast, a photodiode that is configured in the Photoconductive mode has a reverse bias voltage across the photo-sensing element (Figure 4-13). This decreases the diode capacitance, which facilitates high-speed operation (e.g., high-speed digital communications). The design trade-off is increased diode leakage current and linearity errors. The op amp needs to have a wide Gain Bandwidth Product (GBWP). FIGURE 4-13: Photoconductive Mode Detector. MCP60X V1 MCP60X V2 R2 R2 R3 MCP60X R4 R3 R4 VOUT VREF RG + – – + – + V OUT V 1 V 2 – () 1 2R 2 R G --------- + ⎝⎠ ⎛⎞ R4 R 3 ------ ⎝⎠ ⎛⎞ V REF + = MCP60X V2 RG R2 R2 MCP60X R1 VOUT VREF V1 R1 - + - + V OUT V 1 V 2 – () 1 R 1 R 2 ------ 2R 1 R G --------- ++ ⎝⎠ ⎛⎞ V REF + = D1 Light VOUT VDD MCP60X R2 C2 ID1 VOUT = ID1 R2 – + D1 Light VOUT VDD MCP60X R2 C2 ID1 VOUT = ID1 R2 VBIAS VBIAS < 0V – + |
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