Datasheet AD548 (Analog Devices) - 9

Application Hints–AD548 PHOTODIODE PREAMP
The performance of the photodiode preamp shown in Figure 7 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 × IS. Figure 7. An error budget illustrating the importance of low amplifier Figure 9. Low Power Instrumentation Amplifier input current, voltage offset, and offset voltage drift to minimize Gains of 1 to 100 can be accommodated with gain nonlinearities output voltage errors can be developed by considering the equi- of less than 0.01%. Input errors, which contribute an output valent circuit for the small (0.2 mm2 area) photodiode shown in error proportional to in amp gain, include a maximum untrimmed Figure 7. The input current results in an error proportional to input offset voltage of 0.5 mV and an input offset voltage drift the feedback resistance used. The amplifier’s offset will produce over temperature of 4 µV/°C. Output errors, which are indepen- an error proportional to the preamp’s noise gain (I + RF/RSH), dent of gain, will contribute an additional 0.5 mV offset and where RSH is the photodiode shunt resistance. The amplifier’s 4 µV/°C drift. The maximum input current is 15 pA over the input current will double with every 10°C rise in temperature, common-mode range, with a common-mode impedance of over and the photodiode’s shunt resistance halves with every 10°C 1 × 1012 Ω. Resistor pairs R3/R5 and R4/R6 should be ratio rise. The error budget in Figure 8 assumes a room temperature matched to 0.01% to take full advantage of the AD548’s high photodiode RSH of 500 MΩ, and the maximum input current common-mode rejection. Capacitors C1 and C1′ compensate for and input offset voltage specs of an AD548C. peaking in the gain over frequency caused by input capacitance
TEMP
when gains of 1 to 3 are used.
C RSH (M ) VOS ( V) (1+ RF/RSH) VOS IB (pA) IBRF TOTAL
The –3 dB small signal bandwidth for this low power instrumenta- tion amplifier is 700 kHz for a gain of 1 and 10 kHz for a gain of –25 15,970 150 151 µV 0.30 30 µV 181 µV 0 2,830 200 207 100. The typical output slew rate is 1.8 V/ µV 2.26 262 µV 469 µV µs. 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
LOG RATIO AMPLIFIER
75 15.6 350 2.6 mV 320 32 mV 34.6 mV Log ratio amplifiers are useful for a variety of signal conditioning 85 7.8 370 5.1 mV 640 64 mV 69.1 mV applications, such as linearizing exponential transducer outputs Figure 8. Photodiode Preamp Errors Over Temperature and compressing analog signals having a wide dynamic range. The AD548’s picoamp level input current and low input offset The capacitance at the amplifier’s negative input (the sum of the voltage make it a good choice for the front-end amplifier of the photodiode’s shunt capacitance, the op amp’s differential input log ratio circuit shown in Figure 10. This circuit produces an capacitance, stray capacitance due to wiring, etc.) will cause a output voltage equal to the log base 10 of the ratio of the input rise in the preamp’s noise gain over frequency. This can result in currents I1 and I2. Resistive inputs R1 and R2 are provided for excess noise over the bandwidth of interest. CF reduces the voltage inputs. noise gain “peaking” at the expense of bandwidth. Input currents I1 and I2 set the collector currents of Q1 and Q2,
INSTRUMENTATION AMPLIFIER
a matched pair of logging transistors. Voltages at points A and The AD548C’s maximum input current of 10 pA makes it an B are developed according to the following familiar diode excellent building block for the high input impedance instru- equation: mentation amplifier shown in Figure 9. Total current drain for V = (kT/q) ln (I /I ) BE C ES this circuit is under 600 µA. This configuration is optimal for In this equation, k is Boltzmann’s constant, T is absolute tem- conditioning differential voltages from high impedance sources. perature, q is an electron charge, and IES is the reverse saturation The overall gain of the circuit is controlled by RG, resulting in current of the logging transistors. The difference of these two the following transfer function: voltages is taken by the subtractor section and scaled by a factor V ) of approximately 16 by resistors R9, R10, and R8. Temperature + R OUT = 1 + (R1 2 V R IN G REV. D –9–