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OP285 데이터시트(PDF) 10 Page - Analog Devices |
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OP285 데이터시트(HTML) 10 Page - Analog Devices |
10 / 16 page REV. A OP285 –10– thereby reducing phase error dramatically. This is shown in Figure 13 where the 10x composite amplifier’s phase response exhibits less than 1.5 ° phase shift through 500 kHz. On the other hand, the single gain stage amplifier exhibits 25 ° of phase shift over the same frequency range. An additional benefit of the low phase error configuration is constant group delay, by virtue of constant phase shift at all frequencies below 500 kHz. Although this technique is valid for minimum circuit gains of 10, actual closed-loop magnitude response must be optimized for the amplifier chosen. –20 –45 10k 100k 10M 1M –25 –30 –35 –40 –15 –10 –5 0 START 10,000.000Hz STOP 10,000,000.000Hz SINGLE STAGE AMPLIFIER RESPONSE LOW PHASE ERROR AMPLIFIER RESPONSE Figure 13. Phase Error Comparison For a more detailed treatment on the design of low phase error amplifiers, see Application Note AN-107. Fast Current Pump A fast, 30 mA current source, illustrated in Figure 14, takes advantage of the OP285’s speed and high output current drive. This is a variation of the Howland current source where a sec- ond amplifier, A2, is used to increase load current accuracy and output voltage compliance. With supply voltages of ±15 V, the output voltage compliance of the current pump is ±8 V. To keep the output resistance in the M Ω range requires that 0.1% or better resistors be used in the circuit. The gain of the current pump can be easily changed according to the equations shown in the diagram. 1 2 3 A1 5 6 7 VIN1 VIN2 A2 A1, A2 = 1/2 OP285 R2 R1 GAIN = , R4 = R2, R3 = R1 R1 2k R2 2k R5 50 R3 2k R4 2k IOUT = VIN2 – V IN1 R5 VIN R5 = IOUT = (MAX) = 30mA Figure 14. A Fast Current Pump A Low Noise, High Speed Instrumentation Amplifier A high speed, low noise instrumentation amplifier, constructed with a single OP285, is illustrated in Figure 15. The circuit exhibits less than 1.2 µV p-p noise (RTI) in the 0.1 Hz to 10 Hz band and an input noise voltage spectral density of 9 nV/ √Hz (1 kHz) at a gain of 1000. The gain of the amplifier is easily set by RG according to the formula: V V k R OUT IN G =+ 998 2 . Ω The advantages of a two op amp instrumentation amplifier based on a dual op amp is that the errors in the individual am- plifiers tend to cancel one another. For example, the circuit’s input offset voltage is determined by the input offset voltage matching of the OP285, which is typically less than 250 µV. 1 2 3 A2 A1 5 6 7 VIN A1, A2 = 1/2 OP285 RQ 9.98k +2 GAIN = RG( ) OPEN 1.24k 102 10 2 10 100 1000 GAIN R1 4.99k P1 500 DC CMRR TRIM AC CMRR TRIM C1 5pF–40pF + – RG R2 4.99 R3 4.99k R4 4.99k VOUT Figure 15. A High-Speed Instrumentation Amplifier Common-mode rejection of the circuit is limited by the matching of resistors R1 to R4. For good common-mode rejection, these resistors ought to be matched to better than 1%. The circuit was constructed with 1% resistors and included potentiometer P1 for trimming the CMRR and a capacitor C1 for trimming the CMRR. With these two trims, the circuit’s common-mode rejection was better than 95 dB at 60 Hz and better than 65 dB at 10 kHz. For the best common-mode rejection performance, use a matched (better than 0.1%) thin-film resistor network for R1 through R4 and use the variable capacitor to optimize the circuit’s CMR. The instrumentation amplifier exhibits very wide small- and large-signal bandwidths regardless of the gain setting, as shown in the table. Because of its low noise, wide gain-bandwidth product, and high slew rate, the OP285 is ideally suited for high speed signal conditioning applications. Circuit RG Circuit Bandwidth Gain ( ) VOUT = 100 mV p-p VOUT = 20 V p-p 2 Open 5 MHz 780 kHz 10 1.24 k 1 MHz 460 kHz 100 102 90 kHz 85 kHz 1000 10 10 kHz 10 kHz |
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