Datasheet AD648 (Analog Devices) - 9

AD648
The AD648 in this configuration provides a 700 kHz small signal
DUAL PHOTODIODE PREAMP
bandwidth and 1.8 V/ The performance of the dual photodiode preamp shown in µs typical slew rate. The 33 pF capacitor across the feedback resistor optimizes the circuit’s response. The Figure 27 is enhanced by the AD648’s low input current, input oscilloscope photos in Figures 26a and 26b show small and voltage offset, and offset voltage drift. Each photodiode sources large signal outputs of the circuit in Figure 24. Upper traces a current proportional to the incident light power on its surface. show the input signal V R IN. Lower traces are the resulting output F converts the photodiode current to an output voltage equal voltage with the DAC’s digital input set to all 1s. The circuit to RF × IS. settles to ± 0.01% for a 20 V input step in 14 µs. 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 equivalent circuit for the small (0.2 mm2 area) photodiode shown in Figure 27. The input current results in an error pro- portional to the feedback resistance used. The amplifier’s offset will produce an error proportional to the preamp’s noise gain (1+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 MΩ, and the maximum input current and input offset voltage specs of an AD648C. 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 Figure 26a. Response to ±20 V p-p Reference Square excess noise over the bandwidth of interest. CF reduces the Wave noise gain “peaking” at the expense of signal bandwidth. Figure 26b. Response to ±100 mV p-p Reference Square Wave Figure 27. A Dual Photodiode Pre-Amp
TEMP RSH VOS IB
ⴗ
C (M
⍀
) (
␮
V) (1 + RF/RSH) VOS (pA) IBRF TOTAL –25 15,970 150 151
␮
V 0.30 30
␮
V 181
␮
V 0 2,830 225 233
␮
V 2.26 262
␮
V 495
␮
V +25 500 300 360
␮
V 10.00 1.0 mV 1.36 mV +50 88.5 375 800
␮
V 56.6 5.6 mV 6.40 mV +75 15.6 450 3.33 mV 320 32 mV 35.3 mV +85 7.8 480 6.63 mV 640 64 mV 70.6 mV
Figure 28. Photodiode Pre-Amp Errors Over Temperature REV. E –9–