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AD548C bảng dữ liệu(PDF) 7 Page - Analog Devices |
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AD548C bảng dữ liệu(HTML) 7 Page - Analog Devices |
7 / 8 page Figure 29. Low Power Instrumentation Amplifier Gains of 1 to 100 can be accommodated with gain nonlinearities of less than 0.01%. Referred to input errors, which contribute an output error proportional to in amp gain, include a maxi- mum untrimmed input offset voltage of 0.5 mV and an input offset voltage drift over temperature of 4 µV/°C. Output errors, which are independent of gain, will contribute an additional 0.5 mV offset and 4 µV/°C drift. The maximum input current is 15 pA over the common-mode range, with a common-mode impedance of over 1 × 1012 Ω. Resistor pairs R3/R5 and R4/R6 should be ratio matched to 0.01% to take full advantage of the AD548’s high common-mode rejection. Capacitors C1 and C1 ′ compensate for peaking in the gain over frequency caused by input capacitance when gains of 1 to 3 are used. The –3 dB small signal bandwidth for this low power instru- mentation amplifier is 700 kHz for a gain of 1 and 10 kHz for a gain of 100. The typical output slew rate is 1.8 V/ µs. LOG RATIO AMPLIFIER Log ratio amplifiers are useful for a variety of signal condition- ing applications, such as linearizing exponential transducer out- puts and compressing analog signals having a wide dynamic range. The AD548’s picoamp level input current and low input offset voltage make it a good choice for the front-end amplifier of the log ratio circuit shown in Figure 30. This circuit produces an output voltage equal to the log base 10 of the ratio of the in- put currents I1 and I2. Resistive inputs R1 and R2 are provided for voltage inputs. Input currents I1 and I2 set the collector currents of Q1 and Q2, a matched pair of logging transistors. Voltages at points A and B are developed according to the following familiar diode equation: V BE = (kT /q )ln ( IC / I ES ) In this equation, k is Boltzmann’s constant, T is absolute tem- perature, q is an electron charge, and IES is the reverse saturation current of the logging transistors. The difference of these two voltages is taken by the subtractor section and scaled by a factor of approximately 16 by resistors R9, R10, and R8. Temperature Application Hints–AD548 PHOTODIODE PREAMP The performance of the photodiode preamp shown in Figure 27 is enhanced by the AD548’s low input current, input voltage offset and offset voltage drift. The photodiode sources a current proportional to the incident light power on its surface. RF converts the photodiode current to an output voltage equal to RF × I S. Figure 27. An error budget illustrating the importance of low amplifier input current, voltage offset and offset voltage drift to minimize output voltage errors can be developed by considering the equi- valent circuit for the small (0.2 mm 2 area) photodiode shown in Figure 27. The input current results in an error proportional to the feedback resistance used. The amplifier’s offset will produce an error proportional to the preamp’s noise gain (I + RF/RSH), where RSH is the photodiode shunt resistance. The amplifier’s input current will double with every 10 °C rise in temperature, and the photodiode’s shunt resistance halves with every 10 °C rise. The error budget in Figure 28 assumes a room temperature photodiode RSH of 500 M Ω, and the maximum input current and input offset voltage specs of an AD548C. TEMP CRSH (M )VOS ( V) (1+ RF/RSH) VOS IB (pA) IBRF TOTAL –25 15,970 150 151 µV 0.30 30 µV 181 µV 0 2,830 200 207 µV 2.26 262 µV 469 µV +25 500 250 300 µV 10.00 1.0 mV 1.30 mV +50 88.5 300 640 µV 56.6 5.6 mV 6.24 mV +75 15.6 350 2.6 mV 320 32 mV 34.6 mV +85 7.8 370 5.1 mV 640 64 mV 69.1 mV Figure 28. Photo Diode Pre-Amp Errors Over Temperature The capacitance at the amplifier’s negative input (the sum of the photodiode’s shunt capacitance, the op amp’s differential input capacitance, stray capacitance due to wiring, etc.) will cause a rise in the preamp’s noise gain over frequency. This can result in excess noise over the bandwidth of interest. CF reduces the noise gain “peaking” at the expense of bandwidth. INSTRUMENTATION AMPLIFIER The AD548C’s maximum input current of 10 pA makes it an excellent building block for the high input impedance instru- mentation amplifier shown in Figure 29. Total current drain for this circuit is under 600 µA. This configuration is optimal for conditioning differential voltages from high impedance sources. The overall gain of the circuit is controlled by RG, resulting in the following transfer function: V OUT V IN = 1 + ( R 1 + R2 ) R G REV. C –7– |
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